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THE LIBRARY
THE UNIVERSITY OF BRITISH COLUMBIA
MENDEL'S
PRINCIPLES OF HEREDITY
CAMBRIDGE UNIVERSITY PRESS
Hontion: FETTER LANE, E.G.
C. F. GLAY, Manager
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ALSO ILontion: H. K. LEWIS, 136, GOWER STREET, W.C.
(EUinbursb: 10°. PRINCES STREET
JSerlm: A. ASHER AND CO.
ILcipjig: F. A. BROCKHAUS
i^cbJlork: G. P. PUTNAM'S SONS
Bombag anl3 Calcutta: MACMILLAN AND CO., Ltd.
A /I rights reserved
GREGOR MENDEL, 1866
Enl(iri!;ed from a i^roup of the brethren of the Konigskloster
i^h^i^'S
PRINCIPl ? •- <)F HEREDITY'
BY
w BATE SO V
FELLOW OF PROFESSOR OF BIOLOGY
.MF^
pRiNClPLti (*
H
4SC]^
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MENDEL'S
PRINCIPLES OF HEREDITY
BY
W. BATESON, M.A., F.R.S., V.M.H,
FELLOW OF ST JOHn's COLLEGE, PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE
Cambridge :
at the University Press
1909
Published March 1909 Reprinted August 1909
PREFACE
THE object of this book Is to give a succinct account of discoveries in regard to Heredity made by the application of Mendel's method of research. Following the clue which his long lost papers provided we have reached a point from which classes of phenomena hitherto proverbial for their seeming irregularity can be recognized as parts of a consistent whole. The study of Heredity thus becomes an organised branch of physiological science, already abun- dant in results, and in promise unsurpassed.
A translation of Mendel's two papers, together with a biographical note, is appended. The translation of the first, based on a draft prepared for the Society by Mr C. T. Druery, was printed in the Royal Horticultural Society's Journal, 1901. With modifications I published it separately in 1902, giving a brief summary of Mendelism as then developed, under the title Menders Principles of He7'edity : A Defence. The object of that publication was to put Mendel's work before the English speaking peoples and to repel the attack which the late Professor Weldon had recently made on Mendelian methods and the conclusions drawn from them. The edition was at once sold out, but I did not reprint the book. As a defence it had served its purpose. Moreover the progress of experiment with the extension of Mendelian conceptions was rapid, and the account of those conceptions there given was In some Important respects soon out of date. In particular my view of the nature of compound factors was shown to be unnecessarily complex and largely incorrect. Though
vi Preface
obviously in a subject fast extending under the influence of many workers there can be no finahty, yet at the present time our knowledge of the main facts has reached a definite stage, and a useful and relatively permanent pre- sentation of the phenomena can be attempted.
The range and diversity of facts, zoological and botani- cal, from which the material is drawn are so wide that it has been difficult to present them adequately within a moderate compass. Many of the types studied might singly provide the subject of a treatise, and the temptation to annotative excursion has been very great ; but the course which seemed most useful was to admit only such detail as had a clear significance in the exposition of principle, or as a suggestion for further inquiry. The reader therefore will understand that if he turns to the original records specified he will almost always find information, perhaps important, which is omitted here.
In the original plan of the book it was intended to discuss somewhat fully the bearing of the new facts on the great problems of Biology, but it is perhaps more fitting that these theoretical considerations should be detached from a presentation of the concrete phenomena. In 1907 I had the honour of delivering the Silliman Lectures in Yale University, and I then took these wider aspects of Genetics as my theme, showing the bearing of the new knowledge on current theory, especially on that of Evolu- tion, and the nature of Variation. The substance of these lectures I propose to publish separately with amplifications, and on the present occasion allusion to these matters has been restricted to the briefest possible indication of the lines of thought which Mendelism inevitably suggests.
A chapter dealing with practical applications of Mendelian principles has been introduced. Such applications will probably far exceed any limits we can yet perceive. Among them we must foresee not merely advances in the art of
Preface vii
breeding animals and plants, but a control over the destiny of our own species. These things are spoken of in their place. To prevent disappointment, however, it must be at once admitted that for fanciers Mendelism can as yet do comparatively little. "Fancying" provides the chief interest in life for thousands of persons in this country. It is an occupation with which the scientific naturalist should have more sympathy than he has commonly evinced. If the scientific world had kept in touch with the operations of the ''fancy" much nonsense which has passed into scientific orthodoxy would never have been written. The study of Mendelian phenomena will do something to bring about a fruitful interchange of experience. But for the " fancy " our work can as yet do two things only. First, in the study of the w^orkings of the Mendelian system it will provide a most fascinating pursuit, which If followed with assiduous care may at any moment lead to some consider- able advance in scientific knowledge. Secondly, the prin- ciples already ascertained will be found of practical assistance in the formation of new breeds and may save many mistakes and waste of time. But applied to the business of breeding winners In established breeds they cannot materially help, for almost always the points which tell are too fine to be dealt with In our analysis.
In a work of this kind an author must necessarily speak of various subjects on which his knowledge can be super- ficial only, and I trust that If Inaccuracies have been Intro- duced, readers will be good enough to send me corrections.
Much and varied assistance has been given me by many persons. Such help on special points has been acknowledged in the text, but a fuller and more prominent acknowledgment is due to my colleagues. Without their cooperation there would have been, so far as Cambridge Is concerned, but meagre contributions to record. In the early days of Mendelism, and before, Miss E. R. Saunders
viii Preface
collaborated with me. A beautiful series of results, especially relating to the heredity of Stocks [Matthiold), has been the fruit of her labours exclusively. Not only have these results greatly advanced our knowledge of genetic pheno- mena, but I think that at a time when Mendelism was, in England at least, regarded with suspicion, the obvious precision of her work and the persistence of her advocacy did much to convince the scientific world of the reality of our assertions.
In 1904 I had the good fortune to gain Mr R. C. Punnett as a partner. Since that date we have worked in close collaboration, and the work that we have thus done has been in every sense a joint product, both as regards design, execution, and interpretation of results. Though for the presentation of the views contained in this book I am solely responsible, those that apply to the subjects of our own work are often his, or have been arrived at in consequence of interchange of ideas with him.
On some points of general physiology I have received useful suggestions and criticism from Mr F. F. Blackman, and in this respect I am also especially beholden to Miss F. M. Durham.
The Plates of Sweet Peas and Mice are photographic reproductions, on the whole very accurate, of coloured drawings most kindly made for me by Miss Wheldale. The Plate of Primula flowers is taken from an excellent coloured photograph by Mr Walthanv^ For Fig. 9 I am obliged to the New Phytologist.
For several years past I have had an exceptional opportunity of seeing breeding conducted on a large scale
* Since the word "magenta," often used in English for the description of a colour, is not understood on the Continent, I may say that it means a purplish or hhiish red, as distinguished from a crimson or pink red. On IMate VI, Fi.^s. 8, 9, 14, 15, 20, 21 represent shades of magenta, while I'igs. 2, 7, 13, iQ are true reds.
Preface ix
through the great kindness of Messrs Sutton of Reading, who have given me the privilege of watching such parts of their work in raising varieties as seemed especially in- structive, with unrestricted access to their pedigree books. From this I have derived much profit, and many hints which have formed the starting point for fuller experiment. My hearty thanks are due to them for this Important assistance.
W. BATESON.
Grantchester, Cambridge. February^ 1909.
Note to the second impression.
The publication of a reprint gives an opportunity of correcting mistakes which correspondents have been good enough to point out. These concern chiefly the exceptional case In regard to web-foot in Pigeons (p. 36), the dominance of colour in Helix (p. 45), and the age of the subject of Fig. 25. For correction In the formula (p. 60) see Corrigenda.
A serious error respecting the heredity of Colour-blind- ness, which through Mr Nettleshlp's courtesy I was able to withdraw by means of a printed slip inserted at p. 260 of the original Issue, has now been excised. In a note added at p. 195 I have indicated the probable bearing of the special features observed In the case of Colour-blindness upon the theory of Sex, and at p. 231 a diagram Is inserted showing the probable course of descent followed by that condition. This scheme Is in part unverified and must be considered as tentative onlv. I shall be most crrateful for accurate In- formation as to families containing colour-blind members.
W. B.
August^ 1909.
CORRIGENDA
p. 59, last paragraph. For is given read are given, p. 60, line 5. For \n read \n.
ADDENDUM to p. 48
In Naturalist^ 1906, p. 113, C. T. Trechmann records an observation that several sinistral Limtiaea peregra placed in an aquarium produced young both sinistral and dextral, the former slightly preponderating in number.
CONTENTS
PART I.
CHAPTER PAGE
I. Introductory. Mendel's Discovery . . . . i
Introductory — Some pre-Mendelian Writings — Mendel's Discovery — Dominant and Recessive — Segregation. Allelomorphism — Homo- zygote and Heterozygote. Purify of Type.
II. The Material Investigated . . . . . .18
List of Structural Characters in Plants and Animals — List of Types in which the inheritance of Colour has been studied — Preliminary Deductions — Dominance and heterozygous characters — Mendel's system distinguished from that of Galton.
III. Numerical Consequences and Recombinations . . 57
Representations of the Fg Generation and Novelties due to Re-combina- tion of Factors — Compound Characters — Combs of P'owls — Hetero- stylism — White Flowers from Red x Cream.
IV. Heredity of Colour ....... 74
Factors determining Colours : the Ratio 9:3: 4 — The " Presence and Absence " Hypothesis. Epistatic and Hypostatic Factors — Colours of Mice — Pied Types — A Dominant Piebald.
V. Heredity of Colour {continued) ..... 88
Albinos giving Coloured Offspring ; Reversion on Crossing — Various Kinds of Whites — Stocks— Orchids — Pigeons— Fowls — Primula.
VI. Heredity of Colour {contmued) . . . . .107
Eye-Colours. Variations in Colour of the Iris— Deficiency of Eye- Pigments in some Coloured Types.
VII. Heredity of Colour {continued) . . . • - ^^5
The Genetics of Yellow Pigments in certain Animals. Yellow Mice not breeding true — The Case of Basset Hounds and the " Law of An- cestral Heredity." Relation of this Principle to Mendelian Rules.
VIII. Heredity of Colour {continued) 132
Various Specific Phenomena in Colour-Inheritance. Relation of Colour to Hoariness in Stocks. Miscellaneous Cases. Colour of a Special Part controlling that of other Parts— Summary and Discussion— Subtraction-Stages.
Xll
Contents
CHAPTER
IX. Gametic Coupling and Spurious Allelomorphism
PAGE
148
Pollen-Shape and Flower-Colour. Axil-Colour and Sterile Anthers- Hooded Standard and Flower-Colour in Sweet Peas.
X. Heredity and Sex ^^4
Evidence from Breeding Experiments. ^;j^«w— Sex-limited Heredity. The Horns of Sheep— Colour-Blindness— Sex and Spurious Allelo- morphism. The Currant Moth— The Cinnamon Canary— The Silky Fowl — Aglia tau — Cytological Evidence— Summary.
XI. Double Flowers iQ^
Miscellaneous Cases. Recessive and Dominant Doubling— " Hose-in- Hose " Flowers— The Special Case of Double Stocks.
XII. Evidence as to Mendelian Inheritance in Man . 205
Normal Characters— Diseases and Malformations. Dominants— Sex- limited Dominants— Recessives— Notes on collecting Evidence.
XIII. Intermediates between Varieties and the "Pure
Lines" of Johannsen 235
Intermediates as Heterozygous Forms— Subtraction-Stages of Dominants — Interfering Factors — Fluctuational Forms — "Pure Lines."
XIV. Miscellaneous Exceptional and Unconformable Phe-
nomena 245
Crosses breeding true without Segregation. Parthenogenetic or Apo- ganiic Forms. Hieracitun — Sexual Forms — Numerical Aberrations — Irregularities of Dominance — Alternation of Generations — Mater- nal Characters in certain Seeds.
XV. Biological Conceptions in the Light of Mendelian
Discoveries ......••• 266
Nature of Units — Nature of Segregation — Moment of Segregation — Differentiation of Parts compared with Segregation — Reversion and Variation. " Bush " and " Cupid " Sweet Peas — Mendelian Segre- gation and Species — Discontinuity in Variation — Mendelism and Natural Selection.
XVI. Practical Application of Mendelian Principles . 291
Meaning of Pure-bred — Rogueing — Raising Novelties — A Practical Ex- ample— Unfixable Types — Technical Methods — Sociological Appli- cation.
PART II.
1. Biographical Notice of Mendel
2. Translation of the Paper on Hybridisation
3. Translation of the Paper on Hieraciwn .
Bibliography
Supplementary List
Index of Subjects
Index of Authors
309 317 362
369
385 386
394
LIST OF ILLUSTRATIONS
PORTRAITS OF MENDEL.
In 1866 About 1862 About 1880
Frontispiece to face p. 309 to face p. 317
COLOURED PLATES.
Plate I. Lepidoptera .....
11. Mice
III. Reversion in Sweet Peas
IV. Fowls
V. Spurious Allelomorphism in Sweet Peas
VI. Heredity of Colour in Primula Sinensis
to face p. 44
between pp. 80-81
between pp. 93-94
to face p. 1 03
between pp. 154-155
between pp. 294-295
FIGURES.
FIGURE
1. Tall and "Cupid" dwarf Sweet Peas
2. Diagram showing consequences of Segregation .
3. Inheritance of seed-characters in Pea
4. Branched and unbranched forms in Stocks {Mafthiola)
5. Hooded and normal Barley .....
6. Heredity in Wheat .......
7. Fern-leaf and palm-leaf in Primula Sinensis
8. Two-row and six-row Barley
9. Starch-grains in Peas ....
10. Round and wrinkled seed in Maize .
11. Down-colour in Chickens .
12. Types of combs in Fowls .
13. Combs of newly-hatched Chickens
14. Descent of "homostyle" character in Primula
PAGE
9 12
15
20
21
23 24
27
29
30
51 61
62
69
XIV
List of Ilhistratio7ts
FIGURE
15. Diagram of Fo showing ratio 9:3:4
1 6. Diagram of F2 showing ratio 9:7.
17. Diagram of Fo in Sweet Pea showing ratio 27 : 9 : 28
18. Pedigrees of eye-colour in Man ....
19. Pollen grains of Sweet Peas .....
20. Heredity of horns in Sheep .....
21. Heredity of a peculiar form of curly hair
22. Descent of congenital lock of white hair .
23. Brachydactylous hands ......
24. Skiagram of hands .......
25. Hands of brachydactylous child ....
26. Pedigree of brachydactylous family ....
27. Descent from brachydactylous members
28. Drinkwater's pedigree of brachydactyly
29. Descent of prae-senile cataract .....
30. Another cataractous family .....
31. Descent of stationary night-blindness . between pp.
32. Descent of Colour-blindness .....
33. Ideal Scheme of descent of simple sex-limited condition
e.g. horns of Sheep ......
34. Tentative representation of descent of Colour-blindness
35. Polish X Rivet Wheat .......
36. Seeds of Polish x Eivet Wheat .....
37. Reversion in height of Sweet Peas ....
38. Two types of dwarf Sweet Peas ....
PAGE
77 89
91
108
150 171 207 207 211 212 213 214 214
215 216
217
220-221 223
230 231
259 260
282
283
PART I
CHAPTER I
INTRODUCTORY. MENDEL'S DISCOVERY.
Introductory — Some pre-Mendelian Writings — MendeVs Discovejy — Dominant and Recessive — Segregation. Allelomorphism — Homozygote and Heterozygote. Purity of Type,
Among the biological sciences the study of heredity occupies a central position. Whether we be zoologists, botanists, or physiologists, the facts of heredity concern ^t us. Upon this physiological function all the rest in some degree depend. Every advance in knowledge of that central function must affect the course of thought along each several line of biological inquiry.
Moreover though, as naturalists, we are not directly concerned with the applications of science, we must perceive that in no' region of knowledge is research more likely to increase man's power over nature. The science of sociology, and in many of its developments the science of medicine • also, must of necessity form working hypotheses respecting the course of heredity, and we cannot doubt that a percep- tion of the truth in reeard to the function of transmission will greatly contribute to the progress of these sciences. Lastly, to the industrial arts of the breeder of plants or animals, the knowledge we are attempting to provide is of such direct importance that upon this consideration no special emphasis is required. In studying heredity, there- fore, we are examining a vital problem of no mean consequence, and those who engage in that work are happy in the thought that they are assisting one of the main advances in natural knowledge.
But though we may approach this study of genetics — to use the modern designation — from so many different sides, it is especially in their bearing on the problem of
E. H. I
2 Introductory [ch.
the evolution of species that the facts have hitherto been most profitably investigated. It was in the attempt to ascertain the interrelationships between species that experi- ments in genetics were first made. The words '' evolution " and *' origin of species " are now so intimately associated with the name of Darwin that we are apt to forget that the idea of a common descent had been prominent in the minds of naturalists before he wrote, and that, for more than half a century, zealous investigators had been devoting them- selves to the experimental study of that possibility. Promi- nent among this group of experimenters may be mentioned Koelreuter, John Hunter, Herbert, Knight, Gaertner, Jordan, Naudin, Godron, Lecoq, Wichura — men whose names are familiar to every reader of Animals and Plants under Domestication. If we could ask those men to define the object of their experiments, their answer would be that they were seeking to determine the laws of hereditary trans- mission with the purpose of discovering the interrelationships of species. In addition to the observation of the visible structures and habits of plants and animals they attempted by experiment to ascertain those hidden properties of living things which we may speak of as genetic, properties which breeding tests can alone reveal. The vast mass of observation thus accumulated contains much that is of permanent value, hints that if followed might have saved their successors years of wasted effort, and not a few indications which in the light of later discovery will greatly accelerate our own progress.
Yet in surveying the work of this school we are conscious of a feeling of disappointment at the outcome. There are signs that the workers themselves shared this disappointment. As we now know, they missed the clue without which the evidence so laboriously collected remained an inscrutable medley of contradictions.
While the experimental study of the species problem was in full activity the Darwinian writings appeared. Evolution, from being an unsupported hypothesis, was at length shown to be so plainly deducible from ordinary experience that the reality of the process was no longer doubtful. With the triumph of the evolutionary idea, curiosity as to the significance of specific differences was
i] Introchtctory 3
satisfied. The Origin was published In 1859. During the following decade, while the new views were on trial, the experimental breeders continued their work, but before 1870 the field was practically abandoned.
In all that concerns the problem of species the next thirty years are marked by the apathy characteristic of an age of faith. Evolution became the exerclslng-ground of essayists. The number Indeed of naturalists Increased ten- fold, but their activities were directed elsewhere. Darwin's achievement so far exceeded anything that was thought possible before, that what should have been hailed as a long-expected beginning was taken for the completed work. I well remember receiving from one of the most earnest of my seniors the friendly warning that It was waste of time to study variation, for " Darwin had swept the field."
Parenthetically we may notice that though scientific opinion in general became rapidly converted to the doctrine of pure selection, there was one remarkable exception. Systematlsts for the most part kept aloof. Everyone was convinced that natural selection operating In a continuously varying population was a sufficient account of the origin of species except the one class of scientific workers whose labours familiarised them with the phenomenon of specific difference. From that time the systematlsts became, as they still in great measure remain, a class apart.
A separation has thus been effected between those who lead theoretical opinion and those who by taste or necessity have retained an acquaintance with the facts. The con- sequences of that separation have been many and grievous. To it are to be traced the extraordinary misapprehensions as to the fundamental phenomena of specific difference which are now prevalent.
If species had really arisen by the natural selection for impalpable differences, Intermediate forms should abound, and the limits between species should be on the whole indefinite. As this conclusion follows necessarily from the premisses, the selectionists believe and declare that it represents the facts of nature. Differences between species being by axiom Indefinite, the differences between varieties must be supposed to be still less definite. Consequently the conclusion that evolution must proceed by insensible
4 Introductory [ch.
transformation of masses of Individuals has become an established dogma. Systematlsts, entomologists or botan- ists for example, are dally witnesses to variation occurring as an Individual and discontinuous phenomenon, but they stand aside from the debate ; and whoever In a discussion of evolutionary theory appeals to the defmiteness of varietal distinctions In colour for Instance, or In form, as recognizable by common observation without mechanical aid, must be prepared to meet a charge of want of Intelligence or candour. This is no doubt a passing phase and will end so soon as interest In the problems of evolution is combined with some knowledge of variation and heredity.
Genetic experiment was first undertaken, as we have seen, in the hope that It would elucidate the problem of species. The time has now come when appeals for the vigorous prosecution of this method should rather be based on other grounds. It is as directly contributing to the advancement of pure physiological science that genetics can present the strongest claim. We have an eye always on the evolution-problem. We know that the facts we are collecting will help in its solution ; but for a period we shall perhaps do well to direct our search more especially to the immediate problems of genetic physiology, the laws of heredity, the nature of variation, the significance of sex and of other manifestations of dimorphism, willing to postpone the application of the results to wider problems as a task more suited to a maturer stage. When the magnitude and definlteness of the advances already made In genetics come to be more generally known, it Is to be anticipated that workers in various departments of biology will realise that here at last is common ground. As we now know, the conceptions on which both the systematlsts and the specula- tive biologists have based their methods need complete revision in the light of the new facts, and till the possibilities of genetic research are more fully explored the task of reconstruction can hardly be begun. In that work of exploration all classes of naturalists will alike find interest. The methods are definite and exact, so we need not fear the alienation of those systematlsts to whom all theoretical inquiry is repulsive. They are also wide in their scope, and those who would turn from the details of classification
I] Pre-Mendelian IVritings 5
as offering matter too trivial for their attention may engage in genetic Inquiries with great confidence that every frag- ment of soHd evidence thus discovered will quickly take its place in the development of a coordinated structure.
Some pre-Mendeliaji IVritings.
Of the contributions made during the essayist period three call for notice : Welsmann deserves mention for his useful work in asking for the proof that " acquired characters " — or, to speak more precisely, parental ex- perience— can really be transmitted to the offspring. The occurrence of progressive adaptation by transmission of the effects of use had seemed so natural to Darwin and his contemporaries that no proof of the physiological reality of the phenomenon was thought necessary. Weismann's challenge revealed the utter Inadequacy of the evidence on which these beliefs were based. There are doubtless isolated observations which may be Interpreted as favouring the belief In these transmissions, but such meagre indications as exist are by general consent admitted to be too slight to be of much assistance in the attempt to understand how the more complex adaptatlve mechanisms arose. Nevertheless it was for the purpose of elucidating them that the appeal to Inherited experience was made. Weismann's contribution, though negative, has greatly simplified the practical investi- gation of genetic problems.
Though it attracted little attention at the time of its appearance, an honourable place in the history of our science must be accorded to the paper published by de Vrles (1889) under the title Intracelitdare Pangenesis. This essay is remarkable as a clear foreshadow^Ing of that conception of unit-characters which is destined to play so large a part in the development of genetics.
The supreme Importance of an exact knowledge of heredity was urged by Galton in various writings published during the period of which I am speaking. He pointed out that the phenomena manifested regularity, and he made the first comprehensive attempt to determine the rules they obey. It was through his work and influence that the existence of some order pervading the facts became generally
6 Pre-Mendelian Writings [ch.
recognized. In 1897 he definitely enunciated his now famous " Law " of heredity, which declared that to the total heritage of the offspring the parents on an average contribute \, the grandparents ^, and the great-grandparents \, and so on, the total heritage being taken as unity. To this conclusion he had been led by several series of data, but the evidence upon which he especially relied was that of the pedigrees of Basset Hounds furnished him by the late Sir Everett Millais. In that instance the character con- sidered was the presence or absence of black in addition to yellow and white. The colours were spoken of as tri-colour and non-tri-colour, and the truth of the law was tested by the average numbers of the respective colours which resulted from the various matings of dogs of known ancestral composition. These numbers corresponded so well with the expectations given by the law as to leave no reasonable doubt that the results of calculation were in general har- mony with natural fact.
There are features in this important case which need special consideration, and to these I will return. Meanwhile we may note that though there was admittedly a statistical accord between Galton's theory and some facts of heredity, yet no one familiar with breeding or even with the literature of breeding could possibly accept that theory as a literal or adequate presentation of the facts. Galton himself in pro- mulgating it made some reservations ; but in the practice of breeding, so many classes of unconformable phenomena were already known, that while recognizing the value of his achievement, we could not from the first regard it as more than an adumbration of the truth. As we now know, Galton's method failed for want of analysis. His formula should in all probability be looked upon rather, as an occasional consequence of the actual laws of heredity than in any proper sense one of those laws.
Of the so-called investigations of heredity pursued by extensions of Galton's non-analytical method and promoted by^ Professor Pearson and the English Biometrical school it is now scarcely necessary to speak. That such work may ultimat(;ly contribute to the development of statistical theory cannot be denied, but as applied to the problems of heredity the effort has resulted only in the concealment of
I] Rediscovery of Mendel 7
that order which it was ostensibly undertaken to reveal. A preliminary acquaintance with the natural history of heredity and variation was sufficient to throw doubt on the foundations of these elaborate researches. To those who hereafter may study this episode in the history of biological science it will appear inexplicable that work so unsound in construction should have been respectfully received by the scientific world. With the discovery of segregation it became obvious that methods dispensing with individual analysis of the material are useless. The only alternatives open to the inventors of those methods were either to abandon their delusion or to deny the truth of Mendelian facts. In choosing the latter course they have certainly succeeded in delaying recognition of the value of Mendelism, but with the lapse of time the number of persons who have themselves witnessed the phenomena has increased so much that these denials have lost their dangerous character and may be regarded as merely formal.
Rediscovery of Mendel : his Method.
With the year 1900 a new era begins. In the spring of that year there appeared, within a few weeks of each other, the three papers of de Vries, Correns, and Tschermak, giving the substance of Mendel's long-forgotten treatise. Each of these three writers was able from his own ex- perience to confirm Mendel's conclusions, and to extend them to other cases. There could therefore, from the first, be no question as to the truth of the facts. To appreciate what Mendel did the reader should refer to the original paper"^, which is a model of lucidity and expository skill. His success is due to the clearness with which he thought out the problem. Being familiar with the works of Gaertner and the other experimental breeders he surmised that their failure to reach definite and consistent conclusions was due to a want of precise and continued analysis. In order to obtain a clear result he saw that it was absolutely necessary to start with pure-breeding, homogeneous materials, to consider each character separately, and on no account to confuse the different generations together. Lastly he realised
* See Part II.
8 MejideFs Method [ch.
that the progeny from distinct individuals must be separately recorded. All these ideas were entirely new in his day. When such precautions had been observed he anticipated that a regular result would be attainable if the experiments were carried out on a sufficient scale.
After several preliminary trials he chose the edible Pea [Pisttm sativum) for his subject. Varieties in cultivation are distinguished by striking characters recognizable with- out trouble. The plants are habitually self-fertilised, a feature which obviates numerous difficulties.
Following his idea that the heredity of each character must be separately investigated, he chose a number oi pairs of characters, and made crosses between varieties differing markedly in respect of one pair of characters. The case which illustrates Mendelian methods in the simplest way is that in which heredity in respect of height w^as studied. Mendel took a pair of varieties of which one was tall, being 6 — 7 feet high, and the other was dwarf, f to \\ feet. These two were then crossed together. In peas this is an easy operation. The unbroken anthers can be picked out of a bud with a pair of fine forceps and the pollen of the plant chosen for the father may be at once applied to the stigma of the emasculated flower. The cross-bred seeds thus produced grew into plants which were always tall, having a height not sensibly different from that of the pure tall variety. In our modern terminology such a cross-bred, the first filial generation, is called F^. From the fact that the character, tallness, appears in the cross-bred to the exclusion of the opposite character, Mendel called it a dominant character ; dwarfness, which disappears in the F^ plant, he called recessive.
The tall cross-bred, so produced, in its turn bore seeds by self-fertilisation. These are the next generation, F.^ When grown up they prove to be mixed, many being tall, some being short, like the tall and the short grand- parents respectively. Fig. i shows such an F^ family in the Sweet Pea. Upon counting the members of this F^ generation it was discovered that the proportion of tails to shorts exhibited a certain constancy, averaging about three tails to one short, or in other words, 75 per cent, dominants to 25 per cent, recessives.
I]
Mendel s Method
These F^ plants were again allowed to fertilise them- selves and the offspring of each plant was separately sown. It was then found that the offspring, F,, of the recessives
consisted entirely of recessives. Further generations bred from these recessives again produced recessives only, and therefore the recessives which appeared in /^, are seen to
lO
Segregation [ch.
be pure to the recessive character, namely, In the case we are considering, to dwarfness.
But the tall F,^ dominants when tested by a study of their offspring {F}), instead of being all alike (as the dwarfs or recessives were), proved to be of two kinds, viz.
(a) Plants which gave a mixed F^ consisting of both tails and dwarfs, the proportion showing again an average of three tails to one dwarf
(b) Plants which gave tails only and are thus pure to tallness.
The ratio of the impure {a) plants to the pure {b) plants was as 2 to i.
The whole F^ generation therefore, formed by self- fertilisation of 'the original hybrid consists of three kinds of plants :
25% 50 7„ 25 ._
pure dominants impure dominants pure recessives or 3 dominants : i recessive.
Segregation. Allelomorphism.
The conclusion which Mendel drew from these observa- tions is one which will suggest itself to any one who reflects on the facts. The result is exactly what would be expected if both male and female germ-cells of the cross-bred F^ were in equal numbers bearers of either the dominant {D) or recessive {R) character, but not both. If this were so, and if the union of the male and female germ-cells occurs at random, the result would be an F^ family made up of
2c^DD : 2^DR : 2^RD : 2^RR. ^ ^^ : iR.
But, as the first cross showed, when D meets R in fertilisation the resulting individual is in appearance D ; therefore F,, appears as 3Z) : \R. The results of the F^
I] and its Conseqttences 1 1
generation are in exact agreement with this suggestion : for the R plants give R only ; and of the D plants one- third give D only, while two-thirds give the same mixture, '^D : \R, which was produced by F^ (Fig. 2. I).
The descent may be represented diagrammatically thus :
Parents Tall ( TT) x Short (//)
F, - Tall(i7)
F, Tall Tall Tall Short
TT Tt tT tt
pure tall \ \ pure short
F, TT tt t^ It TY~Tt ~tt U
Now since the fertilised ovum or zygote, formed by the original cross, was made by the union of two germ-cells or gametes bearing respectively tallness and dwarfness, both these elements entered into the composition of the original F^ zygote ; but if the germ-cells which that zygote eventually forms are bearers of either tallness or dwarfness, there must at some stage in the process of germ-formation be a separation of the two characters, or rather of the ultimate factors which cause those characters to be developed in the plants. This phenomenon, the dissociation of characters from each other in the course of the formation of the germs, we speak of as segregation, and the characters which segregate from each other are described as allelofuorphtc, i.e. alternative to each other in the constitution of the gametes (Fig. 2).
That this is the true account was proved by further experiments which Mendel made by crossing the F^ with pure dominants and with pure recessives. For DR x DD gave an offspring all dominant in appearance, though in reality consisting of both BR plants and DD plants, on an average in equal numbers. On the other hand DR x RR gives an equal number of dominants and recessives, of which the dominants are all DR plants, and the recessives are all pure recessives. These various experiments illus- trate the composition of the four simple types of Mendelian families, which may be set out thus :
12
Segregation
[CH.
DD X RR gives all DR
DR X RR gives iZ^^T? : i7?y?
DR X Z>i;> gives iZ^i:> : \DR DRxDR gives iZ^Z;> : 2DR : ii?7?
appearing as 3 dominants : i recessive.
X
ID LP
X
»2 F
Raiio
RR
;^:
Medio ID :
II
iR
X
aU D III
Fig. 2. Diagrams showing numerical consequences of segregation.
I. The mating DD x RR^ and Z>i? x DR. II. The mating DR x ^i?.
III. The mating DR x Z>Z>.
The way in which these ratios are produced may be easily represented by means of a number of draught-men. Pairs of draughts then represent zygotes ; single draughts represent germ-cells. That there is a propriety in repre- senting zygotic or somatic cells as double structures and germ-cells as single structures will be evident to biologists ; for we know that each somatic nucleus in plants and animals is a double structure, containing twice the number of chromosomes present in each mature germ-cell. Two black draughts may then be taken to represent a pure black individual, two white draughts a white individual. When they are crossed together F^ is represented by a black
I] and its Conseqtterices 13
draught and a white one (Fig. 2. I). Supposing the black to be a dominant the fact may be represented by putting it on the top. When segregation of the allelomorphs, black- ness and whiteness, takes place in gameto-genesis, the germ-cells of the cross-bred are again bearers of blackness or of whiteness, and it may readily be shown experimentally that the results of their various random combinations give rise to the ratios stated above.
The fact of segregation was the essential discovery which Mendel made. As we now know, such segregation is one of the normal phenomena of nature. It is segregation which determines the regularity perceptible in the here- ditary transmission of differences, and the definiteness or discontinuity so often conspicuous in the variation of animals and plants is a consequence of the same phenomenon. Segregation thus defines the units concerned in the consti- tution of organisms and provides the clue by which an analysis of the complex heterogeneity of living forms may be begun.
There are doubtless limits beyond which such analysis cannot be pursued, but a vast field of research must be explored before they are reached or determined. It is likely also that in certain cases the units are so small that no sensible segregation can be proved to exist. As yet, how- ever, no such example has been adequately investigated ; nor, until the properties and laws of interaction of the segregable units have been much more thoroughly examined, can this class of negative observations be considered with great prospect of success.
The dominance of certain characters is often an impor- tant but never an essential feature of Mendelian heredity. Those who first treated of Mendel's work most unfortu- nately fell into the error of enunciating a '' Law of Domi- nance" as a proposition comparable with the discovery of segregation. Mendel himself enunciates no such law. Dominance of course frequently exists. The consequences of its occurrence and the complications it introduces must be understood as a preliminary to the practical investigation of the phenomena of heredity, but it is only a subordinate incident of special cases, and Mendel's principles of inherit- ance apply equally to cases where there is no dominance
14 Segregation [ch.
and the heterozygous type is intermediate in character between the two pure types.
To the detection of the genetic system of any given case it is however necessary that the results of combinations should be sensibly regular. When, as occasionally happens, a character may sometimes behave as a dominant and sometimes not, we have as yet no satisfactory means of further analysis. These irregularities in dominance may confidently be attributed to the disturbing effects of other factors or of conditions, but the detection of such unknown factors must be a long and perhaps impossible task.
Mendel applied his method to the following seven distinct pairs of characters in peas, and found that in each the inheritance was similar. The dominant character is put first.
1. Height : whether tall or short.
2. Distribution of flowers on the stem : whether
arranged along the axis of the plant, 6»r bunched together at the top so as to form a false umbel *,
3. Colour of unripe pod : whether a shade of green
or bright yellow.
4. Shape of pod : whether simply inflated, or deeply
constricted between the seeds, i.e. as in '' sugar- peas " or '' Pois sans parchemin."
5. Colour of seed-skin : whether various shades of
grey or brown, with or without violet spotting, or white. The "grey" skins are always asso- ciated with coloured flowers and almost always with a purple or red mark in the axils.
6. Colour of cotyledons : whether yellow or green.
7. Shape of seeds : whether rounded or wrinkled.
It will be observed that the first five are plant-characters. In order to see the result of crossing, the seeds must be sown and allowed to grow into plants. The last two characters belong to the seeds themselves. The seeds of course are members of a generation later than that of the plant which bears them. Thus when a cross is made the
* This is a fasciated and semi-monstrous form.
I] and its Conseqtieitces 15
resultant seeds are F^, showing the dominant character yellowness or roundness, but the seed-skins are maternal tissue. Such F^ seeds grow into F^ plants and bear /% seeds which show the typical mixture of dominants and recessives in the pods (Fig. 3). In each case Mendel's
J M 1 II ill)
VR Yv^' (JR Y^ YR
CiR c^vv
9 S i y i >
Fr
(JR 7W Y(? YW YR
rR
Fig. 3. Inheritance of seed-characters in Pea. The seed of a green round variety fertiUsed by pollen of a yellow wrinkled variety are yellow and round (J\). The reciprocal cross would give the same result. Two pods of A seed borne by the J^^ plant are shown. There were 6 yellow round, 3 green round, 3 yellow wrinkled, i green wrinkled.
observations have been substantially confirmed by later observers, and the operation of similar processes has now been recognized in a long series of most diverse characters in both animals and plants.
Consequences of Segregation : Homozygote and
Heterozygote.
Before considering the various extensions of Men- delian research, it may be well to indicate in general terms the chief significance of the facts. The first conception to which we are led is that of tmit-characters, units because they may be treated as such in the cell-divisions of gametogenesis. It is evidently upon some process of
1 6 Purity of Type [ch.
qualitative segregation occurring in one or more of these cell-divisions that allelomorphism depends. The opposite members of each pair of characters being allelomorphic to each other, every zygote"^, or individual produced in ferti- lisation, must, in respect of any such pair, be either a koiuozygote, that is to say, a zygote formed by the union of two gametes each bearing the same allelomorph, as AA and aa, or a heterozygote formed by the union of two germs bearing different allelomorphs, as Aa. Therefore in respect of any pair of allelomorphic characters, the individuals composing the whole population are of three kinds only :
1. Homozygotes of the form A A,
2. Homozygotes of the form aa,
3. Heterozygotes of the form Aa.
The gametes are of two kinds only, A and a. Each kind of homozygote is pure to the character of the gametes which compose it.
Purity of Type.
Purity of type thus acquires a precise meaning. It is dependent on gametic segregation, and has nothing to do with a prolonged course of selection, natural or artificial.
All this is of course consonant with the visible facts that have been discovered by the cytologists, in so far as the nucleus of each somatic cell is a double structure, while the nucleus of each gametic cell is a single structure. It is, in my judgment, Impossible as yet to form definite views as to the relations of the various parts of the cell to the function of heredity. The details of cytology and their interpretation are beyond our present province, but this much is certain : that when in these discussions we idealize the characters as borne by the gamete in an unpaired state and by the zygote in a paired state, we make no assumption which is not in full accord with histological appearances.
From the fact that the development of characters in animals or plants depends on the presence of definite units
* In botany the term zygote is usually restricted to the single cell which results from the process of fertilisation, but by a natural extension the word may be used for the individual which develops by somatic divisions from that cell.
I] Pttrity of Type 17
or factors in their germ-cells, the paradox at once follows that an organism may be pure-bred in respect of a given character though its parents were cross-bred in the same respect. Purity depends on the meeting of two gametes bearing similar factors, and when two similarly-constituted gametes do thus meet in fertilisation, the product of their union is pure. The belief, so long prevalent, that purity of type depends essentially on continued selection is thus shown to have no physiological foundation.
Similarly it is evident that an individual may be pure in respect of one character and cross-bred or impure in respect of others.
As a consequence of the application of Mendel's prin- ciples, that vast medley of seemingly capricious facts which have been recorded as to heredity and variation is rapidly being shaped into an orderly and consistent whole. A new world of intricate order previously undreamt of is disclosed. We are thus endowed with an instrument of peculiar range and precision, and we reach to certainty in problems of physiology which we might have supposed destined to continue for ages inscrutable.
After such a discovery it is obvious that old ideas must be revised. Systematists debating the limits of '' specific rank " or the range of variability, morphologists seeking to reconstruct phylogenetic history, physiologists unravelling the interaction of bodily functions, cytologists attempting to interpret the processes of cell-division — each of these classes of naturalists must now examine the current con- ceptions of his study in the light of the new knowledge. The practical breeder of animals or plants, basing his methods on a determination of the Mendelian units and their properties, will in many of his operations be able to proceed with confidence and rapidity. Lastly, those who as evolutionists or sociologists are striving for wider views of the past or of the future of living things may by the use of Mendelian analysis attain to a new and as yet limitless horizon.
B. H.
CHAPTER II
THE MATERIAL INVESTIGATED.
List of Structural Character's in Plants and Animals — List of Types iri which the inheritance of Colour has been studied — Preliminary Deductions — Dominance and heterozygotis characters — Mendel's system distin- guished from that of Gallon.
Heredity following the general rules described in the last chapter has been witnessed in a great diversity of animals and plants. The characters already proved to follow such rules show an equal diversity. The following is a list of some of them. Adequately to represent the facts respecting each of these cases lengthy description would be needed. In regard to several of them occurrences which do not readily fall into line have been recorded. Of these some are probably due to errors of observation or mistakes of various kinds, but a few will doubtless prove to be genuine exceptions to rule and may constitute points ot departure for fresh and more extended research. In the outline of the phenomena, which is all that this book can profess to offer, it seemed best to restrict as far as possible the enumeration of these details, which can only be thoroughly appreciated by reference to the original papers ; but such annotations as appeared necessary either in elucida- tion of the phenomena or by way of incentive to further work are briefly given with references to the original sources. These annotations will be better understood after the later chapters have been read.
In the following list when one character is conspicuously dominant it is put first, but in several cases the dominance is imperfect.
Plants.
I. Tallness and dwarfness. Peas [Pisiwt) and Sweet Peas [Lathyrzcs odoratus). Runner and French Beans [Phaseolus).
As regards Peas the facts have been recorded by Mendel (195), Tschermak (269, 270, &c.), R.E.C. * (20). When varieties differing
* R.E.C. stands throughout for Reports to the Evolution Committee of the Royal Society, giving an account of the experiments of W. Bateson, E. R. Saunders and R. C. Punnett. Other contributors to these Reports are mentioned by name.
CH. II] Stritctitral Characters : Plants 19
greatly In height are used, dominance is complete, and the two parent forms are represented as three to one in Fc,. No clear exception has yet been observed. Peas {Ptsum) exist in a vast number of distinct horticultural varieties which can roughly be classified as tall (about 5 — 6 ft.), half-dwarfs (about 4 ft), dwarfs (about 9 ins. to 3 ft.). The genetic relations of the half-dwarfs to the others are not fully explored, and further investigation will probably lead to the discovery of important facts. The cross half- dwarf X tall giving tall as dominant has produced some extreme dwarfs in F2, doubtless by recombination {q-v.), R.E.C. 20, p. 69. The cross half-dwarf X dwarf has given intermediates in F^ {ibid.).
The cross between tall and dwarf *' Cupid " Sweet Peas gives complete dominance of tallness and simple segregation in F^., "Cupids" indis- tinguishable from the original "Cupid" parent reappearing (Fig. i).
Phaseolus has been investigated especially by Tschermak (278) who records some apparently anomalous results, de Vries (298, 11. p. 76) states that he found that extracted F.j dwarf Antirrhinum did not breed true, but threw plants of various heights. The experiment should be repeated.
2. Branching habit and the unbranched habit. Sun- flower {^HeliantJucs, Shull, 241) and Cotton (Balls, 6). The branched form of Stock {Matthiola incana) is dominant to the unbranched Brompton type. \n F^ the unbranched type reappears, but the ratio has not been determined (Fig. 4). E. R. Saunders (unpublished).
3. The straggling habit of both the tall and dwarf " Cupid " Sweet Peas, and the much-branched erect habit of the '' Bush" Sweet Peas (R.E.C. 22).
The relation of these two types to each other is not altogether simple. As described (q.v.) F^ from Cupid x Bush is a reversio7iary form exactly like the normal tall variety. Neither the tall varieties nor the Cupids show the profuse branching of the Bush Sweet Peas which gives them their peculiar appearance. This is evidently recessive to the unbranched condition, and the fact thus stands out in contrast to those observed in the case of Sunflower and Cotton. But in the Sweet Pea we have the additional complication that the factor which represses the excessive branching by its presence gives increase of height. The tall and the Bush differ from each other in respect of this factor only. It is present in the tall but absent from the Bush. In the cross between Bush and Cupid two pairs of factors are concerned as explained in the passage referred to.
4. Hairiness and glabrousness. Lychnis. MattJiiola (Stocks). Wheat.
The case of Lych?iis has been studied by de Vries (288) and R.E.C. (19). In crosses between fully hairy and glabrous strains the discon- tinuity Is complete. Various forms intermediate in hairiness may nevertheless be found wild and are by no means rare. Silene inflata
20
Struchiral Characters : Plants
[CH.
often exists in two forms, hairy and glabrous, growing side by side, and doubtless their genetic relations are the same as those found for the corresponding varieties of Lychnis. In this species a third form is found with hairs on the edges only (12).
The case of Matthiola is important and presents features of special interest, R.E.C (19, 20, 21, see also Correns, 61). Between thoroughly hoary and glabrous strains the discontinuity is absolute, and the glabrous
Fig. 4. Matthiola. Branched and unbranched forms in F^. A photo- graph of Miss Saunders' plants, the leaves removed. (Supplied by Miss Killby.)
are entirely destitute of hairs. The dominance is complete and homo- zygotes cannot be distinguished from heterozygotes. A third, or " half hoary " form exists, which is glabrous or nearly so on the upper surface only. Its behaviour has not been fully investigated (19, p. 33.)
The genetics of hairiness in wheat have been studied by Spillman (247), Tschermak (270), Biffen (27). The heterozygotes are sometimes inter- mediate in hairiness.
The Peach and the Nectarine are probably related to each other as hairy dominant and glabrous recessive.
Peculiar results are recorded in Cotton (Balls, 6).
n]
Structural Characters : Plants
21
5. Prickliness and smoothness of fruits. Dattira, (R.E.C. 19, 20.) Ranunculus arvensis (20).
The case of Datura is interesting from the fact that it sometimes has mosaic fruits, one quarter or one half being prickly and the rest smooth. This is perhaps to be regarded as indicative of segregation occurring among zygotic cells (see Chap. xv.).
Ranunculus arvensis has three types, spiny, tuberculated, and smooth. The first is a simple dominant. Tuberculated x smooth gave ^1 partially spiny (21, p. 55).
6. Absence of glands iyMatthiola incana) on leaves was dominant to presence of glands {M. smuata) (R.E.C. 20, p. 40).
Fig.
5. Cross between a normally awned Barley and a variety with " hooded " awns. P, F, the parents. F^ shows partial dominance of hoods. The increase in length of ear is noticeable. The case also illustrates the result of crossing a 2-row type with a 6-row type, showing dominance of the former.
(From Professor Bififen's specimens.)
22 Structural Characters : Plants [ch.
7. Rough and smooth foliage. Wheat. Biffen (27).
8. Keeled glumes and rounded glumes. Wheat. Ibid.
9. Beardless and bearded ears. Wheat. Ibid. Also Spillman (247) and Tschermak (270).
Most, if not all, of the "beardless" varieties exhibit a slight and variable amount of awn especially on the uppermost spikelets (Fig. 4).
10. The "hoods" or '' Kapuzen'' characteristic of certain Barleys show a partial dominance over the normal type. These hoods, Professor Biffen states, are, structurally, aborted florets (Fig. 5). Tschermak (270), Biffen (30).
11. Hollow and solid straw. Wheat. Biffen (27).
This is a structural character of an interesting kind, and one upon which the commercial value of straw very largely depends. It was shown that many factors were concerned in the production of the stem-characters; and in Fc^ by the recombination of these factors a great variety of straws appeared.
12. Blunt and pointed pods. Pisttm. Tschermak (271), R.E.C. (20). Phaseolus. Tschermak (272).
The dominance in this case is complete. Some varieties exist in both a blunt and a pointed type {e.g. Sutton's Continuity). The nature of these cases is discussed later.
13. Lax and dense ears of Wheat and Barley give different results according to the varieties used. Sometimes F^ is lax, sometimes it is intermediate (Spillman, 247; Biffen, 27, 28). See Fig. 6. In Barley an increase in ear- length has been observed (Fig. 5).
14. Development of fibrous parchment-like lining to pods, and the absence of the same which constitutes the ''sugar peas." Pisum. In Phaseolus (kidney-beans), where similar types occur, the evidence is that the dominance is reversed (Emerson, 120, 121).
This is one of the features originally investigated by Mendel. He regarded the parchmented type as a dominant. In our experiments F ^ has always had some parchment but the quantity is so much reduced as to cause the heterozygote to have a very distinct appearance (R.E.C. 20).
15. Much serrated and little serrated edges of leaves. Urtica (cp. Pkyteuma, Correns, 70, p. 197). This cross
n]
Structural Characters : Plants
23
was described by Correns {^^) who gives a striking diagram representing his results. The cross was made between two forms known as Dodartii and Hltdifera, which were
•4-*
»-.
>N O
-4->
tn
a> ex, is >^
J^ 'wi ''*^
<^ !.: 2
<u t* *-■
" O ^
+j (U Oh
OJ - D
'^ .5 ^
^ OJ ^
rt <u -7^
•V S a>
rt ^ CA)
ii c ^
03 «^. c«
n 1^ '-
^ ^ O
= ^-C
S
<u
to . •*-»
t^ t/5 aj
O -^ O
J-i c -^
U (U 73
bb
regarded by Linnaeus as entire-leaved Dodartii has as a variety oi pilulifera.
distinct species, been treated by
The almost later authors
24 structural Characters : Plants [ch.
1 6. Palmatifid or ''palm-leaf" and pinnatlfid or "fern-leaf." Primula Smensis (Fig. 7).
The fern-leaved form arose in English horticulture about i860 as a variation from the normal type. I have had opportunities of seeing its genetic behaviour on a large scale at Messrs Sutton's, and many experiments have been made with it by Mr R. P. Gregory in conjunction with me. Dominance is usually complete, but at Messrs Sutton's I have seen on two occasions strains containing plants of intermediate leaf-shape, which were presumably hetero- zygous, for the two types occurred on sister-plants. The leaf-shape is entirely independent of the colours and other features of the plant, and can be transferred bodily from one colour-type to another. Messrs Sutton's varieties "• Mont Blanc" and "Sirdar," for example, are sold both in the palm-leaved and in the fern-leaved forms.
Fern-leaf {R) Palm-leaf {D)
Fig. 7. The two types of leaf found in Primula Sinensis.
17. Leaves and petals normal or laciniated. Cheli- donium majus. de Vries (290) and (298), i. p. 134.
This case is interesting in comparison with No. 15. In the Nettle, serraiio7i is a dominant, while here laciniation is a recessive. A careful study of the physiological distinction between the two processes would probably lead to important results (cp. Leake, 170, on leaves of Cotton).
II] Strttctitral Characters : Plants 25
18. Certain leaf characters in Capsella bursa-pastoris. Shepherd's Purse. (Unpublished work of Shull ; about to appear as a publication of the Carnegie Institution.)
19. Various characters in the seed of Cotton. Balls (6).
Many of these are of great commercial importance. Balls (6) gives the following list :
Domiiia7it. Recessive.
Long staple. Short staple.
Regular distribution. Irregular distribution.
Coloured lint. White lint.
Silky lint. Harsh lint.
More fuzz. Less fuzz.
He says that all the desirable characters are dominant, and that hence the chances of picking out a stable form by common selection are very small. Individual selection must be adopted.
20. Biennial habit and annual habit. Hyoscyamus, Correns {j'^.
More research on the relations of annuals to biennials is greatly to be desired. Points of the highest physiological interest are involved. In connection with root crops also some questions of commercial importance are raised. In R.E.C. (19, p. 135) I ventured to suggest that the persist- ence of " runners " which go to seed in such plants as Beet and Mangel may be due to want of individual selection of pure dominants, and in view of Correns' observation the probability of this suggestion is increased.
2 1. Normal stem and fasciated stem associated with peculiar distribution of inflorescences. Peas [Pistcm). Mendel (195), R.E.C. (20). In our experience various intermediates occur In /%.
22. Susceptibility to rust-disease [Ptucinia glumaruiii) and resistance to the same. Wheat. BIffen (27, 29).
This is perhaps one of the most important instances to which Mendelian method has yet been applied. Using a variety very susceptible to rust and another practically immune to its attacks Biffen found that F-^ was not perceptibly less attacked than the rusty type. F<, showed ordinary segregation, and the green, resistant plants, standing among the yellow rusty ones, formed a very striking spectacle. The recessives bred true and their progeny has remained rust-proof. It has not yet been shown to what the resistance is due. Working with Professor Biffen, Miss Marryat (193) found that the rust-hyphae are checked after entering the stomata of the resistant plants. If, as may be suspected, the resistance is due to the presence of some anti-toxin, the dominance of "susceptibility" must be taken to indicate that the formation of the anti-toxin is prevented by the prese?ice of a factor in the dominant forms, a conclusion which may lead to definite progress in the physiology of disease-resistance.
26 structural Characters : Plants [ch.
23. Flat standard and hooded standard in the flower of Sweet Pea. R.E.C. (22). See Plate V. Some very curious phenomena have been observed in this case, which are described in Chap. ix. The type known as "Snapdragon," perhaps an extreme form of hood, is also a recessive to the flat type. R.E.C. (20, p. ^'^).
24. Imbricated petals and stellate or '' star "-type. Primula Sinensis. Observations made at Messrs Sutton's and experiments of R. P. Gregory with W. Bateson. (See Fig. 14.)
25. The monstrous condition of the calyx in which it resembles the corolla, seen in " hose-in-hose " Cainpanula is an imperfect dominant to the normal. Associated with this homoeotic variation, the female organs are more or less completely sterile in certain strains. Correns (76). This subject is discussed in Chap. xi.
26. Abortion of the female organs in the lateral florets of Barley, as found in the 2 -rowed types, and the complete or hermaphrodite development of the florets, as in the 6-rowed types (Fig. 8). Tschermak (270) and (275), p. 1 1. Biffen (30).
This case is somewhat complex. There are three types, (i) Six-row, in which 3 perfect hermaphrodite florets are developed in each spikelet. All set seed and the result is that the ear has 6 rows of seeds. (2) Types in which the lateral florets have anthers but no female organs. (3) The " Abyssinian " type in which the lateral florets contain neither male nor female organs. The types (2) and (3), being able to make seeds only in the central florets of each spikelet, alike develop two rows of seeds. When (i) is crossed with (3), F^ is Hke (2) ; and (2) crossed with (3) gives Fx also like (2). Some further complexities have been observed, but in general it appears that the dominant factor has the power of partially preventing the formation of the reproductive organs in the lateral florets. The facts may perhaps be interpreted as bearing on the phenomenon of Sex.
Tschermak (275) describes crosses between a 2-row and a "4-row" type. From Professor Biffen I understand that the latter is in reality a lax-eared 6-row type. F^ is 2-row, and in 7^2 the ratio is 12 2-row : 3 "4-row" : I 6-row. This is a special case of the ratio 9:3:3:1, lax- ear and 2-row being dominants. Tschermak and ShuU (242) regard it as an illustration of the effects of a latent factor.
n]
Structural Characters : Plants
27
27. The two-celled type of fruit is dominant to the many-celled type in Tomato. Price and Drinkard (221).
The case is one of the few in which the genetic behaviour of a meristic or divisional feature has been investigated apart from any complexity introduced by differentiation.
Fig. 8. Cross between Abyssinian 2-row Barley and a club-headed 6-row type. The middle figure shows F^. The length of ear is increased. (From Professor Biffen's specimens.)
28. Style short, associated with large pollen grains, constituting the ''thrum" type, and style long, associated with small pollen grains constituting the "pin-eyed" type. Prhnula Sinensis and acaulis. Bateson and Gregory (17).
The short styled type has been found in the homozygous condition in P. Sinensis but not yet in P. acaulis. For the relations of these types to the " homostyled " form, see p. 68. Dominance is complete.
28 Strudztral Characters : Plants [ch.
29. Long style and short style In Oenothera. (This difference is probably quite distinct in nature from ordinary heterostylism as seen in Prhmtla, &c.) de Vries (290). The same fact has been observed by Balls (6) in Cotton.
30. Normal long pollen grains with three pores, and rounded pollen grains usually with two pores. Sweet Pea {Lathyrus odorahis). R.E.C. (20, 21, 22). See Fig. 18.
31. Normal anthers and sterile anthers. Sweet Pea. R.E.C. (20, 21, 22).
With regard to these two last features numerous complications occur, which are described in later chapters.
32. Roundness of seed connected with the presence of starch in large elongated simple grains, and wrinkledness of seed connected with the presence of peculiar compound starch-grains. Pisum.
This is one of the most familiar of Mendel's original examples (see Fig. 3). It has been re-investigated by many observers. Correns (60) ; Tschermak (269, &c.) ; R.E.C. (20); Hurst (155); Lock (172, 173). The F^ seeds made by fertilising an emasculated flower of a wrinkled variety with pollen from a round variety, or vice versa, are generally ordinary round seeds, and F^^ shows the common ratio 3 round : i wrinkled, the two types being mixed in the pods of the F^ plants.
Among the multitude of varieties of peas now cultivated there is a great diversity both of rounds and of wrinkleds. The interrelations of these several types, even as regards seed-shape, have as yet been imperfectly explored. The degree to which the wrinkles are formed is fairly uniform for any one type, but the various types show different degrees of wrinkling. The differences obviously depend chiefly on the chemical and physical properties of the reserve-materials in the cotyledons, and an analysis of these peculiarities might lead to further discoveries.
Gregory (134) found that the starch in round peas occurs chiefly as large elongated simple grains, whereas in wrinkled peas it is in the form of small grains of irregular shape which are often coinpou?ided together (Fig. 9). Darbishire (94) added the interesting fact that in F^ the grains are intermediate, many being large and simple, but round instead of elongated, with an admixture of compound grains. He confirmed also Denaiffe's observation* that wrinkled take up more water than round, but he found that Fy is intermediate in this respect, and he suggests that the size, the
* Denaiffe, Les Fois potagers, p. 9.
"]
structural Characters : Plants
29
shape of the grains, and their simple or compound nature, may be governed by distinct factors. He regards the absorptive power as again separable from these features.
^O'O^
(7 n^-O
?^o
o
0
O O
Round and Indent.
Wrinkled.
Fig. 9. Outlines of starch grains in the different types of peas. The wrinkled contains many compound grains. (From Gregory.) Magni- fication the same in both figures.
A third type of pea, of which the purple sugar-peas (sa/is parche7nin) are a good instance, may be described as "indent." These also have large, simple starch grains. Such seeds are of irregular flattened shape and may be confounded with true wrinkled peas. Their properties are entirely different, and the two sorts must be carefully distinguished. One of their special properties w411 be discussed in a later chapter, but here it must suffice to say that their genetic properties are essentially those of round peas. Much confusion has been introduced by want of care in distinguish- ing these types.
Intermediates, which on casual sorting, cannot be classed either as round or wrinkled, sometimes occur. Some of the round types {e.g. Victoria Marrow) contain a large proportion of such seeds. Their peculiarity is almost certainly due to environmental influence, though obviously the liability to this affection may be transmitted. When such seeds are found in F^_ from a cross between thoroughly round and wrinkled varieties, the pitting, when it exists, generally affects all the round seeds of \}i\Q. pods in which it occurs. With experience such pitting can immediately be recognized as distinct from the true genetic wrinkling, and in our experiments the results of a further sowing have repeatedly confirmed the judgment made by inspection of the seeds.
A complete account of all the phenomena would run to great length. The interrelations of round and wrinkled seeds are to be recommended as offering perhaps the most favourable example for an investigation of the chemical nature of a genetic factor. The wrinkling is evidently the consequence of a particular method of drying, and this must depend on the nature of the reserve-materials. A first step would be to determine the relative amounts of sugar and starch in the two chief types. It is natural
30
Stntctitral Characters : Plants
[CH.
to suppose that the wrinkled peas are those in which the transformation of sugar into starch has gone less far than in the round peas ; but, as much starch is formed in the wrinkleds, one ferment having this transformative power must be present in them. Hence we are led to suppose that in the round pea a semid ferment is present which can carry the process further. As offering an attractive problem in physiological chemistry the phenomena are recommended to those who have the requisite skill to investigate them.
2i^. Starchy endosperm giving a full, rounded seed, and sugary endosperm giving a shrivelled and wrinkled seed. Maize. Fig. lo. de Vries (290); Correns (63); Lock
(172, 174)-
Fig. 10. A cob of Maize (Zm mays) borne by an F^ plant from the cross round x wrinkled, fertilised with its own pollen, showing the mixture of round (dominant) and wrinkled (recessive) seeds. (From a specimen given by Dr Webber.)
Of the various Mendelian experiments this is one of the most demon- strative. Dominance is perfect so far as external observation goes. Correns records a remarkable excess of round seeds as recurring with great constancy in certain families when F^ is self-fertilised (see later).
It often happens that pollen from one variety of maize is blown by the wind to the stigmas of another variety. If this pollen possesses a dominant factor capable of affecting the seed, seeds exhibiting it are formed. If for instance pollen from a round maize is blown on to a wrinkled or sugar-corn, round seeds will be formed among the normally wrinkled seeds. When formerly it was supposed that the endosperm, which contains the reserve- materials, was a maternal structure, the change in the seed was regarded as an influence exerted by the embryo on the maternal tissues. The effects of such influences were called by Focke ^'' Xenia.''' There are a few examples of such influence which may with probability be regarded as genuine*; but since the discovery of the fact that the endosperm of maize results from a double fertilisation effected by the second nucleus of the
* The phenomena are discussed by Darwin, An, a?td Fits., ed. n. 1885, I. pp. 428-433. It seems likely that in some of these instances the factor introduced by the pollen-grain can influence or infect tissues in contact with the embryo.
ii] Stnichtral Characters : Plants 31
pollen-tube, cases like that of maize are not strictly to be classed as Xenia (see Correns, 58).
34. Glutenous and starchy endosperms. Wheat. Biffen
(27).
Professor Biffen's researches respecting these important features are not yet completed. The glutenous, translucent, hard type has definite dominance over the opaque, soft, starchy type.
35. Single flowers usually behave as dominants to doubles, as in Stocks, Primula, &c. In Carnations the doubleness dominates.
The most extensive researches on the genetics of double- ness are those of Miss Saunders in the case of Stocks {Matt/nola), R.E.C. (20-23). The peculiar phenomena discovered are discussed in a separate chapter {q.v.).
36. In Phaseolits hypo-geal cotyledons are dominant to epi-geal. Various intermediates in P.. Tschermak (278, p. 54).
This list and that which follows make no pretension to completeness. Those features are enumerated which either seem of special interest, or have been studied with some thoroughness. Indications respecting many more are to be found in the original papers (see especially for Peas and PJiaseobts the writings of Tschermak and Lock ; for Cotton, Balls; for Oenothera, &c., de Vries, and Macdougal (186); for Wheat and Barley, Biffen, and Tschermak; for Maize, Correns, and Lock; for various plants, Correns, and de Vries).
In the genus Brassica numerous crosses have been studied by Sutton (262). In his experiments it was found, among other important results, that the bidbing of the Swede, Turnip, and Kohl Rabi disappeared completely in crosses with non-bulbing Kales, and that in P, imperfect bulbing reappeared. Professor Biffen, who is continuing work on the same lines, tells me that in regard to these and similar characters cultural conditions play a great part, and lead to curious and conflicting results.
32 Struchtral Characters : Animals [ch.
Animals. Structural Characters.
Man.
A considerable number of diseases and malformations have been shown to behave usually as dominants. A few conditions may be said, more doubtfully, to behave as recessives. The subject of human inheritance is discussed in Chap. xii. Of normal characteristics, eye-colour is the only one yet studied (Hurst, 161)'^ sufficiently to justify a positive statement as to the existence of a Mendelian system of descent.
Cattle.
'^'] . Absence of horns in polled breeds of Cattle is dominant to the presence of horns (R.E.C. 19; Spillman,
In sheep the inheritance of horns is sex-limited (^.2^.), and from evidence given me by Mr E. P. Boys-Smith I suspect that this is true in the case of Goats also.
Horse.
38. There is little doubt that the gait known as ** pacing" is recessive to the ordinary trotting gait in the American trotters. Trotters bred together may produce pacers, but hitherto I have found no authentic instance of genuine natural pacers, when mated together, producing trotters. Correspondents have sent me word of several apparent exceptions to this rule, but all on inquiry have proved to be erroneous. In the pacing gait the two legs of the same side of the body are moved together or nearly so, while in trotting the foreleg of one side moves almost with the hind leg of the other. Horses may be trained with more or less success to adopt either gait, but the distinction between natural pacers and natural trotters is a fairly sharp one (16). The physiological nature of the difference is quite obscure, but presumably it is of nervous origin.
Mouse.
39. From time to time mice are found hairless, with the skin thrown up into corrugated folds. Experimenting with such mice Mr Archibald Campbell found the condition to
* See also Davenport (107).
II] structural Characters : Animals 33
be a recessive, the presence of normal fur being a dominant. The fur grows at first normally and falls off as maturity is reached. Of 12 /^„ mice 3 lost their hair. I am indebted to Mr Campbell for information respecting this interesting case, and for living specimens. The attempt to breed the recessives together failed, but in Gaskoin's case"^ naked parents produced young like themselves. From his account it appears that the young which he observed never grew their hair, but the fact is not absolutely certain from the description.
40. The normal condition and the ''waltzing" habit in Japanese mice. The waltzers exhibit a peculiar vertiginous movement of the head when they come out into the light, and spin often with extreme rapidity, running after their tails till apparently exhausted.
Our knowledge of this case is derived from Von Guaita (135) and Darbishire (90). The dominance of the normal type is complete, and in F ., waltzers reappear. The F ^ numbers obtained by Darbishire were 458 normals, 97 waltzers, where the expectation is 386 normals, 139 waltzers. The deficiency may perhaps indicate a complication, but more probably it is due to the greater delicacy of the abnormal mice, which was so great that all attempts to breed them together were unsuccessful.
Rabbit, Guinea-pig.
41. Normal short hair and the long "Angora" hair Rabbit, Guinea-pig, and doubtless Cat (see Hurst, 157; Castle, 45 and 48 ; Sollas, itnpublished ; Castle and Forbes, 55).
Castle (48), p. 64, gives important details as to the physiological nature of the distinction between the normal and "Angora'' hair, which he regards as resulting from a special method of growth.
42. The rough or rosetted condition of the coat in the Guinea-pig dominates over the normally smooth condition (Castle, 48 ; Sollas, itnpiiblished).
Castle found occasionally that duxiva\d\% partially rosetted occurred in F...
* For references see Bateson, Materials for Study of Variation, 1894, p. 56. A good figure is given by Gaskoin, Proc. ZooL Soc. 1856.
B. H. 3
34 Stritcttiral Characters : Animals [ch.
43. Polydactylism occurred in a Guinea-pig, the off- spring of normal parents, and ran an irregular course in its subsequent descent (Castle, 49).
Cat.
44. The abbreviated tail of the Manx Cat is a dominant (more or less imperfect) to the normal tail (see Anthony, 2 ; Hind, 151; Davenport, 98; Kennel, 166, «^).
45. Polydactylism is almost certainly dominant; but, as in other types, irregularities doubtless occur.
Fowls.
For the study of heredity Fowls are especially well suited. In addition to their many colour-characteristics the various breeds present a great range and variety of struc- tural features.
Among the long series of offspring which hens of the more fertile breeds produce, the descent of these charac- teristics can be watched in families of ample length. The chief papers dealing with Fowls are R.E.C. (19-22); Hurst (156); Davenport (loi). The following is a list of the principal facts already elicited as to the behaviour of these structural features but much remains to be done.
46. Various shapes of comb, for example the rose comb and the pea comb, are both dominant to the single comb. The double or longitudinally split condition is also dominant to the unsplit.
See pp. 61-7. Many of the finer details in regard to the heredity of comb-shapes are not yet clear. The classification of the comb-types in the newly-hatched chickens is generally very easy, but in occasional strains forms intermediate between the pea and the single occur in F^, which may probably be due to subtraction-stages of the pea factor {q.v,\ Some of the singles extracted in F^ from various crosses have lateral " sprigs " — as fanciers say. It is not impossible that these irregular processes are due to additional minor factors, but they are subject to so much fluctuation that their descent would be very difficult to trace. The comb of the Silky fowl is a rose, + a trifid element which causes its posterior end to be divided into three irregular points. In F^ from Silky x Single, regular rose combs are produced in those individuals which have the rose factor without this trifid element.
Attention may be called to the dominance of the median splitting of the comb found in certain breeds, for the facts may have a bearing on the genetics of meristic characters. Splitting of the comb may occur in one of
ii] Striictttral Characters : A7iimals 35
several distinct ways. It may affect mainly the anterior portion, or the posterior. The split combs of established breeds have possessed ordinary dominance; but a form of posterior splitting somewhat like that of the F^ from Breda x Single occurred apparently as a mutation among extracted singles, and exhibited a curious genetic behaviour suggesting irregularity of dominance (20, pp. 108 and 113).
47. The normally webbed feathers are dominant to the peculiar feathers of the Silky fowl.
48. Extra toe Is usually dominant to the normal four- toed condition, but exceptions occur.
This irregularity of dominance is exhibited by all cases of polydactylism yet studied in birds or mammals. It seems to be a property of certain strains. Some families run a perfectly regular Mendelian course, others contain members with only the normal four toes, which are yet capable of transmitting the extra toe. The numbers in such families are not favour- able to the suggestion that the irregularity is caused by a definite disturbing factor.
49. Crest is dominant to no crest.
F2 may contain individuals with crests far larger than those of the parent crested breed, a fact which suggests that in breeds with small crests \e.g. Silky) the full development of the crest is kept in check by some other factor.
50. Feathered leg partially dominates over clean leg.
Both Hurst (156) and Davenport found dominance very irregular. F ^ is intermediate, and traces of leg-feathering are occasionally seen in the offspring of clean-legged birds.
51. "Frizzling," or turning back of the feathers, Is dominant to the plain straight feathers of the normal.
52. Normal size of feathers on the hocks, or tibio-tarsal region. Is dominant to elongation of these feathers to form quills — the '' Vulture-hock " of fanciers.
53. Muff, or tuft of feathers at sides of the bill and throat, as in Faverolles, is dominant to no muff, as in ordinary breeds.
54. Imperfect development of coccyx and tail-feathers with absence of tail, as in '' Rumpless " fowls, Is dominant to the normal development of those parts.
This interesting case was investigated by Davenport (loi). It is exactly comparable with that of the Manx Cat.
36 Strtictztral Characters : Animals [ch.
55. Certain breeds (Houdan, Polish, Breda) have an extraordinary development of the nostril, which is patulous, with alae horizontal instead of curving downwards. This peculiarity is recessive to the normal (Davenport, 10 1). Hurst has observed the same thing and Mr Punnett and I have similar evidence from the Breda. Davenport states that in his experience the ''high" nostril is never combined with a fully developed comb.
56. The tendency to go broody and sit on eggs dominates over the absence of this instinct, characteristic of several Mediterranean breeds. There is probably segre- gation in regard to these two dispositions, but this cannot yet be asserted positively.
In regard to fertility as measured by egg-production there is as yet no clear evidence.
57. The loud and penetrating shrieks which the cocks (and to a less degree the hens) of an Egyptian breed give out when caught, were reproduced almost exactly by the F^ generation from a cross with a non-shrieking breed. Though numerical data in regard to such a character are scarcely attainable, there is little doubt of the segregation as evidenced by /%.
Pigeons.
58. The normal foot is dominant to the webbed con- dition of the toes which sometimes occurs as an abnormality (Staples-Browne, 254).
Mr J. L. Bonhote tells me that in his experiments webbed birds have produced normal offspring. He is making further experiments with this family.
59. The ''shell," or turning-back of the head-feathers of the Nun is dominant to the normal plain head {ibid.).
60. Birds with normal, 12-feathered tails crossed with the many-feathered Fantail give intermediate numbers in F ^. In F ., 12-feathered tails reappear, but, so far, no real Fan has come from the cross-breds. Mr Staples- Browne, to whom I am indebted for this information, will publish a complete account of his evidence. He tells me that the extracted 12-feathered birds do not breed true, but may throw birds with 13 or 14 feathers.
II] structural Characters : Animals 37
Canaries.
61. Crest Is dominant to plain-head, as the non-crested condition is called by fanciers (R.E.C. 19, p. 131 ; Daven- port, 105).
The type of crest which fanciers admire consists of feathers neatly laid down over the head. To produce such birds crested individuals are bred with plain-heads^ and it is clear that the exhibition type of crest is a heterozygous form. When crested birds are bred together it is said that an ugly, standing crest frequently is produced, and presumably this is the homozygous type of crest. The mating of two crested parents is by several authors said to give rise to some bald birds. Other writers {e.g. Blakston) have ridiculed this statement, and formerly I was inclined to regard it as a mere exaggeration, but Davenport in his recent paper mentions bald heads as sometimes occurring among his crested birds. He has kindly supplemented his published account with the statement that the bald patch is an area "on the back of the head varying from four to six millimetres in diameter practically without feathers and remaining featherless throughout life. The crest, however, on top of the skull remains perfectly evident, and often baldness can only be detected by blowing the feathers." In no case was such a bald patch found in a plain-head.
The bald patch on the occiput is recognized by Blakston {Cage Birds, p. 104) as a property of crested birds, and presumably the "balds" alleged to come from the mating of two crests are birds homozygous for crest- factor, in which the crest stands up and allows the bald patch to be seen. Davenport had a crested bird without any bare patch, and he found that the feathering in this region was due to a separate dominant factor.
Animals and Plants in which Colonr-Characters have been shown to have a Mendelian Inheritance.
The phenomena of colour-inheritance are complicated in several ways. Some of these complications which are of great importance and interest will be considered in subsequent chapters. It is, however, convenient to enu- merate the genera in which Mendelian heredity has been observed In order to illustrate the scope of the principle. The following list of genera contains the chief of those in which heredity according to a Mendelian system has been shown to occur. In some of them as the result of extensive research many Mendelian features of colour have been discovered, and the existence of numerous colour- factors is demonstrated. In others only one such factor for colour has been detected.
38 Colours of Plants [ch.
Plants.
Antirrhinum (Snapdragon). Orchids (several genera).
Atropa. Papaver.
Brassica (Turnips and Swedes). Phaseolus.
Clarkia. Phyteuma.
Coreopsis. Pisum.
Datura. Polemonium.
Gossypium (Cotton). Primula.
Helianthus (Sunflower). Salvia.
Hordeum (Barley). Solanum (Tomato).
Hyoscyamus (Henbane). Triticum (Wheat).
Lathyrus (Sweet Pea). Verbascum (Mullein).
Lychnis. Viola.
Matthiola (Stocks). Zea (Maize, Indian Corn).
Mirabilis.
Animals.
Man. . Fowls.
Mice. Pigeons.
Rats. Canaries.
Rabbits. Axolotl.
Guinea-pigs. Lepidoptera, various (Silkworm ; A-
Horse. braxas grossulariata ; Anger ona
Pigs. prunaria, &c.).
Sheep. Coleoptera {Lina ; Leptinotarsa ;
Cattle. Crioceris).
Cats. Helix.
For the convenience of readers acquainted with the phenomena in outline and desirous of pursuing the subject further the following brief annotations are placed here. Until the chemistry of pigmentation is better understood, a comparison between the behaviour and properties of the several types cannot be instituted with much confidence.
Antirrhinum. Wheldale (303) has shown that the lowest or hypostatic factor dominant to albino gives yellow in the "lips" of the flower; the addition of various other factors produces anthocyan reds which superposed on the yellow give deep crimson red colour. A second series of reds, more purplish or magenta in tint (colour of wild A. majus)^ results from addition of a factor which in absence of anthocyans gives an ivory colour. This ivory is epistatic to yellow. It is remarkable that the lowest anthocyan factor gives red in the tube with a tinge in the lips, while the addition of the next above it gives the self-coloured flower.
There is also a 'whiteA.v\^^A type of each colour-combination ("Delila" of de Vries, 298).
All the factors except those for yellow and ivory can be carried by the albino. Among the reds several heterozygous combinations can be recog- nized. The heredity of striping is still under investigation.
Atropa Belladonna. The normal dark-fruited type is dominant to yellow-fruited (de Vries, 290; Saunders, 19).
II] Colours of Plants 39
Brassica. White chromoplasts dominant to yellow in Swedes and Turnips (Sutton, 262).
Clarkia elegans. Common magenta-red dominant to salmon pink (Bateson and Punnett).
Coreopsis tmctoria. Ordinary yellow type dominant to var. bruimea with brown flowers (de Vries, 290). The brown flowers like those of Cheira7ithus (Wall-flower) are no doubt due to presence of much dark anthocyan, and the case is probably one in which the development of little anthocyan dominates over the development of much anthocyan (cp. Lathyrus, Pri??iula^ &c.).
Datura. Purple in flower or stem dominant to white flower and green stem (de Vries, 290; Saunders, 19).
Gossypiu7n (Cotton). Dominance of many colour-characters in plant, flower, and seed (Balls, 6). F.2 details not yet published.
HeliantJms. Purple disk dominant to yellow disk (Shull, 241).
Hordeum (Barley). Black pigment in paleae dominant to its absence (Tschermak, 270; Biff'en, 30).
Hyoscyaimis fiiger a?i?iuus x H. niger pallidtis were found by Correns (69) to give F^ flowers of intermediate tint.
Lathyrus (Sweet Pea). Anthocyan colours dominant. Purples dominant to reds. Colour depends on two complementary factors. Yellow chromo- plasts recessive to colourless. Facts fully described in later chapters. Plants with coloured flowers have dark seed-coats. Whites have seed-coats colourless.
Lychfiis. F^ between L. diurua and L. vespertina has flowers of inter- mediate tint ranging through many grades (see de Vries, 290 ; Correns, 69; R.E.C. 19). Segregation imperfectly studied.
Matthiola (Stocks). Colours as in Sweet Pea (R.E.C. 19-21; Tschermak, 278; Correns, 61). For colours of seeds see R.E.C. 19.
Mirabilis. Colours consist of a complex series of reds and yellows, the interrelations of which are not yet clear (see Correns, 67, 74, 77). Miss Marryat's experiments (unpublished) prove the existence of a number of heterozygous forms.
Orchids. Dominance of anthocyan colour in Cypripedium is clear. In that genus it results from union of two complementary factors (Hurst, Gard. Chron. 1908, i. p. 173). As regards distribution of colour the facts are complex, but several indications of Mendelian distribution have been recognized (Hurst, 153, 160). See p. 96.
Papaver. Presence of dark purple spot at base of petals dominant to the absence of such colour (de Vries, 290).
Phaseolus. The elaborate researches of Tschermak (271-3, 275, 278) have demonstrated the existence of numerous factors controlling the colour of the flowers and seed-coats in P. vulgaris^ P. multiflorus and their hybrids. The flower-colours are purples, reds, and white, with a bicolour form of the red ("Painted Lady"). Colour has not yet been produced by union of
40 Colours of Plants [ch.
two whites, but on the analogy of the Sweet Pea such a result may be attainable. Otherwise the same rules apply generally. At least two sets of pigments take part in coloration of seed-coats : (i) brown, (2) purple. White- flowered plants have seed-coats unpigmented, and the bicolour flowers go with parti-coloured half-white seeds. The development of purple in the coats and the pattern in which it is deposited depend on various factors which can be carried by the albino. Various complications were met with (see originals). The cross between the two species showed some degree of sterility.
Similar results were obtained by Emerson (120, 121). The seed-coats of heterozygous plants in some cases were distinguishable. He also found green pods dominant to yellow pods (cp. Pisum).
Further facts^ with a scheme elucidating some of the curious ratios which the seed-colours may exhibit {e.g. 18 : t8 : 6 : 6 : 16) in F2, are given by Shull (242).
Phyteiima Halleri (dark violet) x P. spicatum (white) gave two types in F^, 5 plants being bright blue with violet tinge, 4 violet (Correns, 70).
Pisum (Edible Peas). Flower-colours of three types, (i) Purple. Standard a pale purplish white ; wings deep chocolate purple. (2) Pink. Standards pinkish white ; wings a fine salmon-pink. (3) White. F^ containing all three types is the usual 9 : 3 : 4 in order named. Mark in axils of leaves, if present, is purple in (i), red in (2), absent in whites. Tschermak experimented with a purple strain without the axil-mark, and found that, as in Sweet Peas, the factor for that character can be carried by the albino.
Seed-coats colourless or greenish in white-flowered plants. In plants with coloured flowers one or more of three distinct kinds of pigments always present: (i) a purple, occurring in spots, (2) a brown, distributed either generally over the surface, or in bands (as in Maple peas), (3) an insoluble greenish grey, distributed over the whole testa. Neither (i) nor (2) can be developed in the absence of (3), but traces of (2) may sometimes be seen in white-flowered plants. There are separate factors for (i), (2), and (3), of which (i) and (2) may be carried by the whites (Lock, 176).
Cotyledon-colours are yellow, and green. Yellow is a dominant, and heterozygotes are indistinguishable from homozygous dominants. In rare cases green has been seen as exceptions in F^^ but these are probably due to abnormal conditions. Many modern varieties have cotyledons patched with green and yellow. Genetically these are greens which, show a special liability to bleaching. If protected while ripening they remain green.
Colours of Pisum have been chiefly studied by Mendel (195) ; Tscher- mak (269, 271-3) > Correns (60); R.E.C. (20); Lock (172, 173, 175-6); Hurst (155).
Polemonium. Correns (70) found the white var. of P. coeruleum dominant to the yellow of P. flavu7n ; and the blue type oi coeruleum xjlavum gave F^ blue. It may be inferred that the yellow of flavmn is a chromo- plast colour, and that the blue anthocyan dominated as usual. Hybrids sterile.
Pri7nula. P. Sinensis exists in a long series of colour-types the relations of which are still being investigated by R. P. Gregory in con-
II] Colours of Plants 4 1
junction with me. Some of the more striking facts are referred to in later chapters. White flowers with green stem constitute an albino, recessive to all colours. The magenta shades have a factor epistatic to crimson and pink. Blue is hypostatic to all the red shades. The whites which have red or reddish stej?is are dominant whites, showing only a pale shade or tinge of colour in F^. Deep colours cannot appear on stems that are not red except in the white-edged "Sirdar" {g.v.).
Salvia Horniinum. Purple, red, white, related as in Fisum^ &c. Saunders, R.E.C. (20).
Triticum (Wheat). Red chaff is dominant to white chaff. Grey chaff is epistatic to red and dominant to white. Tschermak (270) ; Biffen (27) ; Spillman (247).
Verbascum blattaria. Yellow (a sap-colour) dominant to white. ShuU (241).
Viola. White is recessive to colour (de Vries, 290) in V. cornuta. The brown seed-colour of V. papilionacea is dominant to buff of V. hirsutula, and the purple of the capsule of hirsutiila to the absence of purple vc\ papilionacea- (Brainerd, 41).
Z^a (Maize). Yellow endosperm dominant to white. Blue in aleurone layer an irregular dominant to absence of blue. (Definite exceptions are frequent.) Red pericarp, a plant-character, dominant to absence of red. The relations of the striped types have not been clearly determined. Correns {(i'^\ Lock (172, 174).
Colours of Ani77tals. Man.
Eye-colour {q.v.), Hurst (161), Davenport (107).
Albinism {q.v.) is doubtless recessive, but in man its descent is complex and has not yet been elucidated.
Red hair is recessive to dark hair and perhaps to ordinary brown (see Hurst, 162).
Cattle.
Red-roan is a heterozygote of red and white (Wilson, 311) ; and blue- roan is similarly related to black and white. Spillman (249) suggests that black is dominant to red.
Cats.
Red is dominant to black in males. Tortoiseshell is the corresponding form of the heterozygote in females. Doncaster (109). Dilution-types, blue, and cream, recessive to saturated colours.
As to eye-colour see Przibram (224).
Mice, Fats, Fabbits, Guinea-pigs.
Colours fully discussed in later chapters. Chief papers by Allen (i); Bateson (10); Castle (48, 53); Crampe (83, «); Cuenot (84-9); Darbishire (90); Durham (116); Hurst (157); Mudge (204). Albino recessive in all cases. A piebald type dominant to self-colour exists in mice (Durham, 116).
42 Colours of Animals [ch.
Horse.
Chestnut recessive to bays and browns. Relations of these two domi- nants to each other not clear. Hurst (158).
Pigs.
Several notes published by Spillman (249, 251-2). White is usually a dominant to colour in domesticated races* but piebalds are frequent in F^. The relation of black to red is not yet clear. The white belt, characteristic of certain breeds; is, according to Spillman, due to a complementary pair of factors which may be separately carried by self-blacks. He makes the interesting suggestion that the appearance of the belt may be a "reversion" to a condition like that of the Indian Tapir (251).
The colour of the wild boar is dominant to the red of Tamworth and segregates normally from it (252). The wild colour is presumably due to an " agouti " factor like that of the rodents.
Sheep.
From such fragments of evidence as I can find it seems that the white of ordinary sheep is not, as in the pig, a dominant to colour, but a recessive. From Darwin's record {An. and Fits. 11. p. 4) of the appearance of all black sheep from a cross between white Southdown ewes and a Spanish ram with two black spots, it may perhaps be inferred that the black colour is due to complementary factors.
Black face and white face give a speckled face in the heterozygote. The dark ring round the eyes depends on a separable factor (Wood, 312).
Fowis.
Colours very complicated and genetics imperfectly understood. Whites are of various kinds, one being dominant and at least two recessive. Colour depends on complementary factors which may be borne by whites. Black is an imperfect dominant to black-red. Brown-red a dominant to black-red (Fig. 11). . Blue is a heterozygous colour of black and a splashed white. The red and yellow pigments of the black-red cock may be replaced by white, thus giving the Silver Duckwing, but in the hen the red of the breast is not thus replaced, and the Duckwing hen differs from the black- red in having the yellow of hackle and mantle replaced by white. These replacements may occur as consequence of recombination in F^ from crosses between white and coloured breeds, whence it is to be inferred that the replaced reds and yellows depend on a special factor. Pencilling is a dominant to its absence, and various mottlings are also dominant. The descent of colour is influenced in some cases by sex in ways not yet clear, and in both sexes heterozygous types occur.
The relations of the buff of Cochin (and of other breeds derived from it) to other colours are not yet known.
As regards colour of the down, the brown striped condition is dominant to the pale brown down associated, for example, with Wheaten. Both types of down may occur in the same breed (^.i^. Indian Game), and the fact seems to have no relation to the adult plumage. R.E.C. (21, 22).
* Mr Staples-Browne has given me confirmatory evidence.
II] Colours of Animals 43
The black body-pigmentation of the Silky is a dominant, but may be inhibited by another factor, the descent of which is sex-limited. See Chap. x.
Another sex-limited descent is to be found in the relations of Cuckoo to black, but the details have not been ascertained (Spillman, 249, 253, a).
The daw-eye is recessive to red. The dark iris is usually a dominant to red.
Red ear-lobe is an imperfect dominant to white. R.E.C. (19).
Principal papers dealing with these features are R.E.C. (19-22), Hurst (156); Davenport (loi).
Pigeons.
Black is dominant to blue. Relations of red and yellow not clear. Black and blue are dominant to white of Fantail ; heterozygotes generally, if not always, having some white. Chequering dominant to its absence. The white rump of the Rock-pigeon is dominant to blue rump (Staples- Browne, 255).
Canaries.
Presence of black, as in green and pied types, dominant to absence of black as in the various yellows and cinnamons. The pink eye of cinnamons is recessive to black eye, with a sex-limited inheritance. There are probably several heterozygous colours, but in order to determine these the genetic interrelationships of the various Yellows, Jonque, Mealy, must be worked out. The " cap " and lacing of the Lizard are dominants. (Noorduijn, 213-5; Davenport, 105; F. M. Durham, unpublished.)
Axolotl.
Crosses between normal and albino gave dominance of pigmentation. Subsequent generations showed remarkable and as yet unique features. In F2 dark larvae to white larvae were 3:1; but the white F.^ larvae, though remaining red-eyed, acquired a certain amount of pigment, sometimes distributed as a metameric chequering. No thorough albino occurred in JF^- When however these chequered albinos w^ere bred with a true albino, the latter was found dominant and true albinos were produced (Hacker, 143-4). Hacker compares this case with the phenomena seen in Mice, &c., but there is an essential distinction in the fact that in all other instances true albinos come in Fo and in the dominance of the true albinism over the chequered character. It would be interesting to see whether the development of pigment in the Fc, whites is in any way dependent on conditions.
Lepidoptera.
Bombyx mori (Silkworm). Brown colour seen in a dark variety of the moth was proved to be an imperfect dominant (Coutagne, '^2)1 P- 122).
The larvae have many colour-types. Coutagne used a dark ^'"inoricaiaV variety, a variety with transverse stripes, and ordinary white larvae. Both the coloured types are dominant to white, but when the dark self-colour factor and the stripe-factor are present in the same larva the stripes show on the dark ground-colour {ibid. p. 142). Toyama (268) also made many experiments with the colours of the larvae. He found striping a dominant over plain white. In certain F .^ families from striped x white a new pale
44 Colours of Ajiiinals [ch.
form was produced (p. 349). The ratio of striped : common marked white : pale unmarked white is as ShuU points out (242) 12:3:1 (actually 1463, 363, 126). I incline to interpret this as signifying that the 12 striped were in reality of two kinds, in the ratio 9 : 3, but that the distinction between the common and the pale was not easy to detect in the striped class. On this view the striped parent was a "pale." Shull regards the paleness as " latent " in both parents.
As regards the colour of silk there is a complication. In Toyama's experiments yellow was always a dominant to white. Coutagne sometimes obtained this result, but (/. c. p. 123) the white of a race called " Blanc des Alpes " proved to be dominant to yellow.
Abraxas grossulariata (Currant Moth). The type is dominant to var. lacticolor (q.v.), Doncaster (iii, 114). See Plate I, figs, i, 2. The pecu- liarities of this case are discussed in connection with Sex.
Mr L. W. Newman has been good enough to send me information as to a cross between A. grossulariata and the var. varleyata. This is a nearly black suffused form (see Porritt, Eiit. Rec. xv. p. 10). F^ was typical grossulariata, and in 7^2 there were 24 typical and 7 varleyata (4 d", 3 ? )•
Angerona priuiaria. The dark-banded var. sordiata dominant to the normal, reticulated type. In the heterozygotes the lighter bands are more or less reticulated. Doncaster (m). See Plate I, figs. 7-10.
Xa7ithorhoe ferriigata. The form with purplish band is dominant to that with black band. Prout, L. B. (223).
Hemerophila abruptaria : the dark var. may be inferred to be a dominant to the type, from the experiments of Harris (147). Plate I, figs. 5, 6.
Amphidasys betularia (Peppered Moth). The normal is almost certainly recessive to the black, or doubledayaria form (see, for example, the records of Main and Harrison, 192). Plate I, figs. 3, 4.
Triphaena comes : the reddish form is recessive to the melanic (see Bacot, 3, and Prout, 222).
Callimorpha doi7iinula : red of the hind wings is dominant to yellow. Standfuss (253, /;, p. 222). Mr L. W. Newman has kindly given me in- formation that he bred 34 reds and 10 yellows in 7^2-
Aglia tau: type is recessive to the dark form lugens. Standfuss (253, b^ p. 311), (see Chap, x, for the sex-distribution of these varieties).
Lasiocampa quercus (Oak-egger). The heredity of colours of the hairs of the larvae has been investigated by Bacot (4) and Warburg (302). Several varieties were studied and their genetic interrelationships are not altogether certain, but it appeared that red fur of var. sicula was dominant to the white of the var. meridionalis. When English and French races were crossed, various blend-forms were produced in F-^.
COLEOPTERA.
Most of the observations thus far made relate to Phytophaga. Com- plications were met with in all the cases investigated by Miss McCracken. I have not been able clearly to understand the exact procedure followed in the matings and must refer the reader to the original papers.
Pb
^*
g£
^;
•^
• vw grossulariatu 2. Ditto var. lacticolor. .r. var. /M;/
\ mphidasys betulari i. 5 Dark var. of <ierophii
Male and female var. sordiata of 9. and 10. An^ttona prunaria^
CnJnuTK nf ,^ nimnf :
fCH.
he rat rommon marked
j;, 12:3:1 (actually _..__... . _ -. ._,. _^..ig that the 12 striped kinds, in the ratio 9 : 3, but that the distinction ^he pale was not easy to detect in ' )ed
...nped parenf wns -i -'Dnlf" Sbr,!, . h(^.
'00th parents, colour of silk there is a coiaplication. vas always a dorninant to while. Coi!''^' but (/. c. p. 123) the white of a race • be dominant
ta (Curnait :auL.i/. me type is a> . Lerf^TTT iTj\ See Plate I, figs. 1. ire disr nnection with Sex
;an ha.s
.1. gross ...^ ..„.._,.., ,..., ;
' (see Porritt. Ent. Rec. xv. p. 10). F^ was 1 7^^ tht cal and 7 varleyata (4
.i var. sordiata dominant to the
. — . .. L. . . .ozygotes the lighter bands are
ess reticulated. r (iii). See Pla'te I, figs. 7-10.
XanihorkiH ferrugata. The form with purplish band is dominant to that with black band. Prout, L. B. (223).
/^ the dark var. may be in)' uMt
^■' '• . - . .....iments of Harris (147). i , ., .,.: -,,
<^fuhria (Peppered Moth). The normal is almost the black, or doubledayaria form (see, for example, the did Harrison, 192). Plate I, figs. 3.
: the reddish form is recessive lo iht. mdai • . . 222).
inyfti : rtn^ of the hind wings is don.fnnht to yellow. Ir L. W. Newman has ki en me in-
. brcU 34 reds and 10 yellows in F^
\ form :' ion of t;... . . . ,.
the heredity of colours of t] ed by Bacot (4) and War!
a English and Fn uced in y^, .
i
•(' '•'
^
to PI
•rs.
Cona-
cn. I in
Plate
w¥'0
: -^ >
- "^-.* ^
', ^
• ••
*%.•
I
^^^^^
m'
<'^ W ■^'■'■■■"'
w^»
10
I
I. Abraxas grossulariata. 2. Ditto var. lacticolor. 3. The var. Doubledayaria
of 4. Amphidasys betiilaria. 5. Dark var. of 6. Hemerophila abriiptaria.
7. and 8. Male and female var. sordiata of 9. and 10. Angerona prunaria^ male and female.
n] Colours of Animals 45
Lina lapponica. Two forms occur, one having elytra spotted with black on a brown ground, while in the other the elytra are entirely black. The latter is recessive, and the formation of the brown pigment in the ground is thus due to a dominant factor. McCracken (189, 199).
Melasoma {Lina) scripta. A totally black form was found to be re- cessive to the spotted type. Two intermediate conditions may occur, one of which may be a homozygous type (see original). McCracken (191).
Gastroidea dissimilis. Two forms, either deep blue-black, or shiny bright green. The latter is recessive. Curious complication as regards numerical results. McCracken (190).
Leptinotarsa dece77ilineata (Colorado potato beetle). Evidence obtained by Tower (266, pp. 275-9) indicates that a variety called by him pallida behaves as a recessive. The var. melaiiothorax of the species Z. multi- tae?iiata also proved to be a recessive to its type {ilnd. pp. 284, 292, 293). Various other more complex phenomena are recorded {q. v.).
Crioceris asparagt (Asparagus beetle). Each elytron has three yellow areas or spots on a blue-black ground. The upper spot is sometimes united to the middle one. This condition proved to be recessive to that in which the spots are separate, but all intermediate conditions occur [being presumably heterozygous]. Lutz (182).
MOLLUSCA.
Helix horte?isis and H. 7ie7?ioralis. The U7iba7ided variety is dominant to the banded types in both the species, sometimes completely, some- times partially. Generally also red ground-colour is dominant to yellow or brown, but this effect may diminish with age of the hybrid individual. Lang (167-9). For details as to hybrids between the two species see (169).
As regards dominance of colours very little In the way of general rule can yet be predicated, nor till the chemistry of pigments is much better understood is It likely that such general rules will be discovered. It may, however, be remarked that actual albinism, the total absence of pig- mentation, is always, so far as we know, a recessive character in both animals and plants. Curious cases never- theless are known both in animals and plants where a partial whiteness, which we should a priori Imagine to be a kind of albinism, behaves as a dominant"^'. Another fact of a somewhat paradoxical nature is to be seen in the behaviour of some of the very deep colours, red and purples, characteristic of the flowers and other parts of some garden strains. These more intense colours both In Primula Sinensis, in the Stock, and in the Sweet Pea (and doubtless
* See Chap. v.
46 Preliminary Deductions [ch.
Cyclaine7i) are recessive to the paler and more commonplace tints ^.
Though in the case of colours in plants which are due to the development of pigmented sap, albinos are recessive to the coloured types, the yellow or cream colour due to the presence of yellow chronioplasts is recessive to the colourless condition of the chromoplasts. Hence we find, what at first seems paradoxical, that white flowers are dominant over cream-coloured flowers. Yellow dependent on sap-zo\oMX is dominant to the corresponding white.
With regard to the behaviour of black pigment, which might naturally be supposed to have similar genetic pro- perties in the various animals, no quite satisfactory general rule can be laid down. The presence of black pigment is commonly dominant to the absence of black, as in the race- horse, where chestnut, namely the absence of black "points" is recessive to the presence of such " points " as in bays and browns. Most cases, however, such as that of the mouse, and other animals in which black pigment exists intimately mixed with other pigments are not so simple as this and involve special problems. In so far as the features of those cases can be expressed in the simple terminology hitherto used, these blacks must be classed as recessive to the normal colours. Further particulars will be given in the chapters on Colour.
Prelhninary Dediictio7is from Mendelian Phe^iomena.
It will be observed that animals and plants, as such, do not show any difference in their manner of heredity. Inheritance on simple Mendelian lines may be followed by characters of very diverse kinds, such as height, shape, chemical constitution, colour, and several structural features. In view of such a list the important question arises whether there is any distinct category or class of characters to which the Mendelian system does not apply. Various possible limitations may be discovered when the phenomena have been more fully examined, but it may be stated at once that no such class of characters has hitherto been identified.
■*^ In Antirrhinum Miss Wheldale finds that the deeper magentas are recessive to the ordinary magentas, but in the crimso7i-red series the paler are recessive to the deeper tints.
ii] Meristic Cases 47
As yet only one example of a character which can at all readily be interpreted as meinstic in nature has been shown to have a Mendelian inheritance. This is the case of the reduction in number of the human phalanges in brachydactyly. We speak of a character as '' meristic " when it manifests itself in respect of the number of parts into which the body or one of its organs is divided. Meristic characters are in several ways distinguishable from other features of bodily organisation. The physiological occur- rences which result in meristic variations are in all likelihood distinct from those which produce substantive changes, and exceptional interest would attach to any investigation of the genetic properties of such variations. Polydactylism is of course a meristic feature, but it may involve something more than a divisional change, pure and simple, since change in the number of digits is usually accompanied by change in the distribution of differentiation. A case in which the disturbance of differentiation is not so evident is provided by the cross between Oxalis tetraphylla^ much cultivated in Germany as Glilcksklee, and one of the forms with three leaflets. This cross was partially investigated by Hildebrand"^, who used O. latifolia. He found that the 3-fold character was an imperfect dominant, the leaves being 3-fold with the exception of occasional 4-fold leaves which appeared for the most part at the flowering period. The hybrids were fully fertile, but their progeny has not been studied. Satisfactory meristic cases from which all confusing elements are eliminated must be rare, but it is greatly to be hoped that they will now be searched for. It is most desirable that cases of difference in the ground- plan numbers of some radial type will be found amenable to experimental tests. Here the problem may be seen in a somewhat simplified form on account of the elimination of serial differentiation f.
■**■ Y{\\^t\yc2Ji^^ Jenaische Ztsch. f. Naturiv. 1889, xxni. N. F. xvi. p. 56.
t Since this paragraph was set up Price and Drinkard's (221) evidence has been published showing the dominance of tivo chambers in the fruit of the tomato over the many-chambered condition. More evidence as to such cases would be welcome.
Drinkwater's recent discovery as to the bones of the brachydactylous fingers, showing that the middle phalanx is actually formed as a distinct bone which afterwards unites with the distal phalanx, raises considerable doubt whether the variation in that case is meristic after all.
48 Rights and Lefts [ch.
Respecting the genetics of one most interesting class of variations evidence is scanty. This is right- and left- handedness. From Mayers'^ observations on Partula (Gastropod) we learn that parents of either twist may bear young of either twist. The numbers in the uteri were so small that the absolute numbers are insignificant, and it may be an accident that no mixture of types was found in any one uterus. Langf bred numerous left-handed Helix- pomatia with each other and obtained thousands of young, all right-handed, which in their turn again produced ex- clusively right-handed offspring. Direction of twist is a fundamental meristic phenomenon, being as Crampton and Conklin have proved, determined as early as the first cleavage-plane in the ^<g^ ; and great light on the problems of cell-division might perhaps be obtained if the inheritance of these differences could be determined. The only case we have attempted to study, that of Medicago, in which the fruits are right- or left-handed spirals according to species, proved unworkable, perhaps on account of the minute size of the fiower and the roughness of the manipulations.
Lutz (i8i) has collected facts as to the inheritance of the mode in which the hands are clasped, whether the right or left thumb is placed uppermost. No definite result was obtained, but effects of heredity were somewhat marked, though neither condition bred true. A fuller analysis should be attempted, taking families separately.
When the Mendelian principles were first rediscovered the suggestion was made that though the system might apply to the unions of pure races, there was no certainty that such rules apply to the uncontrolled matings of natural forms. The objection was not one which was likely to have weight with those who had an acquaintance with genetic phenomena, but it had undoubtedly an effect in post- poning general recognition of the importance of Mendel's discovery. Categorical proof of the invalidity of this ob- jection is now provided by one of the cases referred to above — that which concerns the heterostylism of Primula. It is scarcely doubtful that In the Primrose nearly every plant arises by the "legitimate" union of long- and short-
* Mayer, A. G., Mem. Mus. Camp. Zool. Harvard, xxvi. No. 2, 1902. t Lang, A., Vierieljahrs. d. nat. Ges. Zurich, 1896, and 168, p. 42, together with informgition kindly sent in a letter.
"] Cases in Wild Types 49
styled Individuals. Yet the long-styled are always pure. Moreover, all the short-styled plants hitherto tested have proved to be simple heterozygotes, giving equality of longs and shorts when bred with longs. Hitherto no pure DD, viz. short-styled plant, has been found in the case of the Primrose, but no difficulty has been met with in raising pure short-styled plants of Primula Sinensis. Besides this example of Mendelian heredity manifested by a wild type several of the examples of colour-inheritance in insects relate to wild species.
The circumstance that a character has not been pre- viously bred pure does not, so far as is known, in any way influence the mode of transmission of that character. For instance, in the breeding of thoroughbred race-horses the heredity of chestnut colour is that of an ordinary recessive"^, though the various colours, bay, brown, and chestnut have been indiscriminately united together in the breed. No difference is manifested between colour-inheritance of chest- nuts which have had many chestnut ancestors in recent generations, and those that have no chestnut progenitor in the nearer degrees. The same is true for some of the colour-cases seen in Lepidoptera which had not been the subject of any previous selection. A remarkable example of an obviously Mendelian inheritance in a wholly wild form is that of the eye-colour of the Owl — Athene nocttia\.
Abundant examples of characters breeding true, though newly-constituted, will be provided by those cases in which a novelty of structure is brought suddenly into existence by the occurrence of fresh combinations. In spite of their recent origin, such new combinations have just the same genetic properties and powers of transmission that are possessed by the types of long-selected breeds.
The suggestion hazarded by several writers that a dis- tinction may be drawn between inter-racial and intra-racial heredity has no foundation in fact.
* Mr Hurst, who first elucidated the colour-inheritance of race-horses, found that according to the records, chestnuts of various ancestries have exclusively chestnut offspring with about i 7o ^^ exceptions, which are very possibly due to error in the returns (see later).
t Giglioli, Ibis^ 1903) P- i- (See later, p. iio.) B. H. 4
50 Nature of Dominance [ch.
Do^ninance : the Heterozygote Character,
The character of the heterozygote, the "hybrid character" of Mendel, gives no indication as to the system by which the parental characters are transmitted. The expressions, '* blended" inheritance, ''particulate" inheritance and so on, terms formerly devised by Galton for describing the zygotic appearances, are now seen to be descriptive not so much of the mode of transmission as of the consequences of certain groupings of special allelomorphs ; and as it is obviously preferable in all possible cases to use the ultimate descrip- tions reduced to terms of gametic composition, such terms are now seldom requisite. Dominance must be discussed more fully when other facts have been set forth, and in this preliminary notice of the more salient features of the phenomena it will be enough to point out that dominance is no inseparable attribute of Mendelian inheritance. The essential phenomenon is segregation.
The occurrence of dominance is often an assistance to the investigator and may greatly simplify the analysis of the various generations. Seldom however is dominance uniformly complete, and in certain cases, as those of the combs in poultry, where dominance is quite definite, it is still possible for an observer thoroughly familiar with the material to distinguish the homozygous dominants from the heterozygous with fair certainty. Provided the recessives as a class can be identified the application of Mendelian analysis is almost equally easy whether the heterozygotes show definite dominance or some intermediate condition.
The statement made by de Vries that dominance is an attribute of the phylogenetically older character has not been borne out by more extended investigation. In the lists given above many examples to the contrary occur. No one, for instance, can doubt that the various types of dominant comb (rose, pea, &c.) in fowls and the colour called "Brown- breasted " have arisen since domestication. This colour- example is illustrated by Fig. ii, where the distinction between the striped Black-red type and the almost uni- colorous Brown-red, or "Brown-breasted" type is shown. The striped type is practically that of the wild Gallus bankiva, but the unicolorous type of down-colour is a com-
II]
Nature of Dominance
51
plete dominant. Males raised as F^ between the two pure breeds are in their adult plumage almost intermediate, but the F^ hens are indistinguishable from the pure Brown-red hens.
A B
Fig. II. Two newly-hatched chickens in ^2 generation from the cross Brown-red Game Bantam x Black-red. A is the Black-red type having dark stripes on a light ground. B is the Brown-red (or '* Brown-breasted ") type, a dark, almost unicolorous, blackish brown. A is the recessive and B is the dominant. The whole ^2 family consisted of 58 like B and 18 like A.
As an example in which the heterozygotes are inter- mediate the inheritance of colour in the Andalusian fowl may be taken. Andalusians are in general colour what fanciers call blue — namely a diluted black. In the cocks the hackles and saddle-feathers are full black, and the feathers of the breast are edged or '' laced" with black. The hens are blue, laced with black more or less, all over. This breed is recognized by the fanciers as never breeding true to colour. When blue is bred with blue three colours are produced, blacks, blues, and a peculiar white "^ splashed with grey. Experimenting with this breed we have found
* These splashed whites are quite distinct from actual whites. They are in reahty coloured birds as regards composition, and their down-colour is a faint bluish, very like that of the White Rosecomb bantam.
4—2
52
Andalusian Fowl
[CH.
that the numbers from blue x blue average about i black : 2 blues : i splashed white. Both the blacks and the whites extracted from blues breed true to their respective types, black X black giving all blacks ; white x white giving all whites again. When however black is bred with one of these whites the offspring are all blues. There is thus no doubt that the blue is the heterozygous form, while the gametes bear either the blackness, or the whiteness. Ob- viously in such a case, continued selection of blues will not make them breed true. This can only come to pass if it shall be found possible to get a blue bird in the gametes of w^hich the blue or intermediate character is carried as a definite factor. These results may be represented in tabular form thus :
|
Blue X Blue 1 |
|||||
|
n Bh |
icks only |
2n Blues |
n Splashe |
d whites |
|
|
Blacks |
n Blacks |
2n Blues |
n Whites Splashed whites only |
||
|
Blacks only, &c. |
Splashed whites only, &c. |
||||
|
Black X Splashed 1 |
white |
||||
|
1 Blues only |
|||||
|
n |
Blacks |
2n Blues |
71 Splashed whites |
Such a case as this shows well what Mendel meant by the " hybrid-character." It is that character, or appearance, or quality, which is produced by the meeting of the opposite allelomorphs of the same pair in one zygote or individual. The hybrid-character is a thing apart, which must always be separately determined by experiment. Sometimes it is indistinguishable from the dominant, sometimes, as here, it is an appearance recognizably distinct from that of either dominant or recessive.
Prof. J. Wilson (311) has shown that a similar rule pro- bably holds in the case of shorthorn colours, where red-
n] Imperfect Dofninance 53
roan is a heterozygous character, caused by the meeting of the factors for red and white '^. The blue-roan so often seen In the cross between black Aberdeen-Angus cattle and white shorthorns Is presumably the corresponding hetero- zygote form for black and white.
With further knowledge of the details and closer exami- nation of material probably many such cases will be found. Darblshire, for example, has lately shown that though it is usually Impossible in the case of peas to tell pure round seed from the heterozygous rounds, by external appearances, yet on microscopical examination the two classes can be distinguished at once by the different structure of the starch grains (94).
Many such cases where dominance Is Imperfect are now known. This phenomenon has no bearing on the more important question of the degree of perfection with which segregation is accomplished. The supposition that domi- nance w^as an essential phenomenon of Mendelism was of course a delusion. Imperfection of dominance does not even obscure the application of Mendelian analysis. The cases In which difficulty does arise are those in which dominance Is irregular and the recessive class cannot be distinguished with certainty. In the fowl, for instance, the extra toe Is usually a dominant, but in some strains there Is irregularity, and birds without the extra toe may neverthe- less transmit it. So also the blue colour of maize seeds, though usually a dominant, may sometimes be carried on by seeds which appear white (Lock, 174). Even in these examples, however, there Is no reason to think that such irregularities are indications of imperfect segregation. It is not impossible that they may be ascribed to inter- ference caused by the presence of other factors in various combinations, and sometimes, no doubt, to disturbance by external conditions.
All observations point to a conclusion of great import- ance, namely that a dominant character is the condition
* Further information based on a long series of observations of the Sittytown herd of Shorthorns has since been piibHshed by Mr Robert Bruce in Breeder's Gazette, 25 Nov. 1908. Full statistics are given, affording with rare exceptions evidence strongly confirmatory of Prof. Wilson's views. I am obliged to Mr Alexander Bruce for a copy of this paper.
54 Presejtce and Absence [ch.
due to the presence of a definite factor, while the corre- sponding recessive owes its condition to the absence of the same factor. This generaHsatlon, which so far as we yet see, is appHcable throughout the whole range of Mendelian phenomena, renders invaluable assistance in the interpreta- tion of the phenomena of Heredity. The green pea, for instance, owes its recessive greenness to the absence of the factor which, if present, would turn the colouring matter yellow, and so forth. With the examination of further evidence the significance of this principle will become readily apparent.
Mendel's System distingztished fro7n that of Galton.
From the outline of the evidence now set forth the essential aims and methods of Mendelian inquiry will have been understood. By this method we reach reality and concrete fact among phenomena that had become almost proverbial for their irregularity. The key to the problems of genetics and, as w^e confidently believe, to that of Species also, lies in the recognition of the character-units, or factors as we often call them. Their allelomorphism is a pheno- menon of gametogenesis, and is a consequence of those attractions and repulsions by which the germinal cell- divisions are effected. Discontinuity in variation — to use the word variation in its old, comprehensive sense — results from the existence of these units. We recognize therefore that this discontinuity — Galton's "Organic Stability" — is ultimately dependent on the physiology of gametogenesis, and not as we formerly supposed on some feature in the physiology of zygotes. How this simple conclusion was missed we may in vain surmise. The discovery at one stroke replaces all previous disquisitions regarding the laws of inheritance. The magnitude of the discovery and the novelty of its consequences have indeed delayed general recognition of its truth. To this may have been due the curious fact that the famous Nageli failed altogether to realise the importance of Mendel's work. Nageli was of course especially devoted to the study of heredity, and even made it the subject of elaborate mathematical treatment. As we now know, he was in correspondence with Mendel,
ii] Gallon's System compared 55
from whom he received a considerable series of letters and Illustrative specimens (197). These must have utterly failed to arouse his Interest, for when In 1884, the year of Mendel's death, he published his great treatise on heredity, no refer- ence was made to Mendel or his work. That this neglect was due to want of comprehension Is evident from a passage where he describes an experiment or observation on cats, which as It happens, gave a simple Mendellan result. The Angora character (recessive) disappeared In a cross with a certain common cat whose hair-character is, as we now know, dominant. The cross-breds were mated together and the Angora character reappeared In one Individual among a litter of common cats"^. This typically Mendellan fact was thus actually under Nageli's own observation, but from the discussion which he devotes to the occurrence It Is clear that Mendel's work must have wholly passed from his memory, having probably been dismissed as something too fanciful for serious consideration.
It may be useful to specify the distinctive features of Mendellan inheritance which differentiate the cases ex- hibiting it from those to which Galton's system of calculation — or any other systems based on ancestral composition — can apply.
(i) In Mendellan cases. In which the characters behave as units, the types of Individuals considered with respect to any pair of allelomorphic characters are three only, two being homozygous and one heterozygous ; while according to such a system as Galton's the number of possible types is regarded as indefinite.
(2) The Mendellan system recognizes that purity of type may be absolute, and that it may arise In individuals of the /% or any later generation bred from heterozygotes. The views based on ancestry regard purity of type as relative, and arising by the continued selection of numbers of individuals.
(3) In Galton's system no account Is taken of domi- nance, a phenomenon which plays so large a part In the practical application of any true scheme of heredity.
These distinctions are so definite and striking that at first sight it seemed likely that the two methods might be
* C. Nageli, Mechanisch-physiologische Theorie der Adstafn?nungslehre, 1884, p. 199.
56 Zygotes are Double Structures [ch. ii
applicable to two physiologically distinct classes of pheno- mena. It was anticipated that some characters, or possibly even some forms of life, might follow the one system and others the other.
The results of further researches make this supposition increasingly Improbable ; and though undoubtedly there are cases which cannot yet be subjected to Mendellan analysis, it is fairly certain that there is no large group of facts in heredity to which the Galtonian system or any modification of it exclusively applies.
There are however numerous examples where the arith- metical results predlcable by either system are nearly or quite the same, though further breeding would of course reveal that even in these cases the applicability of the Galtonian method was only superficial.
The first aim of genetics must now be to determine the magnitude, number and ultimately the nature of those units which together make up the visible fact we call heredity; and so to discover the consequences of their several com- binations In zygosis or fertilisation. For the power thus to formulate our purpose and for the development of a method by which it may be successfully pursued we are beholden to Mendel's genius.
The difficulty which some feel in realising the signifi- cance of Mendelism arises from the habit of looking on the bodies of animals and plants as single structures. So soon as the mind becomes thoroughly accustomed to the fact that all individuals, at least those of the higher and more familiar types, are double, It becomes easy to think in Mendellan terms, and the world of gametes, whose pairings have brought into existence the individuals we see, comes naturally and persistently before the mind. Henceforth we have to penetrate behind the visible appearances of the individual, and endeavour to reconstruct first those pro- cesses of cell-division which produced the germ-cells or gametes, distributing the characters or factors among them according to definite systems ; and then the subsequent process of union of those gametes pair by pair, in fertilisa- tion to form zygotes, each developing and manifesting in its development those properties of structure. Instinct and conduct conferred upon it by that particular complement of factors which Its two original gametes contained.
CHAPTER III
NUMERICAL CONSEQUENCES AND RECOMBINATIONS.
Representations of the F^ Generation and Novelties due to Re- combination of Factors — Compound Characters — Combs of Fowls — Heterostylism — White Flowers from Red X Ci^eam,
The unity of characters being recognized, we may next examine some of the statistical consequences of this pheno- menon. In order to determine the number of units and allelomorphic pairs which are concerned in any practical case, we have to be guided first by the visible statistical composition of F^ ; and next by such tests of the gametic constitution of the several /% individuals as can be made by breeding from them.
To those who are familiar with algebraical methods, the employment of /% numbers to discover the number of terms in the gametic series may present small difficulty, but to others the following graphic method of demonstration may be of service. For the Introduction of this system, which greatly simplifies difficult cases, I am indebted to Mr Punnett.
Take the simplest example, of one pair of allelomorphs, say Tall {T) and Dwarf (t). The parent zygotes of the pure strains are then TT and tt. Their gametes are 7", Z", and t, t, respectively. The F^ heterozygote is 77, and Its gametes are all either T or t in equal numbers. As this is true both of the female germs and the male germs, there are four possible combinations. Make therefore four
58 * Represent atioit of Z^. Generation [ch.
squares, representing the female gametes or germ-cells, similar cells being placed vertically, thus :
|
r? |
/? |
|
r? |
t9. |
Similarly the male germs may be represented by four squares :
|
T$ |
T$ |
|
tS |
t6 |
Here the similar germs are placed horizontally instead of vertically. If therefore the first set of squares be superposed on the second, all the four possible zygotic combinations are represented, thus :
Thus there are iTT+2Tt-\- iU\ and since T \s domi- nant, the visible appearance of /% is
We may next deal similarly with the case of two allelo- morphic pairs, A a and Bd. We shall now require i6 squares. Writing each set of four similar female germs in vertical rows and the corresponding sets of male germs in horizontal rows, keeping to the same order, the sixteen possible combinations are represented. In each square the
Ill]
Representation of F^ Generation
59
upper expression indicates the nature of the female gamete, the lower one that of the male oramete :
|
AB AB |
Ab AB |
aB AB |
ab AB |
|
AB Ab |
Ab Ab |
aB Ab |
ab Ab |
|
AB aB |
Ab aB |
aB aB |
ab aB |
|
1 AB ab |
Ab ab |
aB ab |
ab ab |
The constitution of each of the sixteen types which are produced in 7% is thus displayed, and since A is dominant over a, and B over b, the visible appearance of F., is
<^AB : ^Ab : ^y^^B : \ab.
Similarly when three pairs of factors are concerned, Aa, Bb, Cc, the F^ type will exhibit them all, and be in appearance ABC. In /% there must then be eight visibly distinct types, and the ratios in which they severally appear will be as follows :
27 ABC ^ gaBC+ ^ABc + ^AbC-V zAbc + laBc
+ ^abC-\- labc.
I am obliged to a mathematical friend for the following scheme by which the number of types and the ratios in which each will appear is given for any number of pairs of factors, one factor of each pair being dominant and the other recessive.
4 = 3 + 1
i6 = (3 4-i)-^ = 3' + 3 + 3+ I =9 + 3 + 3 + 1
64 = (3 4-i)^ = 3' + 3-3' + 3. 3 + 1 =27 + 27 + 9+ I
256 = (3 + 0'=3' + 4-3' + 6.3' + 4-3 + i = 81 + 274-27 + 27 + 27
+9+9+9+9+9+9 + I.
6o Compound Characters [ch.
So in general
4'^ = i'
+ i'^'^ + f^ + ^ times
+ 3"''+3""'+ \n (n-\) times
+ 3""' + 3"''+ \n {n- i) (;e-2) times
+ &c.
Compound Characters and Novel Types produced by
Re-combinations.
Thus far we have dealt only with cases in which the characters of each allelomorphic pair have independent effects on the visible appearance of the zygotes. In peas, for instance, we have seen a pair of characters, tallness and dwarfness, producing their effects quite independently of other pairs of characters, such as those which determine the flower colour or the seed-shapes. Each can be separately perceived by its effects, and the presence of the one in no way influences the development of the other. Such are the imaginary characters A^ a, and B, b, whose distribution is represented above, and the four types pro- duced by their several combinations are each distinguished without difficulty, as AB, aB, Ab, ab.
We now pass to a class of cases manifesting greater complexity. The essential phenomenon in these cases is that definite characters are produced by the mutual inter- action of factors belonging to distinct allelomorphic systems. Such interactions, as we now know, are of the greatest importance in heredity, and the progress of genetics will consist largely in disentangling the elements to which these combination-effects are due.
We may speak of characters thus produced as compound characters. The nature of such compound characters is well exemplified by phenomena which have been observed respecting the inheritance of several types of combs seen in various breeds of poultry, and as these cases are illustra- tive of many others I propose to consider them in some detail.
in]
Compotmd Characters
6i
The Combs of Foivls.
In the first place we are concerned with the following types :
Single co77ib. The high, serrated comb which is familiar to everybody is called by fanciers a single comb (Fig. 1 2 A). It is the type found in Galhis bankiva and I believe all the wild species, and we are fairly safe in regarding it as the primitive or original form from which all the others have been derived. It is the characteristic comb of Leghorns, Minorcas, and many other breeds.
Fig. 12. Various types of combs in Fowls.
A. Single Comb : cock.
B. Pea Comb ; cock.
C. Pea Comb : hen.
D. Rose Comb : (Bantam) cock.
E. Walnut comb in a young cock. This is the type in Malays, and
can be produced by crossing Rose x Pea.
Pea Comb. This comb (Fig. \2 B, C) differs from the single in being much lower and closer to the head. It has no spike-shaped serrations, but merely rounded lumpy pro- jections along the middle line. In addition to these there is a characteristic development of similar lumps or tubercles on each side usually uniting to form a more or less definite
62
Combs of Fowls
[CH.
lateral ridge. These two lateral ridges together with the median one constitute the three ridges commonly spoken of as the essential feature of the pea comb. The size and details of development of these combs differ a good deal with individuals and with strains. Pea comb is especially characteristic of Indian Game, Aseel, and the Brahma breeds.
The F^ from pea x single is pea, that character mani- festing a definite dominance. The heterozygous pea combs are generally higher than the pure pea and may usually, though not always, be distinguished from them. Sometimes the heterozygous pea comb is so large and has the ridges so ill-defined that it approaches the single type, but combs which cannot at once be referred to one class or the other are extremely rare. The distinction is especially sharp in the case of the newly hatched chicks, becoming somewhat less marked with later development. F^ from this cross is of the usual form, 3 pea : i single.
Fig. 13. The combs as they appear in newly hatched chickens.
In the top row from left to right : — Walnut comb in a light- coloured bird, showing the peculiar band of hairs ; ditto in a dark- coloured bird ; Rose comb.
In lower row from left to right : — Pea comb ; ditto ; Single comb ', ditto.
iiij Combs of Fowls 63
Rose Comb, The next type to be taken into account is the rose comb, which consists of a triangular mass of small spikes or papillae. The apex of the triangle, called the peak or pike, points backwards and is free from the head. Such a comb is characteristic of Hamburghs, Rose-combed Dorkings and many other breeds. A particular form of rose comb with the pike curved downwards is a peculiarity of Wyandottes.
F^ from rose x single is rgse. The dominance of the rose comb is very definite, and it is frequently quite im- possible to distinguish pure rose from the heterozygous type containing single. 7% consists of the usual 3 rose : I single.
Rose X Pea. When a pure rose-combed bird is crossed with a pure pea the resulting comb is very different from either. It has no distinct papillae like the rose, or ridges like the pea. In the newly hatched chick the region of the comb is covered with a nearly flat or somewhat warty- looking skin. At the beginning of the posterior third there is generally a most curious band of bristles or hairs crossing the comb. Some F^ birds have hairs scattered over the posterior part of the comb, either with or without a definite transverse band. The hairs usually increase in quantity and definiteness as maturity approaches. It seldom happens that a rose-pea ("walnut") bird has none of these hairs, though with age they may get worn off As the chicken grows, the skin of the comb itself increases in size and becomes more or less corrugated. Such corrugations may become very large in males and are especially developed anteriorly. In this region the comb is often widened so as to form a lobe on each side, but the part behind the band remains single, so that the whole comb has a 3-lobed appear- ance when seen from above. One of the corrugations very often appears as a furrow separating the flatter posterior lobe from the anterior and more elevated part of the comb. It is from the corrugated surface of this comb that it is called by fanciers a " walnut " comb. The only pure breed in which such a comb occurs is the Malay. In Fig. 13 the hairs can be seen fairly well in the top left-hand bird, which has light plumage. In the bird next to it the plumage is dark and the band of hairs is not so distinct in the photograph.
64 The Walnut Comb [ch.
It was not a little surprising to see so striking and characteristic a structure as the walnut or Malay comb appear with strict regularity as the product of two such dissimilar parents as the rose and the pea. Not only is the general appearance of the walnut quite distinct from these, but the presence of the hairs constitutes a feature of absolute difference, for no hairs are found on combs of the usual types. Were the walnut, rose, pea, and single combs found as characteristics of wild birds no naturalist would hesitate to regard them as four distinct specific characters ; and even as the special properties of domesticated birds I suppose they would by many be regarded as evidence of long-continued selection. Nevertheless, as will be seen, these four forms stand to each other in a simple genetic relation and the fact suggests wide possibilities in regard to many hitherto unexceptionable differences ''of specific value " recognized among animals and plants.
The interpretation of the facts was at first by no means easy, and I am sorry to have been responsible for the promulgation of a quite erroneous suggestion regarding them. Further knowledge of kindred phenomena has, however, made the elucidation of this case now perfectly clear and simple.
To return to the experimental results. Having found that rose x pea gives walnut, the next thing to be done was to test the genetic properties of birds thus produced. This was done in two ways (i) by breeding the walnuts together, (2) by breeding them with singles. In what follows, the names may be abbreviated thus : i?, rose ; P, pea ; RP, walnut ; and S, single.
Experiment showed that RP x RP gave an /% family RP, R, Py and S. The appearance of S, which was not known to have been put in, is not at first sight intelligible. Repeated trials proved that the ratio in which these combs appeared was
<^RP : zR : zP : iS.
It was further proved by experiment that the R birds were either pure R or contained the recessive S, but gave no more P or RP ; that the P birds similarly could only give P, or P and 5 ; while the 5 were all pure to that character.
Ill]
The Walnut Comb
65
Consistent with this result were the offspring obtained from first-cross RP x S, for this mating gave the ratio
iRP \ \R : iP : iS.
As it was already established that R and P were each dominant to S, the inference was certain that the gametes produced by the F^ {RP) birds were RP, R, P, and 5" in equal numbers.
The whole series of phenomena may be represented as due to the combinations of two pairs of allelomorphic characters or factors, namely
1. Rose (domt) R, absence of rose (rec.) r.
2. Pea (domt) P, absence of pea (rec.)/.
|
RP |
RP |
RP |
RP |
|
RP |
Rp |
rP |
rp |
|
walnut |
walnut |
walnut |
walnut |
|
pure |
giving rose |
giving pea |
giving all 4 |
|
Rp RP |
Rp Rp |
Rp rP |
Rp rp |
|
walnut |
rose |
walnut |
rose |
|
giving rose |
pure |
giving all 4 |
giving single |
|
rP |
rP |
rP |
rP |
|
RP |
Rp |
rP |
rp |
|
walnut |
walnut |
pea |
pea |
|
giving pea |
giving all 4 |
pure |
giving single |
|
rp RP |
rp Rp |
rp rP |
rp rp |
|
walnut |
rose |
pea |
single |
|
giving all 4 |
giving single |
giving single |
pure |
Reference to the diagram of combinations shows that the two " absences " of rose and pea respectively will meet once on an average in 16 times, and to such a meeting without doubt the appearance of the single combs as a novelty, in /%, is to be ascribed.
In the early years of this experiment, knowing that^i?
B. H.
66 The Breda Comb [ch.
and P were allelomorphic to 5, I came to regard them as also allelomorphic to each other. This idea led to con- fusion, but we know now that no case justifies such an application of the principle of allelomorphism. A rose comb is not due to an elemental factor which can segregate from the pea comb factor. The two factors belong to distinct allelomorphic pairs and each in the gametogenesis of the heterozygote segregates from its own allelomorph, which is simply the absence of the factor in question. The single comb contains neither R nor P, The rose comb is a single comb modified by the presence of R, while the pea comb is produced by the presence of P. We may therefore describe the rose as R no P, and the pea diS P no R. It is convenient to use capital letters for dominants and small letters for recessives, the rose being thus written Rp, and the Pea, rP. The walnut comb is the RP, while rp gives the single.
The allelomorphism of the elements which go to the constitution of the shapes of combs in fowls may without doubt be carried very much further. For example there are indications that the size of the comb depends to some extent at least on other pairs of factors. Another curious set of phenomena, perhaps worth investigating further, may be studied in a cross between a single comb breed and the '' Breda " fowl. The Breda is usually said to have " no comb." As a matter of fact it has two very minute tubercles which represent the comb. When this breed is crossed with the single comb, F^ has what may be called a *' double " single comb. It consists of two large lobes or leaves diverging outwards from a common base^. Such a comb is evidently due to the introduction by the Breda of a factor which may be called " bifidity." This factor acts on the large comb brought in by the single-combed parent and the result of the combination is a large, double comb. F., from this cross has not yet been raised, but there can be no doubt that it will contain members having the '' no comb " of the Breda and the ''absence of bifidity" of the single- combed breed. Such birds probably have only a minute tubercle at the posterior end of the comb region.
* The two lobes sometimes unite anteriorly to a greater or less extent.
Ill] The Breda Comb 67
Mr Hurst suggested to us that an excellent confirmation of the truth of the analytical method by which the composi- tion of the rose and pea combs has been represented could be obtained by a cross between the rose comb and the Breda, which, as has been stated, has the bifidity-factor but practically no comb at all. The elements involved are
Rose, R, No rose, r.
Comb present, C. Comb absent, c.
Bifidity, B. No bifidity, b.
The 7\ generation has the composition Rr, Cc, Bb, namely a dottble rose comb. /% generation contains a great variety of forms, of which those having B and C, but no R, have the high bifid comb like that of the F^ raised in the cross Breda x Single, and those which contain only r and b in combination with C have single combs of the ordinary high type (Fig. 12). Both these kinds occurred in /%, and their appearance is entirely confirmatory of the scheme of representation adopted. Since neither the rose nor the Breda have outwardly any suggestion of single comb in their appearance, were it not for a knowledge of Mendelian analysis such a result must have seemed utterly unaccount- able.
When therefore we look at such an organ as the comb of a fowl and attempt to conceive its genetic properties, we have to remember that the structure as a whole may be composite in origin, and that the visible appearances and properties may result from the interaction of a number of distinct elements each transmitted independently in gameto- genesis.
Everything however points to the conclusion that the number of these elements is finite, and that their properties are not beyond the reach of orderly analysis.
In the example of the walnut combs the interaction of two dominant factors is such that the 9 members of the /% series of 16 have a feature, the walnut comb, distinct from those of the original parents, while the i member of the series which contains neither dominant factor, also has a feature, the single comb, distinct from anything visibly
5—2
68 Heterostylism [ch.
introduced. The next case also illustrates the appearance of a novelty in /^,, but, as will be seen, it is one of the groups of 3 members which manifests it.
Heterostylism in Primula.
The dimorphic, heterostyled condition of Primula plants is well known and need not be described in detail. The two types are distinguished thus :
A. Thrum, or Short-styled type.
1. Style short, the stigma standing at the level of
constriction of the tube.
2. Anthers at the mouth of the tube.
3. Pollen grains large.
B. Pi7t, or Long-styled type.
1. Style long, the stigma usually standing in the mouth
of the tube.
2. Anthers at the level of constriction.
3. Pollen grains small.
Experiments made by Mr R. P. Gregory In conjunction with me showed that the Inheritance of these two types is Mendellan^, the short-styled or thrum behaving as dominant. The long-styled, being recessive, always breed true to that type on self-fertilisation or when bred inter se.
In connection with this fact it Is Interesting to observe
* I know no authentic case of the presence of both long- and short- styled flowers on the same plant. Such occurrences are frequently- announced, but so far as I can discover the records are based on mistakes. In occasional flowers on long-styled plants, especially in the beginning of the flowering period, the style does not attain its proper length. Such flowers are through carelessness sometimes misdescribed as short- styled. The anthers however are at the lower level, and the pollen-grains are small, so the essentially long-styled nature of these plants is quite clear.
The statement is also sometimes made that pin plants have produced thrum-eyed offspring without the intervention of a cross. This mistake is due to the appearance of a type with "exsert" anthers. Such anthers project from the mouth of the tube and give a thrum-like look to the flower. But careful examination shows that the anther-filaments are inserted at the lower level, and the pollen-grains are small. Such plants are therefore long-styled.
Ill] Heterostylism 69
that Primula Sinensis, which is preferred by fanciers in the pin form, was easily bred true to that type, and is always so maintained in good strains. For our experiments it was with considerable difficulty that we procured any thrum plants. On the other hand it was decided long ago that the Auricula and the Polyanthus for exhibition purposes must always be thrums, but though thrums have thus been largely selected for breeding, pin-eyed plants are continu- ally reproduced, as must be expected, for the thrum is a dominant, and therefore liable to contain the recessive type. The facts about to be described relate to an experiment made with a peculiar race of P. Sinensis grown by Messrs
Fig. 14. Some of the types of flowers in F^_ from the cross short-style (thrum) ; small eye x liomostyle ; large eye.
A. The long-styled flower : with small eye.
B. The homost) led : large eye.
C. The short-styled : small eye.
D. The short-styled : large eye.
In the two upper flowers the corolla is of the "star" type. D is the ordinary, imbricated type of Sine?isis. C is more or less inter- mediate in corolla-shape. This shape is the usual heterozygote formed between star type and Sinensis type. The corolla -shapes are of course quite independent of the style and " eye " characters.
70 Heterostylism [ch.
Sutton under the name '' Primrose Queen." To the casual observer this race differs from an ordinary Primula in the fact that the yellow '' eye," instead of forming a small and well-defined pentagon, is continued as a yellow flush ex- tending far over the limb of the corolla. The anthers of these flowers stand at the lower level and the pollen-grains are small ; but the style instead of projecting into the mouth of the tube stops at the anther-level, being thus practically the same length as in the short-styled type. Such a form of flower was called by Darwin komostyled.
Crosses were made betw^een this homostyle race with the yellow flush and an ordinary thrum with the pentagonal or small eye. /\ is thrum with the small eye, showing that the yellow flush is recessive. /% gave the following series :
9 thrum or short-style with small eye,
3 ,, ,, with the large eye,
3 ordinary ^//^ 07^ long-style with small eye,
I homostyle with the large eye like Primrose Queen.
The long-styled or pin type, which apparently was not put in, is evidently due to the re-combination of the charac- ters. The two pairs of characters are
Thrum type (domt). Pin type (rec).
Small eye (domt). Large eye (rec).
The homostyle is the form which the pin type assumes when the large eye is developed ; but when in 7% the pin type meets the small eye, the ordinary pin or long-styled form is produced.
/% gave a simple and confirmatory result ; for of the 7^2 long-styled plants, some proved pure to the long-styled character, while others threw the recessive homostyle.
White Flowers in F., from Redx Cream.
Exactly comparable with the foregoing case is the paradoxical appearance of wkite-^o-w^r^d individuals in the /% from the cross of a sap-coloured variety with a variety having cream-coloured flowers. For example, in Sweet Peas or Stocks [Matthiola) when a red-flowered type is crossed with a cream, F^ is red, without any cream-colour.
Ill]
Novelties by Re-combination
71
7% consists of 9 reds without cream, 3 reds with cream, 3 whites, I cream.
When the allelomorphs are correctly distinguished the significance of this series is obvious. The operations may be shown in a tabular form, thus :
Parents Red variety x Cream variety
Allelomori?hs [ ^^^ '^P ^^^ Colourless sap {R)
■^ [Colourless corps. (Z?) Yellow corpuscles {R) { Red sap
F^
[Colourless corpuscles
I
P ( Red sap
^ [Colourless corps.
Red sap Yellow corps.
Appearance 9 Red 3 Red Cream
Colourless sap Colourless corps.
3 White
Colourless sap Yellow corps.
I Cream
These cases of novelties resulting through a re-combi- nation of the factors brought in by the original pure types are striking because it is not at first sight evident how the novelty has been produced. Generally speaking, however, the re-combinations form in /% a series of types many of which are obviously new combinations of features which could be recognized on inspection as present in the pure parents. Thus the cross between a bearded, rough chaff, red w^heat, and a beardless, smooth chaff, white wheat give in F^ a beardless, rough chaff, red. But in /% all the different possible combinations occur, such as bearded, smooth, red ; beardless, rough, white ; bearded, rough, white, &c., each in their appropriate numerical proportions. In the Guinea-pig, starting from albino, smooth coat, long hair, and crossing it with coloured, rough coat, short hair, F^ is coloured, rough coat, short hair. But 7% contains the various re-combinations of these three pairs of characters, such as albino, rough coat, short hair ; coloured, smooth coat, long hair, &c. Thus by selecting any desired type in i% any of these new combinations can be fixed and perpetuated. Basing his procedure on a knowledge of the dominance or recessiveness of each character the breeder may thus guide his operations with certainty.
That this has been the mode by which most of the new breeds of domesticated plants and animals have been created is obvious. The traces of it remain in many cases. For
72 Practical Examples [ch.
instance, Sutton's '' Nonpareil," one of the marrow peas, consists in about equal numbers of yellow-cotyledon seeds and green-cotyledon seeds. Like all the new peas it must have arisen at some definite moment by the selection of an individual yellow-seeded plant which was true for the various good qualities of Nonpareil — being homozygous for them in other words — but in cotyledon-colour it was heterozygous. As the diversity of colour was not thought objectionable it persists. If any one wishes to make an exclusively green-seeded Nonpareil, all he has to do is to take green seeds from a sack of that variety and sow them, saving the seeds they bear. If he desires the yellow type pure, he may similarly sow yellow seeds from the same sack. Most of these by now will presumably be pure to yellow, but some may not. By keeping the seeds of any plant which gives only yellow seeds a pure yellow Nonpareil will be constituted.
Similarly Sutton's ''Continuity" is a pea which is allowed to be either pointed in pod or blunt. The variety is true in other respects and it is clear that its original progenitor was a plant homozygous for the peculiarities of Continuity but heterozygous in respect of the pod-shape. The pointed-pod plants would be found true for that character, since it is recessive, while the blunt-pod plants might or might not be true for it.
So in the Chinese Primrose, several varieties, e.g, Sutton's Mont Blanc, Sirdar, &c., distinguished by peculi- arities of colour have been fixed both in a palm-leaved and in a fern-leaved form, these having of course been saved in F^ or later generations from a heterozygote in which the palm and fern-leaved characters were combined. In Sweet Peas, after the original dwarf '' Cupid " was found — accord- ing to tradition a chance seedling among tall plants — it was easy to transfer the characteristic colour of the various tall types on to a Cupid foundation, and now any colour almost can be had either as a tall or as a Cupid.
Possible Limits to Re-combination.
These illustrations might be extended indefinitely. It will probably occur to many that there are limits to these possibilities of transference, and so undoubtedly there are.
Ill] Limits to Interchangeability 73
The detection of these limits is one of the more important tasks still awaiting us. Though on this head little can yet be asserted with confidence it is likely that such limitations are constituted in two distinct ways : First, from all we know of the capacities of animals and plants we must anticipate that some characters are incompatible in the same individual. For example in cattle the highest milk- production is not to be found in the breeds which make the best beef. Meat-production and milk-production are to some extent alternative and can only be combined by compro- mising one quality or both. That such an alternative distribution is merely a result of allelomorphism seems on the whole unlikely, though certainly not impossible.
Then again we must surely expect that these transferable characters are attached to or implanted upon some basal organisation, and the attributes or powers which collectively form that residue may perhaps be distinguishable from the transferable qualities. The detection of the limits thus set upon the interchangeability of characters would be a dis- covery of high importance and would have a most direct bearing on the problem of the ultimate nature of Species.
CHAPTER IV
HEREDITY OF COLOUR.
Factoids deter^niniitg Colours : the Ratio 9:3: 4. — The ''Presence and Absence^' Hypothesis. Epistatic and Hypostatic Factors — Colours of Mice — Pied Types — A Dominant Piebald,
With regard to the application of the Mendelian system to problems of colour inheritance the evidence is now considerable. The fact that in both animals and plants albinism behaved as a recessive to colours was soon dis- covered. Several examples among plants are mentioned by de Vries in his first paper on this subject, and shortly after similar facts were recorded in regard to animals. Since in the course of a large range of experiments with many species of animals and plants no case to the contrary has been met with, it may perhaps be asserted as a general truth that pigmentation is always dominant to total absence of pig- ment"^. When however we proceed to the investigation of the genetic properties of varieties which are so far deficient in pigment as to be called sometimes partial albinos, we find that various specific rules are followed and no uni- formity of behaviour has yet been discovered. White fowls for instance are thus commonly spoken of as partial albinos, but the pigmentation of their eyes sharply distinguishes them from albinos which are destitute of pigment, and many of their genetic properties are found experimentally to be quite distinct from those of real albinos. The same is true of certain varieties of plants, which though varying from the specific type by possessing white flowers have yet some red or purple sap in the stem or elsewhere. With-
* The Axolotl is perhaps an exception. See p. 43. The fact that in plants colourless chromoplasts are dominant to yellow chromoplasts scarcely constitutes an exception, for the yellow of the chromoplasts is not pigment in the usual acceptation of that term.
CH. IV] The Ratio 9:3:4 75
out experiment no prediction can be made with confidence as to the behaviour of such types in their crosses.
Albinism being recessive in all ordinary cases, 7% from the cross colour x albino contains i albino to 3 coloured members. As regards the characters of the dominant or coloured mem- bers various complications have to be considered.
In the simplest cases the coloured F, individuals are all of the same colour. For example on crossing a grey rabbit with an albino, F^ is grey and F^^ may be 3 greys : i albino. But frequently it is found that in addition to the greys and albinos blacks appear in /%. Repeated experiments, for example those of Hurst, have shown that in such families the /% ratio is
9 greys : 3 blacks : 4 albinos
' , '
12 -4
3 • I
The relation of this ratio to the ordinary 9:3:3:1 was first pointed out by Cuenot (86)^. As represented by him two pairs of allelomorphs are concerned, namely :
Donii7iant. Recessive.
1. Colour (C). Albinism [A).
2. Grey determiner (G). Black determiner {B).
The presence of one or other of the determiners G or B is only perceptible w^hen it exists in combination with the colour-factor. If 6^ is present together with C, the colour is grey ; if ^5* is present with C but without G, the colour is black. If a coloured individual contain both G and B, being thus heterozygous in the second pair of factors, the colour is grey, for the effects of grey dominate. But since in the absence of colour (C) neither determiner produces a perceptible effect, albinos may exist of the forms AAGG, AAGB, or A ABB, and without breeding tests it will not be possible to distinguish between these several forms. On crossing with a black of course each albino can be known by its effects. For GG will then give greys only, GB will give equal numbers of greys and blacks, while BB albinos will give only blacks.
* Shortly before the publication of Cuenot's paper Mr R. H. Lock wrote to me from Ceylon with the same suggestion.
76 Presence and Absence [ch.
The ''Presence and Absence'' Hypothesis applied to the
Case of Colour,
So long as attention Is restricted to crosses like these involving only two sorts of colours besides the albinos, the system suggested by Cuenot is adequate, but when a third colour has to be considered, as In the case of mice, some modification is required. The simplest notation by which these and other complex Mendelian phenomena can be expressed is provided by what is spoken of as the Presence and Absence hypothesis already illustrated in the case of the combs of fowls.
Mendel himself probably conceived of allelomorphism as depending on the separation of a definite something responsible for the dominant character from another some- thing responsible for the production of the recessive character. It Is however evidently simpler to imagine that the dominant character is due to the presence of something which in the case of the recessive is absent. As yet there is no absolute proof that this mode of de- scribing the facts is correct, but everything points that way, and no phenomena have yet been encountered which cannot be thus formulated when their nature is understood. In cases where the pure dominants are recognizably distinct from the heterozygous dominants, it must naturally be sup- posed that two " doses " of the active factor are required, one from the paternal, and another from the maternal side, in order to produce the full effect.
Applying the presence and absence system to the case of the colours of rabbits, the first pair of allelomorphs can obviously be represented as
Do7nina7tt. Recessive,
I. Presence of Colour (C). Absence of Colour {c).
The second pair we have so far spoken of as the grey determiner and the black determiner, regarding these two as allelomorphic to each other. But it is equally possible to describe them thus
2. Grey determiner [G), Absence of ditto {g).
IV]
Presence and Absence
77
'
Then in the case where grey x albino gives in F^ 9 g^^y • 3 black : 4 albino,
we simply have to regard B, the black determiner, as common to both parents, and the same numerical result is produced. Such a case may usefully be represented in a tabular form, thus :
Parents Grey x Albino
Gametic Composition... CGB
CgB
F,
Grey
CcGgBB
F^ 9 Grey
12 3
3 Black
4 Albino
4 I
|
CGB CGB grey |
cGB CGB grey |
CgB CGB grey |
CgB CGB grey |
|
CGB cGB grey |
cGB cGB albino |
CgB cGB grey |
CgB cGB albino |
|
CGB CgB grey |
cGB CgB grey |
CgB CgB black |
CgB CgB black |
|
CGB CgB grey |
cGB CgB albino |
CgB CgB black |
CgB CgB albino |
Fig. 15. Distribution of grey, black, and albino individuals in ^2 from the cross grey, CGB, with albino cgB, showing the meaning of the ratio 9 grey : 3 black : 4 albino.
yS Presence and Absence [ch.
In this diagram the 9 squares containing C, G, B, are the 9 greys, the 3 squares containing C and B only are the
3 blacks and the 4 squares containing no C at all are the
4 albinos.
Proceeding to the case of mice we write the composition as follows :
Grey C, G, B, Ch.
Black C, g, B, Ch.
Chocolate C, g, b, Ch.
We thus regard the black mouse as one from which the grey determiner, G, has been removed. In the chocolate mouse the process of removal has been carried further and the black determiner, B, is also gone.
A proof that this system of representation is so far correct is obtained by crossing the grey mouse with the chocolate. Such a cross, if G is not allelomorphic to B, must give blacks in /%. This experiment has been lately carried out by Miss F. M. Durham, to whose work our knowledge of the genetics of mice is largely due. The result is that, as expected, /% does contain blacks, and though the num- bers as yet obtained are small, there can be little doubt that the F^ ratio is
12 grey : 3 black : i chocolate.
Some interesting questions arise in regard to the greys. Obviously we should expect 9 greys containing G and B 4- 3 greys without B. Now fanciers are well aware of a dis- tinction between two kinds of greys or ''agoutis" as they are called. These are known as "golden agoutis" and "cinna- mon agoutis," the former containing black pigment, the latter being without it. In the /% from grey x chocolate both these kinds of agoutis appear, and evidently the cinnamon agoutis are the expected greys wanting In the determiner B.
Thus far all is clear. Certain difficulties however remain unexplained. These will be described later. At this stage in the discussion it is convenient to notice that in view of the facts now stated the use of the term domi- nance must be more carefully restricted than has hitherto been necessary. When we speak of the colour as being dominant over the absence of colour we mean that if the colour is present it will appear, and that If the factor for
IV] Epistatic mid Hypostatic 79
colour Is absent the Individual will be devoid of colour. The term Is thus used correctly to denote the relation between allelomorphic features belonging to the same pair. But confusion will be Introduced If we extend the same term to the relationship between various determining factors which belong to distinct allelomorphic pairs.
Hitherto we have spoken of the determiner for such a colour as grey In rabbits and mice as " dominant " over the colours lower In the scale, such as black or chocolate. Nevertheless we are here dealing with a relationship quite different In order from that subsisting between the coloured and the albino. Pending a more precise knowledge of the nature of this relationship It will be enough to regard those factors which prevent others from manifesting their effects as higher, and the concealed factors as lower. In accord- ance with this suggestion the terms epistatic and hypostatic may conveniently be Introduced. We shall then speak of the determiner for grey as epistatic to that for black ; that for black as epistatic to the determiner for chocolate, and so on.
When the facts are thus clearly represented we perceive that the variation by which, for example, a black mouse came originally Into existence, consisted In the omission of the de- terminer for grey . The chocolate mouse similarly owed Its origin to the successive omission of the determiner for black.
The Important question what the effect of the grey determiner, for example, actually Is, remains undecided. A further serious difficulty also arises In regard to the relation of the colour ji/^//^z£/ to the other colours. Neither of these points Is yet satisfactorily understood In the case of mice. The recent papers of Castle {^^ and of Hurst "^ have made the phenomena In rabbits comparatively clear, though even there, however, an unexplained difficulty remains. The special problems raised by the behaviour of yellow pigment In these animals will be discussed In a sub- sequent section! (see Chap. vii).
* Read at the Internat. Congr. Zool. Boston, 1907 : not yet published.
t According to the number of factors involved and to the definiteness with which their several combinations can be distinguished, an indefinite variety of ratios may of course be produced in F<, families. Some of the most interesting are those in which some of the heterozygous combinations can be distinguished from the homozygous dominants. (See for examples Shall, 242.)
8o Sattirated and Dihtte Colours [ch.
The F^ ratio 9:3:4, the significance of which we have been considering, is one which very frequently recurs in MendeHan analysis. For example, as Tschermak found, when a pink-and-white flowered eating pea {Pisttm sativum) is crossed with a white flowered type, F^ is often of the original purple flowered type. Then F^ will be
9 purple : 3 pink-and-white : 4 white.
Similarly pink Salvia HorminuTn x white may give F^ purple, and /% 9 purple : 3 pink : 4 white. In these cases the factor for the purpleness is of course brought in by the albino, but exactly the same /% may result from a cross between the ptirple type and an albino not carrying the factor for purpleness. All that is essential for the produc- tion of this ratio in /% is that /^, should be heterozygous for two factors, of which one is perceptible whenever present, while the other needs the presence of the first in order that its own effects may be manifested. Such cases are very numerous and in practical breeding are to be looked out for continually. Care must be taken to distin- guish them from families like those of the Andalusian fowl (p. 52) in which the commonest term in the F^ series is a heterozygous type. There the numbers will be 1:2:1, which in a practical example may give results not obviously distinguishable from 3 : 9 : 4. To decide between the two possibilities it is necessary to breed the 7% types again. If neither of the scarcer types when bred inter se can throw the other, and the commoner type cannot be bred pure, the latter is a heterozygous type; but if one of the scarcer types can throw the other, then the ratio is presumably 9:3:4, and in such a case it will be possible to raise a breed true to the type occurring as 9.
Saturation and Dilution of Colours.
Omitting yellow from our consideration, we thus re- cognize that in the mouse the colours, grey (agouti), black, chocolate, which the fur visibly presents, result from the interaction of several factors, and that these factors can in great measure be shown to be distributed in gametogenesis according to Mendelian allelomorphic systems. The ex-
I. Agouti.
2. Black.
3. Chocolate.
Cinnamon Agouti, viz. Agouti without black.
5. Dluc -dilute black.
6. Silver fawn : = dilute chocolate.
■ifa^.-TT A f^\ 1^4^
y;wTgr'"f«fri->^^:-.^
I>»
' • ^ #,
K-*
Plate II
Sc
4. Cinnamon Agouti, viz. Agouti without black.
5. Blue : = dilute black.
6. Silver fawn : = dilute chocolate.
IV] Colottrs of Mice 8i
periments of Miss Durham (ii6) have shown that not only the particular pigment is thus constituted, but also that the intensity or degree of saturation in which it is formed can be represented as determined by similar factors.
For example the black colour may exist in the saturated condition, when the mouse is called black, or in a more dilute form, when it gives the "blue" appearance. Similarly the chocolate colour when diluted gives what fanciers call "silver-fawn." Experiment shows that the cross black x silver fawm gives exactly the same result (in F^ and /%) as blue X chocolate. (See Plate II.)
The following are experimental results illustrating these points.
The allelomorphs concerned may be represented as B, the black determiner, b the absence of B, leaving the colour chocolate. D the dense or saturated condition of the colour, d, the absence of D, leaving the colour dilute. (In the case of the introduction of the albino we should have also to take cognizance of C, the presence of colour, c, its absence.)
The actual results may then be expressed in a tabular form.
Blue X Chocolate
{Bd) ! {bD)
F^ Black
(yBbDd)
I ^ 1
7^2 •••Black Blue Chocolate Silver-fawn
{BD) {Bd) {bD) {bd)
Observed 44 17 17 8
Calculated ^i*-^ 16' i 16' i ^'4
Black X Silver-fawn
{BD) \ {bd)
i?i Black
{BbDd)
I ^ n
7^2... Black Blue Chocolate Silver-fawn
{BD) {Bd) {bD) {bd)
Observed 67 21 20 5
Calculated <5j-<5 21-2 21-2 yi
It is thus immaterial whether the factor for saturation is brought in together with the black determiner or with
B. H. 6
82 Colours of Mice [ch.
the chocolate (more strictly, with the ''absence of the black determiner"). So long as the same factors are introduced, the consequences in F^ and the results of re-combination in 7% are the same. But when the dilution is introduced from each side, /% is of course the usual 3 dominants : i recessive, thus :
Blue X Silver-fawn {Bd) I {bd) F, Blue
{Bbdd)
J- ^
i^2...Blue {Bd) Silver-fawn (bd)
Observed 46 17
Calculated 47'2^ i5'75
It is evident that an extracted albino cannot be carrying a determiner for a colour higher in the scale than that of its coloured parents. Moreover if the parents from which an albino is extracted are alike, and if they throw no offspring with colours other than their own (besides the albinos), then the albinos so extracted must be all bearers of the determiner for their parental colour. If such albinos are crossed with forms of a colour lower in the scale than that borne by the albinos, F^ must be of the colour deter- mined by the albinos. For example. Miss Durham obtained the following :
Silver-fawn x Albino (extracted from chocolates) {Cbd) I {cbD)
F^ Chocolate
{CcbbDd)
I \ 1
7^2 •••Chocolate Silver- fawn Albino
{CbD) {Cbd) (various)
Observed 19 4 6
Calculated i6'3 ^'4 7'2
Black X Albino (extracted from chocolates) {CBD) I {cbD)
A Black
{CcBbDD)
r -^ n
F2... Black Chocolate Albino
Observed 76 24 27
Calculated 77"^ 2j-8 ji-y
IV] Colours of Mice 83
|
A. |
Blue X Albino {CBd) 1 {cbD) Black {CcBbDd) 1 |
(extracted j |
from chocolates) |
||
|
A. |
..Black |
Blue |
Chocolate |
Silver-fawn |
Albino |
|
Obs. |
{CBD) 3?> |
{CBd) 10 |
{CbD) 8 |
{Cbd) 2 |
(various) 12 |
|
Calc. |
27-4 |
pz |
g-i |
3'o |
l6'2 |
On microscopical examination the dilution of the pig- ment seems to consist in a diminution in the number of the pigment granules, and not In a reduction of their size. It Is interesting to notice that many different animals have varieties In which the dilution has proceeded to the same extent. For example the particular dilution of black which we call blue is known as characterizing definite varieties in both rabbits, cats and mice, not to mention other cases less certainly comparable.
As was mentioned in the description of the Andalusian fowl and its genetic features the blue colour of that breed is not comparable with the blues we have been discussing. In the mouse the blue is gametic, being a condition which can be carried by the germ cells, while in the Andalusian the blue is zygotic and depends on the collocation In one individual of one germ-cell bearing black with another which does not bear black. It is interesting, as exemplify- ing the danger of reasoning from analogy where genetic phenomena are concerned, that the blue-roan of cattle should not follow the same rules as the other mammalian blue varieties, but should conform rather to the Andalusian system. Of course blue roan is, even to the eye, not the homogeneous blue of the blue cat or mouse, but a mixture of white or whitish hairs among blue and black ones. Still on analogy we might have expected the blue of cattle to be capable of representation In the germ-cells, but the facts, so far as I can discover, afford no support to that supposition"^.
* From such meagre evidence as I have obtained it is likely that the blue, or blue dapple, of Dachshunds and other hounds is also a heterozygous combination. As to the blue of Greyhounds and Great Danes I have no information, but I suspect it to be a dilute black capable of being bred true.
6—2
84 Colours of Mice [ch.
Wholly Coloured and Pied Varieties,
In the analysis of the relationship between the whole or self-coloured forms and the varieties which differ from them in having an admixture of white many curious and specific phenomena are met with. It is probably true to say that generally the whole-colour is dominant to the pied, but several examples to the contrary are already known. In all the cases yet studied the genetic properties of the pied types can be represented factorially by regarding the pattern or distribution of the colour as due to a distinct factor or to its absence. Where the whole-colour is a dominant, the presence of the factor must be taken as causing that distribution, so that in the absence of that factor the individual is pied. Conversely if the pied type is dominant the presence of the pattern-factor acts by re- straining the distribution of the colour, and in the absence of the restraining factor the whole-colour prevails.
One of the clearest cases is that studied by Hurst in the rabbit, where the pattern known as "Dutch-marked" was proved to be a recessive. In the Dutch rabbit the hind quarters are coloured, for example with grey or black, while the front half of the body is white except for a patch of colour— grey or black as the case may be — surrounding the eyes and covering the ears. This pattern though fluctuating in minor respects is fairly definite and is at once distinguishable both from the self-colours and from the various other pied forms. The cross between a pure self- colour and a Dutch gives F^ nearly self-colour with /% consisting of 3 selfs : i Dutch in the ordinary way. An interesting feature is however to be observed in the fact that the heterozygotes between self and Dutch generally (? always) have some small amount of white collar, marked especially on the back behind the head. Apart from Mendelian experiment it might have been supposed that such a white mark showed that the animal contained some albino blood. Experiment shows on the contrary that the actual self-colours may be carrying albino as a recessive, while the small white mark is an indication of heterozygosis with Dutch pattern only.
The behaviour is in all respects as if the whole-colour
IV] Pied Varieties 85
pattern depended on the existence of a dominant. So also, exactly as in the case of dilution and saturation, an albino may carry either the whole-colour factor, or its absence. Consequently when a Dutch rabbit is crossed with an albino bearing self-colour, F^ is self-colour and /% gives 9 self-colour : 3 Dutch : 4 albinos.
In rats there is a type of colouring w^hich rather closely corresponds to the Dutch of rabbits. This has a "hood" of colour over the back of the head and shoulders continued down the back in a stripe which may either be entire or broken into spots. This behaves towards the self-colour just as does the Dutch in the rabbit ; and just as in the rabbit the heterozygote between the self-pattern and this hooded type always has some white. Crampe i^-^, a) observed this fact before the days of Mendelian analysis. He noticed that when a wild grey rat was crossed with an albino, F^, as we call it, might be a real self-colour, or might have some white; but that subsequently hooded rats only occurred as offspring of those which had some white. Such rats, which are nearly whole-colour but have a little white, are know^n in the fancy as the "Irish" type. Experiments made by Doncaster and also by Mudge indicate that these may be again divided into two subordinate classes, distin- guished by the amount and distribution of the white, and there is some evidence to show that these two types of Irish rats have distinct gametic compositions.
Though the Dutch rabbit and the hooded rat are each such clearly recognizable types, yet within these types there is great fluctuation, and it is practically certain that the fancier's ideal Dutch-pattern rabbit, with the demarcation between the colour and the white passing in a sharp trans- verse line across the middle of the animal, does not exist as a gametic entity. Such individuals of course come into existence from time to time, but selection will not fix their type. As Castle and MacCurdy (183) have shown In the case of rats, selection may nevertheless to a considerable extent be effective In producing hooded types with more colour, and with less colour, which are evidently gametic possibilities.
In mice no pied type exists which Is quite so definite as the Dutch pattern of rabbits or the hooded type of rats,
86 Pied Varieties [ch.
and by taking pains every gradation In amount of white could be found represented among fancy mice. Experience, however, soon shows that some at least of these are gametic types, while presumably others are the consequence of various heterozygous combinations. Cuenot's work with mice led him to the conclusion that in mice the several degrees of piedness are recessive to each other In the order of the amount of white, those with more white being re- cessive to those with less. In general terms this is a true account, but we have not found the rule to apply quite strictly even in mice, perhaps through the existence of the complication next to be considered. It should be remarked also that no general statement can be made as to dominance of self-colours over pied which is applicable to animals In general, and on a wide survey of the results of breeding many paradoxical occurrences are met with. Especially curious are the cases, by no means very rare, in which a cross between a domesticated and a wild animal, e.g. dog and wolf — has produced a partially pied offspring.
Dominant Pied Pypes.
So far we have been considering the behaviour of pied patterns recessive to the whole-coloured types. Though several points remain for Investigation the genetic relations of these patterns are fairly clear. A remarkable complication has next to be mentioned. Both In the rabbit and the mouse it is now known that in addition to the pied types which are recessive there are others which are domi- nant to the whole-coloured form. Hurst has proved this for the variety of rabbit called " English pattern." This animal Is white with spots of colour (black, grey, or other- wise) generally of small size on the sides of the body, a patch over the eyes, and a " chain " of spots sometimes nearly continuous down the middle of the back. In ideal specimens the spots should be of a special form and have a definite distribution, but neither of these features seems to be gametic. The English type Is not now much in fashion and looks uncommon to those used to modern rabbits, but formerly it was very abundant.
IV] A Dominant Piebald Type 87
Hurst's experiments (160) showed that this pattern is an ordinary dominant to self-colour — a definite but most unexpected result.
More lately Miss Durham (116) has found a dominant, and doubtless analogous, pied type in mice. So far no criterion has been discovered which distinguishes this dominant pied pattern externally from the recessive ones, but in breeding the distinction is perfectly sharp. It is likely that the factor for this new dominant was brought into Miss Durham's strains by the introduction of the type called ''black-eyed white," but the evidence is not perfectly clear on this point. As may be supposed, the combination of the dominant pied with the recessive pied in the same strain gave results which it was impossible to disentangle, though when each of these types was isolated the course of descent was perfectly clear. The case is interesting not merely as exemplifying a new^ kind of factor, but as illus- trating a type of complication that may very possibly have to be reckoned with in other difficult and as yet incom- prehensible sets of phenomena.
Another example of a pied condition dominant to self- colour has been seen in our poultry experiments (22); but since in fowls some of the wholly white breeds are prac- tically dominant in whiteness, it is only to be expected that some of the partial whites should also show dominance.
In plants, as in animals, no general rule can be laid down as to the dominance of self-coloured or parti-coloured flowers. In the Sweet Pea, for example, the old-fashioned " Painted Lady" (see Plate V) has a red standard and wings nearly white. It is thus a bi-colour type, but it is dominant to the self-coloured reds and pinks. On the other hand in Antirrhinum the self-coloured types are always dominant to the "Delilah" forms which have the lips coloured and the tube or throat white, as was first shown by de Vries. Miss Wheldale who has since worked on a large scale with Antirrhimun (303) found that for each shade of flower which exists as a self-colour, a corre- sponding Delilah or white-tubed type can be made which behaves to the corresponding self-colour as a simple reces- sive.
CHAPTER V
HEREDITY OF COLOUR— continued.
Albinos giving Coloured Offspring ; Reversion on Crossing — Various Kinds of Whites — Stocks — Orchids — Pigeons — Fowls — Primula.
We have seen that albinos, both animal and vegetable, though devoid of pigments, may yet bear factors which are capable of determining the quality and distribution of colour when they meet with colour in the zygote. Particular colours may thus be due to the co-existence of several distinct determining factors, each with an independent distribution among the germ-cells. The grey of the rabbit for instance is caused by the presence (i) of a colour- element or elements ; (2) of a factor which determines them to be the mixture we call grey, and not for instance, black or yellow. In the case of certain plants this analysis can be carried a step further, and the formation of colour at all in the flowers can be proved to depend on the co-existence of two complementary factors in the individual.
The first indication of this phenomenon was found in the fact that two plants each totally devoid of colour in the flowers and stems, and each breeding true to albinism, may when crossed together give purple flowers in F^. The two white parents each contain a factor which, alone, is incap- able of forming colour. Each of these factors is indepen- dently transmitted in gametogenesis, and thus in 7% the ratio of coloured individuals to whites is 9 : 7. This pro- portion depends on the fact that a series of 16 individuals is necessary to exhibit all the possible combinations of germ-cells, for, as in any example of hybridisation involving two pairs of allelomorphs, there will be four types of female cell and four types of male cell produced by F^. Of these
CH. V]
Reversion in Colour
89
sixteen individuals 9 will contain both the dominant or present factors, while of the remaining 7 individuals, 3 will contain the one dominant, 3 will contain the other, and i will contain neither. There will therefore be 9 which are coloured and 7 which are albino. In the diagrammatic scheme C and R are the symbols representing the two complementary factors, c and r being their respective allelo- morphic absences.
Fig. 16. Composition of the 9 coloured and 7 albino offspring in F.2 from the cross between albino Cr with albino cR, showing the ratio 9 coloured : 7 albinos.
Two examples of this phenomenon have been studied in detail. The first is that provided by the Sweet Pea [Lathy 7^zis odorahts) which has formed the subject of ex- periments carried on jointly by Mr Punnett and myself for some years.
The work was begun by crossing two white sweet peas belonging to the variety Emily Henderson. These plants were alike in every respect so far as could be perceived, excepting that the shapes of their pollen grains differed, the one having the normal long pollen grains of the species, while the other had roundish grains. The object of the experiment was to trace the descent of the pollen-character and at the beginning no question of colour was entertained. When F^ was grown however it was clear that here was a
90 Reversion in Colour [ch.
remarkable opportunity of studying a reversion in colour due to crossing, for these plants instead of being white were purple like the wild Sicilian plant from which our cultivated sweet peas are descended.
The facts respecting the colour inheritance will now be given. With regard to the pollen-shape it must suffice at present to state that the long shape is a dominant and the round a recessive. The details as to the distribution of these tw^o shapes among the /% individuals, which are interesting and have greatly aided the development of genetic theory, are given in the chapter dealing with the phenomenon of "gametic coupling." The present section is concerned with colour only.
When the reversionary F^ generation was first seen its nature was entirely mysterious. When 7% was raised from these F^ plants the series consisted of a mixture of plants, some coloured and some white. In some cases the series of coloured plants consisted of two kinds only, purples like F^, and a red bicolour type, the w^ell-known old variety called '' Painted Lady."
In other cases F^ contained besides those mentioned, two quite distinct additional types of purples and two corresponding additional types of reds.
The phenomena, though, as will be shown, in reality very simple, presented superficially an appearance of great complexity. Further difficulties were met with in the fact which was soon discovered, that the cross between long- pollened whites and round-pollened whites does not always give the coloured types, but may result in ordinary whites only.
It is unnecessary to go through the long series of steps by which the analysis of the phenomena was carried out. The meaning of the facts is now perfectly clear and they can all be arranged in one consistent scheme.
Of the two white parents originally used the one possessed one of the two factors we have called C and R, the other introducing the other complementary factor. The meeting of these two elements produces colour in the flower. If no other epistatic factor is present their colour is red. As a matter of experiment however one of the parents, proved afterwards to be that which had the long pollen, did carry
V]
Colours of Sweet Peas
91
such an epistatic factor whose property is to make the colour purple, just as the factor B, in the mouse, makes the coat colour black. F^ was therefore purple, and 7% con- sisted of 27 purples : 9 reds : 28 whites, as shown below. The factor w^hich determines the colour to be purple is represented as B, the blue factor.
The diagram (Fig. 17) exhibits in a tabular form the composition of the various /% plants. The distribution of the
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Fig. 17. The F^ family from the cross white Emily Henderson Sweet Pea, long poilen X white ditto, round pollen. The 27 cross-hatched squares represent purple plants, all containing B. The 9 single- hatched squares without B are the reds. The 27 squares lacking either C, or R, or both, are all whites of various compositions. The numbers at the foot show the ratio of long to round pollens in the types of individuals represented in the columns above.
92 Extracted Whites [ch.
pollen-shapes among them will be the subject of further consideration (Chap. ix).
White (long-pollened) x White (round-pollened) CrB cRb
F^ Purple
CcRrBb i^2 • • • 2 7 purple : 9 red : 28 whites
CRB CRb of various compositions, but
none containing both Candi?
36 : 28
9 '- 7
When these facts are made out we have no longer any difficulty in understanding how it can be that various results may follow the union of two white plants which would breed true to whiteness in perpetuity if left to self-fertili- sation. For example, white x white will always give only whites unless one of the complementary factors C and R is present in each parent. Whites of the form Crb and cRb when crossed together will give reds only, with 9 reds to 7 whites in /%. Whites of the form CrB and cRB crossed together will give purple in F^, and in /% 9 purples : 7 whites, no reds being present in such a family because the blue factor B occurs on each side of the parentage.
Extracted Whites,
We have so far represented each type of white used for a parent as if it were homozygous for those factors, but of course it may be heterozygous for one or more of them, in which case F^ will contain a mixture of types. For example, CCrrBb x ccRRbb will give in F^ purples and reds in equal numbers. Ccrrbb x ccRrbb will give in F^ a family consist- ing of 3 whites : i red. Various other combinations are possible and most of them have now been met with in the course of experiment.
The fact that the extracted whites, that is to say, those which appear in /%, have not in all cases the same pro- perties as the original parental types is readily intelligible. In the early stages of the research it seemed strange that whereas the original long-pollened white crossed with the round-pollened white gave a coloured result, it was possible and indeed more usual to find whites exclusively produced by the cross of two extracted 7% whites, long-pollened
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V] Colours of Sweet Peas 93
and round-pollened respectively, which outwardly were indistinguishable from the two original parents. As we now know, by the redistribution of factors a great variety of whites is in reality produced in /%, and only those pairs which bear the complementary factors C and R can give coloured offspring. This apparent dissimilarity between the behaviour of the extracted forms and that of the pure types from which they are derived has been adduced as being inconsistent