liBRAi MACMILLAN AND CO., LIMITED LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN COMPANY NEW YORK • BOSTON • CHICAGO ATLANTA • SAN FRANCISCO THE MACMILLAN CO. OF CANADA, LTD. GREGOR MENDEL ABBOT OK BRUNN Frontispiece MENDELISM .10; 26. Scheme of Inheritance of Colour-blindness . . 1 08 27. Single and Double Stocks . . . . .113 28. F2 Generation ex Silky Hen x Brown Leghorn Cock . i f8 29. Pedigree of Eurasian Family . . . . . 121 30. Curve illustrating Influence of Selection . . 147 ILLUSTRATIONS FIG. 31. Curve illustrating Conception of pure Lines 32. Brachydactylous and Normal Hands . 33. Radiograph of Brachydactylous Hand 34. Pedigree of Brachydactylous Family . 35. Pedigree of Haemophilic Family . Xlll PAGE 149 '57 158 159 161 Mendtlisw. ERRATA Page 47, line 26, for " c" read " C." 50, line I, for " white" read " while." 81, line 18, for "zygote" read "gamete." 90, line 2 of table, for " 49 " read "41." 90, line 2 of text, for " (n + i)" read " (« - i)." 103, fig. 25, for " Ffppli[^]" read " ffppli[^]." 118, line 6, for "fpi" read " fpL'" 126, line 7, for "of comparatively" read "of a com- paratively." 159, line 24, for " Medelian" read " Mendelian." For although it be a more new and dif- ficult way, to find out the natureof things, by the things themfelves ; then by read- ing of Books, to take our knowledge up- on truft from the opinions of Philofo- phers : yet muft it needs be confef- fed, that the former is much more open, and lefle fraudulent, efpecially in the Se- crets relating to Natural Philofophy. WILLIAM HARVEY, Anatomical Exercitations, 1653. xiv CHAPTER I THE PROBLEM A CURIOUS thing in the history of human thought, so far as literature reveals it to us, is the strange lack of interest shown in one of the most interesting of all human relationships. Few if any of the more primitive peoples seem to have attempted to define the part played by either parent in the formation of the offspring, or to have assigned peculiar powers of transmission to them, even in the vaguest way. For ages man must have been more or less con- sciously improving his domesticated races of animals and plants, yet it is not until the time of Aristotle that we have clear evidence of any hypothesis to account for these phenomena of heredity. The pro- duction of offspring by man was then held to be similar to the production of a crop from seed. The seed came from the man, the woman provided the soil. This remained the generally accepted view for many centuries, and it was not until the recognition of woman as more than a passive agent that the physical basis of heredity became established. That recognition was effected by the microscope, for only with its advent was actual observation of the minute l B 2 MENDELISM CHAP. sexual cells made possible. After more than a hundred years of conflict lasting until the end of the eighteenth century, scientific men settled down to the view that each of the sexes makes a definite material contribution to the offspring produced by their joint efforts. Among animals the female con- tributes the ,ovum and the male the spermatozoon ; among plants the corresponding cells are the ovules and pollen grains. As a general rule it may be stated that the re- productive cells produced by the female are relatively large and without the power of independent move- ment. In addition to the actual living substance which is to take part in the formation of a new individual, the ova are more or less heavily loaded with the yolk substance that is to provide for the nutrition of the developing embryo during the early stages of its existence. The size of the ova varies enormously in different animals. In birds and reptiles, where the contents of the egg form the sole resources of the developing young, they are very large in comparison with the size of the animal which lays them. In mammals, on the other hand, where the young are parasitic upon the mother during the earlier stages of their growth, the eggs are minute and only contain the small amount of yolk that enables them to reach the stage at which they develop the processes for attaching themselves to the wall of the maternal uterus. But whatever the differences in the size and appearance of the ova produced by different animals, they are all comparable in that each is a distinct and separate sexual cell which, as a rule, is unable to develop i THE PROBLEM 3 into a new individual of its species unless it is fertilised by union with a sexual cell produced by the male. The male sexual cells are always of microscopic size and are produced in the generative gland or testis in exceedingly large numbers. In addition to their minuter size they differ from the ova in their power of active movement. Animals present various mechanisms by which the sexual elements may be brought into juxtaposition, but in all cases some distance must be traversed in a fluid or semi- fluid medium (frequently within the body of the female parent) before the necessary fusion can occur. To accomplish this latter end of its journey the spermatozoon is endowed with some form of motile apparatus, and this frequently takes the form of a long flagellum, or whip-like process, by the lashing of which the little creature propels itself much as a tadpole with its tail. In plants as in animals the female cells or ovules are larger than the pollen grains, though the disparity in size is not nearly so marked. Still they are always relatively minute cells since the circumstances of their development as parasites upon the mother plant render it unnecessary for them to possess any great supply of food yolk. The ovules are found surrounded by maternal tissue in the ovary, but through the stigma and down the pistil a potential passage is left for the male cell. The majority of flowers are hermaphrodite, and in many cases they are also self-fertilising. The anthers burst and the contained pollen grains are then shed upon the stigma. When this happens, the pollen cell slips 4 MENDELISM CHAP. through a little hole in its coat and bores its way down the pistil to reach an ovule in the ovary. Complete fusion occurs, and the minute embryo of a new plant immediately results. But for some time it is incapable of leading a separate existence, and, like the embryo mammal, it lives as a parasite upon its parent. By the. parent it is provided with a protective wrapping, the seed coat, and beneath this the little embryo swells until it reaches a certain size, when as a ripe seed it severs its connection with the maternal organism. It is important to realise that the seed of a plant is not a sexual cell but a young individual which, except for the coat that it wears, belongs entirely to the next generation. It is with annual plants in some respects as with many butterflies. During one summer they are initiated by the union of two sexual cells and pass through certain stages of larval development — the butterfly as a caterpillar, the plant as a parasite upon its mother. As the summer draws to a close each passes into a resting-stage against the winter cold — the butterfly as a pupa and the plant -as a seed, with the difference that while the caterpillar provides its own coat, that of the plant is provided by its mother. With the advent of spring both butterfly and plant emerge, become mature, and themselves ripen germ cells which give rise to a new generation. Whatever the details of development one cardinal fact is clear. Except for the relatively rare instances of parthenogenesis a new individual, whether plant or animal, arises as the joint product of two sexual cells derived from individuals of different sexes. Such sexual cells, whether ovules or ova, sperma- i THE PROBLEM 5 tozoa or pollen grains, are known by the general term of gametes, or marrying cells, and the individual formed by the fusion or yoking together of two gametes is spoken of as a zygote. Since a zygote" arises from the yoking together of two separate gametes, the individual so formed must be regarded throughout its life as a double structure in which the components brought in by each of the gametes remain intimately fused in a form of partnership. But when the zygote in its turn comes to form gametes, the partnership is broken and the process is reversed. The component parts of the dual structure are resolved with the formation of a set of single structures, the gametes. The life cycle of a species from among the higher plants or animals may be regarded as falling into three periods: (i) a period of isolation in the form of gametes, each a living unit incapable of further development without intimate association with another produced by the opposite sex ; (2) a period of association in which two gametes become yoked together into a zygote, and react upon one another to give rise by a process of cell division to what we ordinarily term an individual with all its various attributes and properties ; and (3) a period of dissociation when the single structured gametes separate out from that portion of the double structured zygote which constitutes its generative gland. What is the relation between gamete and zygote, between zygote and gamete ? how are the properties of the zygote represented in the gamete, and in what manner are they distributed from the one to the other ? — these are questions which serve to indicate 6 MENDELISM CHAP, i the nature of the problem underlying the process of heredity. Owing to their peculiar power of growth and the relatively large size to which they attain, many of the properties of zygotes are appreciable by observa- tion. The colour of an animal or of a flower, the shape of a seed, or the pattern on the wings of a moth, are all zygotic properties, and all capable of direct estimation. It is otherwise with the properties of gametes. While the difference between a black and a white fowl is sufficiently obvious, no one by inspection can tell the difference between the egg that will hatch into a black and that which will hatch into a white. Nor from a mass of pollen grains can anyone to-day pick out those that will produce white from those that will produce coloured flowers. Nevertheless, we know that in spite of apparent similarity there must exist fundamental differences among the gametes, even among those that spring from the same individual. At present our only way of appreciating those differences is to observe the properties of the zygotes which they form. And as it takes two gametes to form a zygote, we are in the position of attempting to decide the properties of two unknowns from one known. Fortunately the problem is not entirely one of simple mathematics. It can be attacked by the experimental method, and with what measure of success will appear in the following pages. CHAPTER II HISTORICAL To Gregor Mendel, monk and abbot, belongs the credit of founding the modern science of heredity. Through him there was brought into these problems an entirely new idea, an entirely fresh conception of the nature of living things. Born in 1822 of Austro-Silesian parentage, he early entered the monastery of Briinn, and there, in the seclusion of the cloister garden, he carried out with the common pea the scries of experiments which has since become so famous. In 1865, after eight years' work, he published the results of his experiments in the Proceedings of the Natural History Society of Brtinn, in a brief paper of some forty pages. But brief as it is, the importance of the results and the lucidity of the exposition will always give it high rank among the classics of biological literature. For thirty-five years Mendel's paper remained unknown, and it was not until 1900 that it was simultaneously discovered by several distinguished botanists. The causes of this curious neglect are not altogether without interest. Hybridisation experiments before Mendel there had been in plenty. The classificatory work of Linnaeus in the latter half of the eighteenth 7 8 MENDELISM CHAP. century had given a definite significance to the word species, and scientific men began to turn their attention to attempting to discover how species were related to one another. And one obvious way of attacking the problem was to cross different species together and see what happened. This was largely done during the earlier half of the nineteenth century, though such work was almost entirely con- fined to the botanists. Apart from the fact that plants lend themselves to hybridisation work more readily than animals, there was probably another reason why zoologists neglected this form of investi- gation. The field of zoology is a wider one than that of botany, presenting a far greater variety of type and structure. Partly owing to their importance in the study of medicine, and partly owing to their smaller numbers, the anatomy of the vegetable was far better known than that of the animal kingdom. It is, therefore, not surprising that the earlier part of the nineteenth century found the zoologists, under the influence of Cuvier and his pupils, devoting their entire energies to describing the anatomy of the new forms of animal life which careful search at home and fresh voyages of discovery abroad were continually bringing to light. During this period the zoologist had little inclination or inducement to carry on those investigations in hybridisation which were occupying the attention of some botanists. Nor did the efforts of the botanists afford much encouragement to such work, for in spite of the labour devoted to these experiments the results offered but a confused tangle of facts, contributing in no apparent way to the solution of the problem n HISTORICAL 9 for which they had been undertaken. After half a century of experimental hybridisation the determina- tion of the relation of species and varieties to one another seemed as remote as ever. Then in 1859 came the Origin of Species, in which Darwin pre- sented to the world a consistent theory to account for the manner in which one species might have arisen from another by a process of gradual evolution. Briefly put, that theory was as follows : — In any species of plant or animal the reproductive capacity tends to outrun the available food -supply, and the resulting competition leads to an inevitable struggle for existence. Of all the individuals born, only a portion, and that often a very small one, can survive to produce offspring. According to Darwin's theory, the nature of the surviving portion is not determined by chance alone. No two individuals of a species are precisely alike, and among the variations that occur some enable their possessors to cope more successfully with the competitive conditions under which they exist. In comparison with their less favoured brethren they have a better chance of surviving in the struggle for existence and, conse- quently, of leaving offspring. The argument is completed by the further assumption of a principle of heredity, in virtue of which offspring tend to resemble their parents more than other members of the species. Parents possessing a favourable variation tend to transmit that variation to their offspring, to some in greater, to others in less degree. Those possessing it in greater degree will again have a better chance of survival, and will transmit the favourable variation in even greater degree to io MENDELISM CHAP. some of their offspring. A competitive struggle for existence working in combination with certain principles of variation and heredity results in a slow and continuous transformation of species through the operation of a process which Darwin termed natural selection. The coherence and simplicity of the theory, sup- ported as it was by the great array of facts which Darwin had patiently marshalled together, rapidly gained the enthusiastic support of the great majority of biologists. The problem of the relation of species at last appeared to be solved, and for the next forty years zoologists and botanists were busily engaged in classifying, by the light of Darwin's theory, the great masses of anatomical facts which had already accumulated, and in adding and classifying fresh ones. The study of comparative anatomy and embryology received a new stimulus, for with the acceptance of the theory of descent with modification it became incumbent upon the biologist to demonstrate the manner in which animals and plants differing widely in structure and appearance could be conceivably related to one another. Thenceforward the energies of both botanists and zoologists have been devoted to the construction of hypothetical pedigrees suggest- ing the various tracks of evolution by which one group of animals or plants may have arisen from another through a long-continued process of natural selection. The result of such work on the whole may be said to have shown that the diverse forms under which living things exist to-day, and have existed in the past so far as palaeontology can tell us, are consistent with the view that they are all ii HISTORICAL ii related by the community of descent which the accepted theory of evolution demands, though as to the exact course of descent for any particular group of animals there is often considerable diversity of opinion. It is obvious that all this work has little or nothing to do with the manner in which species are formed. Indeed, the effect of Darwin's Origin of Species was to divert attention from the way in which species originate. At the time that it was put forward his explanation appeared so satisfy- ing that biologists accepted the notions of variation and heredity there set forth and ceased to take any further interest in the work of the hybridisers. Had Mendel's paper appeared a dozen years earlier it is difficult to believe that it could have failed to attract the attention it deserved. Coming as it did a few years after the publication of Darwin's great work, it found men's minds set at rest on the problems that he raised and their thoughts and energies directed to other matters. Nevertheless, one interesting and noteworthy attempt to give greater precision to the term heredity was made about this time. Francis Galton, a cousin of Darwin, working upon data relating to the breeding of Basset hounds, found that he could express on a definite statistical scheme the proportion in which the different colours appeared in successive generations. Every individual was conceived of as possessing a definite heritage which might be ex- pressed as unity. Of this, ^ was on the average derived from the two parents (i.e. ^ from each parent), ^ from the four grandparents, \ from the eight great- grandparents, and so on. The Law of Ancestral 12 MENDELISM CHAP. Heredity, as it was termed, expresses with fair accu- racy some of the statistical phenomena relating to the transmission of characters in a mixed population. But the problem of the way in which characters are distributed from gamete to zygote and from zygote to gamete remained as before. Heredity is essen- tially a physiological problem, and though statistics may be suggestive in the initiation of experiment, it is upon the basis of experimental fact that progress must ultimately rest. For this reason, in spite of its ingenuity and originality, Galton's theory and the subsequent statistical work that has been founded upon it failed to give us any deeper insight into the nature of the hereditary process. While Galton was working in England the German zoologist, August Weismann, was elabora- ting the complicated theory of heredity which eventually appeared in his work on The Germplasm (1885), a book which will be remembered for one notable contribution to the subject. Until the pub- lication of Weismann's work it had been generally accepted that the modifications brought about in the individual during its lifetime, through the varying conditions of nutrition and environment, could be transmitted to the offspring. In this biologists were but following Darwin, who held that the changes in the parent resulting from increased use or disuse of any part or organ were passed on to the children. Weismann's theory involved the conception of a sharp cleavage between the general body tissues or somato- plasm and the reproductive glands or germplasm. The individual was merely a carrier for the essential germplasm whose properties had been determined ii HISTORICAL 13 long before he was capable of leading a separate existence. As this conception ran counter to the possibility of the inheritance of " acquired characters," Weismann challenged the evidence upon which it rested and showed that it broke down wherever it was critically examined. By thus compelling biologists to revise their ideas as to the inherited effects of use and disuse, Weismann rendered a valuable service to the study of genetics and did much to clear the way for subsequent research. A further important step was taken in 1895, when Bateson once more drew attention to the problem of the origin of species, and questioned whether the accepted ideas of variation and heredity were after all in consonance with the facts. Speaking generally, species do not grade gradually from one to the other, but the differences between them are sharp and specific. Whence comes this prevalence of discon- tinuity if the process by which they have arisen is one of accumulation of minute and almost imper- ceptible differences ? Why are not intermediates of all sorts more abundantly produced in nature than is actually known to be the case ? Bateson saw that if we are ever to answer this question we must have more definite knowledge of the nature of varia- tion and of the nature of the hereditary process by which these variations are transmitted. And the best way to obtain that knowledge was to let the dead alone and to return to the study of the living. It was true that the past record of experimental breeding had been mainly one of disappointment. It was true also that there was no tangible clue by which experiments might be directed in the present. 14 MENDELISM CHAP, n Nevertheless in this kind of work alone seemed there any promise of ultimate success. A few years later appeared the first volume of de Vries' remarkable book on The Mutation Theory. From a prolonged study of the evening primrose (Oenothera) de Vries concluded that new varieties suddenly arose from older ones by sudden sharp steps or mutations, and not by any process involving the gradual accumulation of minute differences. The number of striking cases from among widely different plants which he was able to bring forward went far to convincing biologists that discontinuity in varia- tion was a more widespread phenomenon than had hitherto been suspected, and not a few began to question whether the account of the mode of evolu- tion so generally accepted for forty years was after all the true account. Such, in brief, was the outlook in the central problem of biology at the time of the rediscovery of Mendel's work. CHAPTER III THE task that Mendel set before himself was to gain some clear conception of the manner in which the definite and fixed varieties found within a species are related to one another, and he realised at the outset that the best chance of success lay in working with material of such a nature as to reduce the problem to its simplest terms. He decided that the plant with which he was to work must be normally self-fertilising and unlikely to be crossed through the interference of insects, while at the same time it must possess definite fixed varieties which bred true to type. In the common pea (Pisum sativum} he found the plant he sought. A hardy annual, prolific, easily worked, Pisum has a further advantage in that the insects which normally visit flowers are unable to gather pollen from it and so to bring about cross fertilisation. At the same time it exists in a number of strains presenting well-marked and fixed differences. The flowers may be purple, or red, or white ; the plants may be tall or dwarf; the ripe seeds may be yellow or green, round or wrinkled, — such are a few of the 15 16 MENDELISM CHAP. characters in which the various races of peas differ from one another. In planning his crossing experiments Mendel, adopted an attitude which marked him off sharply from the earlier hybridisers. He realised that their failure to elucidate any general principle of heredity from the results of cross fertilisation was due to their not having concentrated upon particular characters or traced them carefully through a ^sequence of generations. That source of failure \ he was careful to avoid, and throughout his ex- I periments he crossed plants presenting sharply I contrasted characters, and devoted his efforts to I observing the behaviour of these characters in successive generations. Thus in one series of ex- periments he concentrated his attention on the transmission of the characters tallness and dwarf- ness, neglecting in so far as these experiments were concerned any other characters in which the parent plants might differ from one another. For this purpose he chose two strains of peas, one of about 6 feet in height, and another of about i|- feet. Previous testing had shown that each strain bred true to its peculiar height. These two strains were artificially crossed l with one another, and it was found to make no difference which was used as the pollen parent and which was used as the ovule parent. In either case the result was the same. The result of crossing tall with dwarf was in every case nothing but tails, as tall or even a little taller than the tall parent. For this reason Mendel termed tallness the dominant and dwarfness the 1 Cf. note on p. 171. m MENDEL'S WORK 17 recessive character. The next stage was to collect and sow the seeds of these tall hybrids. Such seeds in the following year gave rise to a mixed generation consisting of tails and dwarfs but no intermediates. By raising a considerable number of such plants Mendel was able to establish the fact that the number of tails which occurred in thisi generation was almost exactly threejjiaes as great! as the number of the dwarfs. As in the previous year, seed were carefully collected from this, the second hybrid generation, and in every case the seeds from each individual plant were harvested separately and separately sown in the following year. By this respect for the individuality of the different plants, however closely they resembled one another, Mendel found the clue that had eluded the efforts of all hi; predecessors. The seeds collected from the dwai recessives bred true, giving nothing but dwarfs. A: this was true for every dwarf tested. But with tlfie tails it was quite otherwise. Although indistinguish- able in appearance, some of them bred true, while others behaved like the original tall hybrids, giving a generation con- sisting of tails T X D - and dwarfs in F • the proportion of three of the for- i — T mer to one of the / T(D) > T(D) . Dr--F2 latter. Counting i 1 — H 1 i — r — \ 1 | showed that the T TT(D) T(D) DTT(D) T For this is the only ratio which satisfies the conditions that the tails should be to the dwarfs as 3 : i; and at the same time the coloured should be to the whites as 3:1. And these are the proportions that Mendel found to obtain actually in his experiments. Put in a more general form, it may be stated that when two indi- viduals are crossed which differ in two pairs of differentiating characters the hybrids (Ft) are all of the same form,, exhibiting the dominant character of each of the two pairs, while the F2 generation produced by such hybrids consists on the average of 9 showing both dominants, 3 showing one dominant and one recessive, 3 showing the other dominant and the other recessive, and i showing both recessive characters. And, as Mendel pointed out, the principle may be extended indefinitely. If, for example, the parents differ in three pair of characters A, B, and C respectively dominant to a, b, and c, the Fl individuals will be all of the form ABC, while the F2 generation will consist of 27 ABC, 9 ABc, 9 AbC, 9 aBC, 3 Abe, 3 aBc, 3 abC, and i abc. When individuals differing in a number of alternative characters are crossed together, the hybrid generation, provided that the original parents were of pure strains, consists of plants of the same form ; but when these are bred from, a redistribution of the various characters occurs. That redistribution follows the same definite rule for each character, and if the constitution of the original parents be known, the nature of the F2 generation, i.e. the number of possible forms and the proportions in which they 24 MENDELISM CHAP. occur, can be readily calculated. Moreover, as Mendel showed, we can calculate also the chances of any given form breeding true. To this point, however, we shall return later. Of Mendel's experiments with beans it is sufficient to say here that they corroborated his more ample work with peas. He is also known to have made experiments with many other plants, and a few of his results are incidentally given in his series of letters to Nageli the botanist. To the breeding and cross- ing of bees he also devoted much time and attention, but unhappily the record of these experiments appears to have been lost. The only other published work that we possess dealing with heredity is a brief paper on some crossing experiments with the Hawk- weeds (Hieradum\ a genus that he chose for working with because of the enormous number of forms under which it naturally exists. By crossing together the more distinct varieties, he evidently hoped to pro- duce some of these numerous wild forms, and so throw light upon their origin and nature. In this hope he was disappointed. Owing in part to the great technical difficulties attending the cross-fer- tilisation of these flowers he succeeded in obtaining very few hybrids. Moreover, the behaviour of those which he did obtain was quite contrary to what he had found in the peas. Instead of giving a variety of forms in the F2 generation, they bred true and continued to do so as long as they were kept under observation. More recent research has shown that this is due to a peculiar form of parthenogenesis (cf. p. 123), and not to any failure of the characters to separate clearly from one another in the gametes. in MENDEL'S WORK 25 Mendel, however, could not have known of this, and his inability to discover in Hieradum any indication of the rule which he had found to hold good for both peas and beans must have been a source of considerable disappointment. Whether for this reason, or owing to the utter neglect of his work by the scientific world, Mendel gave up his experimental researches during the latter part of his life. His closing years were shadowed with ill -health and embittered by a controversy with the Government on a question of the rights of his monastery. He died of Bright's disease in 1884. Note. — Shortly after the rediscovery of Mendel's paper a need was felt for terms of a general nature to express the constitution of individuals in respect of inherited characters, and Bateson accordingly proposed the words homozygote and heterozygote. An individual is said to be homo- zygous for a given character when it has been formed by two gametes each bearing the character, and all the gametes of a homozygote bear the character in respect of which it is homozygous. When, however, the zygote is formed by two gametes of which one bears the given character while the other does not, it is said to be heterozygous for the character in question, and only half the gametes produced by such a heterozygote bear the character. An individual may be homozygous for one or more characters, and at the same time may be heterozygous for others. CHAPTER IV THE PRESENCE AND ABSENCE THEORY IT was fortunate for the development of biological science that the rediscovery of Mendel's work found a small group of biologists deeply interested in the problems of heredity, and themselves engaged in experimental breeding. To these men the extra- ordinary significance of the discovery was at once apparent. From their experiments, undertaken in ignorance of Mendel's paper, de Vries, Correns, and Tschermak were able to confirm his results in peas and other plants, while Bateson was the first to demonstrate their application to animals. Thence- forward the record has been one of steady progress, and the result of ten years' work has been to establish more and more firmly the fundamental nature of Mendel's discovery. The scheme of in- heritance, which he was the first to enunciate, has been found to hold good for such diverse things as height, hairiness, and flower colour and flower form in plants, the shape of pollen grains, and the structure of fruits ; while among animals the coat colour of mammals, the form of the feathers and of the comb in poultry, the waltzing habit of Japanese 26 CH. iv PRESENCE & ABSENCE THEORY 27 mice, and eye colour in man are but a few examples of the diversity of characters which all follow the same law of transmission. And as time went on many cases which at first seemed to fall without the scheme have been gradually brought into line in the light of fuller knowledge. Some of these will be FIG. 2. A wing feather and a contour feather of an ordinary and a silky fowl. The peculiar ragged appearance of the silky feathers is due to the absence of the little hooks or barbules which hold the barbs together. The silky condition is recessive. dealt with in the succeeding chapters of this book. Meanwhile we may concern ourselves with the single modification of Mendel's original views which has arisen out of more ample knowledge. As we have already seen, Mendel considered that in the gamete there was either a definite something corresponding to the dominant character or a definite something corresponding to the recessive character, 28 MENDELISM CHAP. and that these somethings whatever they were could not coexist in any single gamete. For these somethings we shall in future use the term factor. The factor, then, is what corresponds in the gamete to the unit- character that appears in some shape or other in the development of the zygote. Tallness in the pea is a unit-character, and the gametes in which it is FIG. 3. Two double and an ordinary single primula flower. This form of double is recessive to the single. represented are said to contain the factor for tallness. Beyond their existence in the gamete and their mode of transmission we make no suggestion as to the nature of these factors. On Mendel's view there was a factor correspond- ing to the dominant character and another factor corresponding to the recessive character of each alternative pair of unit-characters, and the characters were alternative because no gamete could carry more iv PRESENCE & ABSENCE THEORY 29 than one of the two factors belonging to the alter- native pair. On the other hand, Mendel supposed that it always carried either one or the other of such a pair. As experimental work proceeded, it soon became clear that there were cases which could not be expressed in terms of this conception. The nature of the difficulty and the way in which it was met will perhaps be best understood by considering a set of experiments in which it occurred. Many of the different breeds of poultry are characterised by a particular form of comb, and in certain cases the inheritance of these has been carefully worked out. It was shown that the rose comb (Fig. 4, B) with its flattened papillated upper surface and back- wardly projecting pike was dominant in the ordinary way to the deeply serrated high single comb (Fig. 4, C) which is characteristic of the Mediterranean races. Experiment also showed that the pea comb (Fig. 4, A), a form with a low central and two well-developed lateral ridges such as is found in Indian game, behaves as a simple dominant to the single comb. The inter- esting question arose as to what would happen when the rose and the pea, two forms each dominant to the same third form, were mated together. It seemed reasonable to suppose that things which were alter- native to the same thing would be alternative to one another — that either rose or pea would dominate in the hybrids, and that the F2 generation would consist of dominants and recessives in the ratio 3:1. The result of the experiment was, however, very different. The cross rose x pea led to the production of a comb quite unlike either of them. This, the so-called walnut comb (Fig. 4, D), from its resemblance to MENDELISM the half of a walnut, is a type of comb which is normally characteristic of the Malay fowl. Moreover, when these F: birds were bred together, .a further unlooked-for result was obtained. As was expected, B FIG. 4. Fowls' combs. A, pea ; B, rose ; C, single ; D, walnut. there appeared in the F2 generation the three forms walnut, rose, and pea. But there also appeared a definite proportion of single combed birds, and among many hundreds of chickens bred in this way the proportions in which the four forms walnut, rose, pea, and single appeared was 9:3:3: I. Now this, iv PRESENCE & ABSENCE THEORY 31 as Mendel showed, is the ratio found in an F2 generation when the original parents differ in two pairs of alternative characters, and from the propor- tions in which the different forms of comb occur we must infer that the . , . , ,, Rose X Pea walnut contains both i dominants, the rose and ' ' the pea one dominant Walnut each, while the single is pure for both reces- .„ [~ T1 „,. . Walnut Rose Pea Single sive characters. Ihis ^ ^ ^ ^ accorded with subse- quent breeding experiments, for the singles bred perfectly true as soon as they had once made their appearance. So far the case is clear. The difficulty comes when we attempt to define these two pairs of characters. How are we to express the fact that while single behaves as a simple recessive to either pure rose or to pure pea, it can yet appear in F from a cross between these two pure forms, tHougri neither of them should, on. Mendel's view, contain the single ? An explanation which covers the facts in a simple way is that which has been termed the " Presence and Absence " theory. On this theory the dominant character of an alter- native pair owes its dominance to the presence of a factor which is absent in the recessive. The tall pea is tall owing to the presence in it of the factor for tallness, but in the absence of this factor the pea remains a dwarf. All peas are dwarf, but the tall is a dwarf plus a factor which turns it into a tall. Instead of the characters of an alternative pair being due to two separate factors, we now regard them as 32 MENDELISM CHAP. the expression of the only two possible states of a single factor, viz. its presence or its absence. The conception will probably become clearer iC we follow its application in detail to the case of the fowl's combs. In this case we are concerned with the transmission of the two factors, rose (R\ and pea (T3), the presence of each of which is alternative to its absence. The rose-combed bird contains the factor for rose but not that for pea, and the pea -combed bird contains the factor for pea but not that for rose. When both factors are present in a bird, as in the hybrid made by crossing rose with pea, the result is a walnut. For convenience of argument we may denote the presence of a given factor by a capital letter and its absence by the corresponding small letter. The use of the small letter is merely a symbolic way of intimating that a particular factor is absent in a gamete or zygote. Represented thus the zygotic constitution of a pure rose-combed bird is RRpp ; for it has been formed by the union of two gametes both of which contained R but not P. Similarly we may denote the pure pea-combed bird .as rrPP. On crossing the rose with the pea, union occurs between a gamete Rp and a gamete rP, resulting in the formation of a heterozygote of the constitution RrPp. The use of the small letters here informs us that such a zygote contains only a single dose of each of the factors R and P, although, of course, it is possible for a zygote, if made in a suitable way, to have a double dose of any factor. Now when such a bird comes to form gametes a separation takes place between the par.t of the zygotic cell containing R and the part which <3oes iv PRESENCE & ABSENCE THEORY 33 not contain it (r). Half of its gametes, therefore, will contain R and the other half will be without it (r}. Similarly half of its gametes will contain P and the other half will be without it (/>). It is obvious that the chances of R being distributed to a gamete with or without P are equal. Hence the gametes containing R will be of two sorts, PR and Rp, and these will be produced in equal numbers. Similarly the gametes without R will also be of two sorts, rP and rp, and these, again, will be produced in equal numbers. Each of the hybrid walnut- combed birds, therefore, gives rise to a series con- sisting of equal numbers of gametes of the four different types RP, Rp, rP, and rp ; and the breeding together of such Fa birds means the bringing together of two such series of gametes. When this happens an ovum of any one of the four types has an equal chance of being fertilised by a spermatozoon of any one of the four types. A convenient and simple method of demonstrating wha% happens under such circumstances is the method sometimes termed the " chessboard " method. For two series each con- sisting of four different types of gamete we require a square divided up into 16 parts. The four terms of the gametic series are first written horizontally across the four sets of four squares, so that the series is repeated four times. It is then written vertically four times, care being taken to keep to the same order. In this simple mechanical way all the possible combinations are represented and in their proper proportions. Fig. 5 shows the result of applying this method to our series RP, Rp, rP, rp, and the 16 squares represent the different kinds of D 34 MENDELISM CHAP. zygotes formed and the proportions in which they occur. As the figure shows, 9 zygotes contain both R and P, having a double or a single dose of either or both of these factors. Such birds must be all walnut combed. Three out of the 16 zygotes contain R but not P, and these must be rose-combed birds. Three, again, con- tain P but not R, and must be pea- combed birds. Finally one out of the 1 6 contains neither R nor P. It cannot be rose — it cannot be pea. It must, therefore, be some- thing else. As a of RP R£ x RP Rp RP rP , RP rP Walnut Walnut Walnut Walnut Rp RP * Rp Rp Rp rP Rp «P Walnut Rose Walnut Rose rP RP rP Rp rP rP rP >P Walnut Walnut Pea Pea rP RP rp Rp T> rP rp rp Walnut Rose Pea Single FIG. 5. Diagram to illustrate the nature of the F2 generation from the cross of rose comb x pea comb. is single. Why it should be single and not something else follows from what we already know about the behaviour of these various forms of comb. For rose is dominant to single ; therefore on the Presence and Absence theory a rose is a single plus a factor which turns the single into a rose. If we could remove the " rose " factor from a rose-combed bird the underlying single would come into view. Similarly a pea comb is a single plus a factor which turns the single into a pea, and a walnut is a single which possesses two additional modifying factors. Singleness, in fact, underlies all these combs, iv PRESENCE & ABSENCE THEORY 35 and if we write their zygotic constitution in full we must denote a walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as rrppSS. The crossing of rose with pea results in a reshuffling of the factors concerned, and in accordance with the principle of segregation some zygotes are formed in which neither of the modifying factors R and P are present, and the single character can then become manifest. The Presence and Absence theory is to-day generally accepted by students of these matters. Not only does it afford a simple explanation of the remarkable fact that in all cases of Mendelian in- heritance we should be able to express our unit- characters in terms of alternative pairs, but, as we shall have occasion to refer to later, it suggests a clue as to the course by which the various domesti- cated varieties of plants and animals have arisen from their wild prototypes. Before leaving this topic we may draw attention to some experiments which offer a pretty confirma- tion of the view that the rose comb is a single to which a modifying factor for roseness has been added. It was argued that if we could find a type of comb in which the factor for singleness was absent, then on crossing such a comb with a rose we ought, if singleness really underlies rose, to obtain some single combs in F2 from such a cross. Such a comb we had the good fortune to find in the Breda fowl, a breed largely used in Holland. This fowl is usually spoken of as combless for the place of the comb is taken by a covering of short bristle- like feathers (Fig. 6, D). In reality it possesses the MENDELISM CHAP. vestige of a comb in the form of two minute lateral knobs of comb tissue. Characteristic also of this breed is the high development of the horny nostrils, a feature probably correlated with the almost com- FIG. 6. Fowls' combs. A and B, Fj hen from rose X Breda ; C, an Fj cock from the cross of single X Breda ; D, head of Breda cock. plete absence of comb. The first step in the experiment was to prove the absence of the factor for singleness in the Breda. On crossing Breda with single the Fl birds exhibit a large comb of the form of a double single comb in which the two iv PRESENCE & ABSENCE THEORY 37 portions are united anteriorly, but diverge from one another towards the back of the head (Fig. 6, C). The Breda contains an element of duplicity which is dominant to the simplicity of the ordinary single comb. But it cannot contain the factor for the single comb, because as soon as that is put into it by crossing with a single the comb assumes a large size, and is totally distinct in appearance from its almost complete absence in the pure Breda. Now when the Breda is crossed with the rose duplicity is dominant to simplicity, and rose is dominant to Rose X Breda Duplex v Duplex Rose Rose i I Duplex Rose Duplex Single Breda Rose Single (Duplex and Simplex) lack of comb, and the F1 generation consists of birds possessing duplex rose combs (Fig. 6, A and B). On breeding such birds together we obtain a genera- tion consisting of Bredas, duplex roses, roses, duplex singles, and singles. From our previous experiment we know that the singles could not have come from the Breda, since a Breda comb to which the factor for single has been added no longer remains a Breda. Therefore it must have come from the rose, thus confirming our view that the rose is in reality a single comb which contains in addition a dominant modifying factor (A*) whose presence turns it into 38 MENDELISM CHAP, iv a rose. We shall take it, therefore, that there is good experimental evidence for the Presence and Absence theory, and we shall express in terms of it the various cases which come up for discussion in succeeding chapters. CHAPTER V INTERACTION OF FACTORS WE have now reached a point at which it is possible to formulate a definite conception of the living organism. A plant or animal is a living entity whose properties may in large measure be expressed in terms of unit-characters, and it is the possession of a greater or lesser number of such unit-characters renders it possible for us to draw sharp distinctions between one individual and another. These unit- characters are represented by definite factors in the gamete which in the process of heredity behave as indivisible entities, and are distributed according to a definite scheme. The factor for this or that unit- character is either present in the gamete or it is not present. It must be there in its entirety or completely absent. Such at any rate is the view to which recent experiment has led us. But as to the nature of these factors, the conditions under which they exist in the gamete, and the manner in which they produce their specific effects in the zygote, we are at present almost completely in the dark. The case of the fowls' combs opens up the im- portant question of the extent to which the various factors can influence one another in the zygote. 39 40 MENDELISM CHAP. The rose and the pea factors are separate entities, and each when present alone produces a perfectly distinct and characteristic effect upon the single comb, turning it into a rose or a pea as the case may be. But when both are present in the same zygote their combined effect is to produce the walnut comb, a comb which is quite distinct from either and in no sense intermediate between them. The question of the influence of factors upon one another did not present itself to Mendel because he worked with characters which affected different parts of the plant. It was unlikely that the factor which led to the production of colour in the flower would affect the shape of the pod, or that the height of the plant would be influenced by the presence or absence of the factor that determined the shape of the ripe seed. But when several factors can modify the same structure it is reasonable to suppose that they will influence one another in the effects which their simultaneous presence has upon the zygote. By themselves the pea and the rose factors each produce a definite modification of the single comb, but when both are present in the zygote, whether as a single or double dose, the modification that results is quite different to that produced by either when present alone. Thus we are led to the conception of characters which depend for their manifestation on more than one factor in the zygote, and in the present chapter we may consider a few of the phenomena which result from such interaction be- tween separate and distinct factors. One of the most interesting and 'instructive cases in which the interaction between separate factors has v INTERACTION OF FACTORS 41 been demonstrated is a case in the sweet- pea. All [white sweet- peas breed true to whiteness. And generally speaking the result of crossing different whites is to produce nothing but whites whether in FX or in succeeding generations. But there are certain strains of white sweet -peas which when crossed together produce only coloured flowers. The colour may be different in different cases, though for our present purpose we may take a case in which the colour is red. When such reds are allowed to self- fertilise themselves in the normal way and the seeds sown, the resulting F2 generation consists of reds White * White and whites, the former being R ' _ p rather more numerous than i the latter in the proportion „•"" IT"1. c -ru • • r Red White-— F2 of 9 : 7. The raising of a ^ ^ further generation from the seeds of these F2 plants shows that the whites always breed true to whiteness, but that different reds may behave differently. Some breed true, others give reds and whites in the ratio 3:1, while others, again, give reds and whites in the ratio 9:7. As in the case of the fowls' combs, this case may be interpreted in terms of the presence and absence of two factors. Red in the sweet-pea results from the interaction of two factors, and unless these are both present the red colour cannot appear. Each of the white parents carried one of the two factors whose inter- action is necessary for the production of the red colour, and as a cross between them brings these two complementary factors together the Fx plants must all be red. As this case is of considerable 42 MENDELISM CHAP. importance for the proper understanding of much that is to follow, and as it has been completely worked out, we shall consider it in some detail. Denoting these two colour factors by A and B respectively we may proceed to follow out the consequences of this cross. Since all the Fa plants were red the constitution of the parental whites must have been A Abb and aaBB respectively, and their gametes White Wgg consequently Ab /\ y\ and aB. The / \ / \ constitution of ' .. D r> eametes Ab Ab aB «•# nf narpnts fV,^ F _ , must, therefore, Red r-i be AaBb. Such AaBb a plant being heterozygous for \ £Ab Ab u7 two factors pro- p's a-? a-f « ° duces a series of ab ab "3 S % gametes of the four kinds AB, Ab, aB, ab, and produces them in equal numbers (cf. p. 33). To obtain the various types of zygotes which are produced when such a series of pollen grains meets a similar series of ovules we may make use of the same " chessboard " system which we have already adopted in the case of the fowls' combs. An examination of this figure (Fig. 7) shows that 9 out of the 16 squares contain both A and B, while 7 contain either A or B alone, or neither. In other words, on this view of the nature of the two white sweet -peas we should in the F2 generation look for the appearance of coloured and INTERACTION OF FACTORS 43 white flowers in the ratio 9 : 7. And this, as we have already seen, is what was actually found by experiment. Further examination of the figure shows that the coloured plants are not all of the same constitution, but are of four kinds with respect to their zygotic constitution, viz. A ABB, *AABb, AaBB, and AaBb. Since AABB is homozygous for both A and B, all the gametes which it produces must contain both of these factors, and such a plant must therefore breed true to the red colour. A plant of the constitution AABb is homo- zygous for the factor Ab, Ab ab \ aB aB Ab aB ab rabi 'ab FIG. 7. Diagram to illustrate the nature of the F2 generation but from the two white sweet peas which give a , c coloured Fj. heterozygous tor B. All of its gametes will contain A, but only one- half of them will contain B, i.e. it produces equal numbers of gametes AB and Ab. Two such series of gametes coming together must give a generation consisting of x AABB, 2x AABb, and x AAbb, that is, reds and whites in the ratio 3:1. Lastly the red zygotes of the constitution AaBb have the same constitution as the original red made from the two whites, and must therefore when bred from give reds and whites in the ratio 9 : 7. The existence 44 MENDELISM CHAP. of all these three sorts of reds was demonstrated by experiment, and the proportions in which they were met with tallied with the theoretical explanation. The theory was further tested by an examination into the properties of the various F2 whites which come from a coloured plant that has itself been produced by the mating of two whites. As Fig. 7 shows, these are, in respect of their constitu- tion, of five different kinds, viz. AAbb, Aabb, aaBB, aaBb, and aabb. Since none of them produce any- thing but whites on self-fertilisation it was found necessary to test their properties in another way, and the method adopted was that of crossing them together. It is obvious that when this is done we should expect different results in different cases. Thus the cross between two whites of the constitution AAbb and aaBB should give nothing but coloured plants ; for these two whites are of the same con- stitution as the original two whites from which the experiment started. On the other hand, the cross between a white of the constitution aabb and any other white can never give anything but whites. For no white contains both A and B, or it would not be white, and a plant of the constitution aabb cannot supply the complementary factor necessary for the production of colour. Again, two whites of the constitution Adbb and aaBb when crossed should give both coloured and white flowers, the latter being three times as numerous as the former. Without going into further detail it may be stated that the results of a long series of crosses between the various F2 whites accorded closely with the theoretical explanation. v INTERACTION OF FACTORS 45 From the evidence afforded by this exhaustive set of experiments it is impossible to resist the deduction that the appearance of colour in the sweet- pea depends upon the interaction of two factors which are independently transmitted according to the ordinary scheme of Mendelian inheritance. What these factors are is still an open question. Recent evidence of a chemical nature indicates that colour in a flower is due to the interaction of two definitive substances : (i) a colourless " chromogen," or colour basis ; and (2) a ferment which behaves as an activator of the chromogen, and by inducing1 some process of oxidation, leads to the formation of a coloured substance. But whether these two bodies exist as such in the gametes, or whether in some other form we have as yet no means of deciding. Since the elucidation of the nature of colour in the sweet -pea phenomena of a similar kind have been witnessed in other plants, notably in stocks, snapdragons, and orchids. Nor is this class of phenomena confined to plants. In the course of a series of experiments upon the plumage colour of poultry, indications were obtained that different white breeds did not always owe their whiteness to the same cause. Crosses were accordingly made between the white Silky fowl and a pure white strain derived from the white Dorking. Each of these had been previously shown to behave as a simple recessive to colour. When the two were crossed only fully coloured birds resulted. From analogy with the case of the sweet -pea it was anticipated that such ¥l coloured birds when bred together would produce an F2 generation consisting 46 MENDELISM CHAP. of coloured and white birds in the ratio 9 : 7, and when the experiment was made this was actually shown to be the case. With the growth of know- ledge it is probable that further striking parallels of this nature between the plant and animal worlds will be met with. Before quitting the subject of these experiments, attention may be drawn to the fact that the 9 : 7 ratio is in reality a 9:3:3:1 ratio in which the last three terms are indistinguishable owing to the special circumstances that neither factor can produce a visible effect without the co-operation of the other. And we may further emphasise the fact that although the two factors thus interact upon one another they are nevertheless transmitted quite independently and in accordance with the ordinary Mendelian scheme. One of the earliest sets of experiments demon- strating the interaction of separate factors was that made by the French Agouti X Albino zoologist Cuenot on the ! 1 1 coat colours of mice. It Agouti X Agouti was shown that in cer- tain cases agouti, which I I , [71 is the colour of the Agouti Black Albino (9) (3) (4? ordinary wild grey mouse, behaves as a dominant to the albino variety, i.e. the F0 generation from such a cross consists of agoutis and albinos in the ratio 3:1. But in other cases the cross between albino and agouti gave a different result. In the Fl generation appeared only agoutis as before, but the F2 generation consisted of three distinct types, viz. INTERACTION OF FACTORS 47 agoutis, albinos, and blacks. Whence the sudden appearance of the new type ? The answer is a simple one. -The albino parent was really a black. But it lacked the factor without which the colour is unable to develop, and consequently it remained an albino. If we denote this factor by C, then the constitution of an albino must be cc, while that of a coloured animal may be CC or Cc, according as to whether it breeds true to colour or can throw albinos. Agouti was previously known to be a simple dominant to black, i.e. an agouti is a black rabbit plus an additional greying factor which modifies the black into agouti. This factor we will denote by G, and we will use B for the black factor. Our original agouti and albino parents we may therefore regard as in constitution GGCCBB and ggccBB respectively. Both of the parents are homozygous for black. The gametes produced by the two parents are GCB and gcB, and the constitution of the Fj animals must be GgCcBB. Being heterozygous for two factors they will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB, and gcB. The results of the mating of two such similar series of gametes when the Fr animals are bred together we may determine by the usual " chessboard " method (Fig. 8). Out of the 1 6 squares 9 contain both j£and G in addition to 0 B. Such animals must be agoutis. Three squares contain C but not G. Such animals must be coloured, but as they do not contain the modifying agouti factor their colour will be black. The remain- ing four squares do not contain C, and in the absence of this colour-developing factor they must all be albinos. Theory demands that the three classes 48 MENDELISM agouti, black, and albino should appear in F2 in the ratio 9:3:4; experiment has shown that these are the only classes that appear, and that the proportions in which they are produced accord closely with the . . . theoretical ex- pectation. Put briefly, then, the explanation of this case is that all the animals are black, and that we are dealing with the presence and absence of two factors, a colour developer (C), and a colour modifier (G), both acting, as it were, Diagram to illustrate the nature of the F2 generation which may arise from the mating of agouti with Upon 3 Substratum albino in mice or rabbits. of black. The F2 generation really consists of the four classes agoutis, blacks, albino agoutis, and albino blacks in the ratio 9:3:3: i . But since in the absence of the colour developer C the colour modifier G can produce no visible result, the last two classes of the ratio are indistinguishable, and our F2 generation comes to consist of three classes in the ratio 9:3:4, instead of four classes in the ratio 9:3:3:1. This explanation was further tested by experi- ments with the albinos. In an F0 family of this nature there ought to be three kinds, viz. albinos homozygous for G (GGccBB\ albinos heterozygous v INTERACTION OF FACTORS 49 for G (GgccBB\ and albinos without G (ggccBB}. These albinos are, as it were, like photographic plates, exposed but undeveloped. Their potentialities may be quite different, although they all look alike, but this can only be tested by treating them with a colour developer. In the case of the mice and rabbits the potentiality for which we wish to test is the presence or absence of the factor G, and in order to develop the colour we must introduce the factor C. Our developer, therefore, must contain C but not G. In other words, it must be a homozygous black mouse or rabbit, gg CCBB. Since such an animal is pure for C it must, when mated with any of the albinos, produce only coloured offspring. And since it does not contain G the appearance of agoutis among its offspring must be attributed to the presence of G in the albino. Tested in this way the F2 albinos were proved, as was expected, to be of three kinds : (i) those which gave only agouti, i.e. which were homozygous for G ; (2) those which gave agoutis and blacks in approximately equal numbers, i.e. which were heterozygous for G ; and (3) those which gave only blacks, and therefore did not contain G. Though albinos, whether mice, rabbits, rats, or other animals, breed true to albinism, and though albinism behaves as a simple recessive to colour, yet albinos may be of many different sorts. There are in fact just as many kinds of albinos as there are coloured forms — neither more nor less. And all these different kinds of albinos may breed together, transmitting the various colour factors according to the Mendelian scheme of inheritance, and yet the visible result will be nothing but albinos. Under E 50 MENDELISM the mask of albinism is all the wWtc occurring that segregation of the different colour factors which would result in all the varieties of coloured forms, if only the essential factor for colour development were present. But put in the developer by crossing with a pure coloured form and their variety of con- stitution can then at last become manifest. So far we have dealt with cases in which the production of a character is dependent upon the interaction of two factors. But it may be that some characters require the simultaneous presence of a greater number of factors for their manifestation, and the experiments of Miss Saunders have shown that there is a character in stocks which is unable to appear except through the interaction of three distinct factors. Coloured stocks may be either hoary with the leaves and stem covered by small hairs, or they may lack the hairy covering, in which case they are termed glabrous. Hoariness is dominant to glabrousness ; that is to say, there is a definite factor which can turn the glabrous into a hoary plant when it is present. But in families where coloured and white stocks occur the white are always glabrous, while the coloured plants may or may not be hoary. Now colour in the stock as in the sweet -pea has been proved to be dependent upon the interaction of two separate factors. Hence hoariness depends upon three separate factors, and a stock cannot be hoary unless it contains the hoary factor in addition to the two colour factors. It requires the presence of all these three factors to produce the hoary character, though how this comes about we have not at present the least idea. v INTERACTION OF FACTORS 51 A somewhat different and less usual Jform of inter- action between factors may be illustrated by a case in primulas recently worked out by Bateson and Gregory. Like the common primrose, the primula exhibits both pin-eyed and thrum-eyed varieties. In the former the style is long, and the centre of the eye is formed by the end of the stigma which more or less plugs up the opening of the corolla (cf. Fig. 9, A) ; in the latter the style is short and hidden by ABC FIG. 9. Sections of primula flowers. The anthers are shown as black. A, " pin " form with long style and anthers set low down; B, "thrum" form with short style and anthers set higher up ; C, homostyle form with anthers set low down as in " pin," but with short style. This form only occurs with the large eye. the four anthers which spring from higher up in the corolla and form the centre of the eye (cf. Fig. 9, B). The greater part of the " eye " is formed by the greenish -yellow patches on each petal just at the opening of the corolla. In most primulas the eye is small, but there are some in which it is large and extends as a flush over a considerable part of the petals (Fig. 10). Experiments showed that these two pairs of characters behave in simple Men- delian fashion, short style ( = "thrum") being dominant to long style ( = " pin ") and small eye dominant to 52 MENDELISM CHAP. large. Besides the normal long and short styled forms, there occurs a third form, which has been termed homostyle. In this form the anthers are placed low down in the corolla tube as they are in the long-styled form, but the style remains short instead of reaching up to the corolla opening (Fig. 9, C). In the course of their experiments Bateson FIG. 10. Two primula flowers showing the extent of the small and or the large eye. and Gregory crossed a large-eyed homostyle plant with a small-eyed thrum ( = short style). The FI plants were all short styled with small eyes. On self-fertilisation these gave an F2 generation consist- ing of four types, viz. short styled with small eyes, short styled with large eyes, long styled with small eyes, and homostyled with large eyes. The notable feature of this generation is the appearance of long- styled plants, which, however, occur only in associa- tion with the small eye. The proportions in which these four types appeared shows that the presence or absence of but two factors is concerned, and at v INTERACTION OF FACTORS 53 the same time provides the key to the nature of the homostyled plants. These are potentially long styled, and the position of the anthers is that of normal long- styled plants, but owing to some interaction between the factors the style itself is unable to reach its full development unless the factor for the small eye is present. For this reason long -styled plants with Short style \ / Homo style small eye / \ large eye Short style small eye Short style Short style Long style Homo style small eye large eye ("pin") large eye (9) (3) (3) (i) the large eye are always of the homostyle form. What the connecting-link between these apparently unrelated structures may be we cannot yet picture to ourselves, any more than we can picture the relation between flower colour and hairiness in stocks. It is evident, however, that the conception of the inter- action of factors, besides clearing up much that is paradoxical in heredity, promises to indicate lines of research which may lead to valuable extensions in our knowledge of the way in which the various parts of the living organism are related to one another. CHAPTER VI REVERSION As soon as the idea was grasped that characters in plants and animals might be due to the interaction of complementary factors, it became evident that this threw clear light upon the hitherto puzzling pheno- menon of reversion. We have already seen that in certain cases the cross between a black mouse or rabbit and an albino, each belonging to true breeding strains, might produce nothing but agoutis. In other words, the cross between the black and the white in Certain instances results in a complete reversion to the wild grey form. Expressed in Mendelian terms, the production of the agouti was the necessary conse- quence of the meeting of the factors C and G in the same zygote. As soon as they are brought together, no matter in what way, the reversion is bound to occur. Reversion, therefore, in such cases we may regard as the bringing together of complementary factors which had somehow in the course of evolution become separated from one another. In the simplest cases, such as that of the black and the white rabbit, only two factors are concerned, and one of them is brought in from each of the two parents. But in 54 PLATE 1. o S 2 £ 3 H CHAP, vi REVERSION 55 other cases the nature of the reversion may be more complicated owing to a larger number of factors being concerned, though the general principle remains the same. Careful breeding from the reversions will enable us in each case to determine the number and nature of the factors concerned, and in illustration of this we may take another example from rabbits. The Himalayan rabbit is a well-known breed. In appearance it is a white rabbit with pink eyes, but the ears, paws, and nose are black (PI. I., 2). The Dutch rabbit is another well-known breed. Generally speaking, the anterior portion of the body is white, and the posterior part coloured. Anteriorly, how- ever, the eyes are surrounded by coloured patches extending up to the ears, which are entirely coloured. At the same time the hind paws are white (cf. PI. I., i). Dutch rabbits exist in many varieties of colour, though in each one of these the distribution of colour and white shows the same relations. In the experiments about to be described a yellow Dutch rabbit was crossed with a Himalaya. The result was a reversion to the wild agouti colour (PI. I., 3). Some of the F^ individuals showed white patches, while others were self-coloured. On breeding from the Fl animals a series of coloured forms resulted in F^ These were agoutis, blacks, yellows, and sooty yellows, the so-called tortoise- shells of the fancy (PL I., 4-7). In addition to these appeared Himalayans with either black points or with lighter brownish ones, and the proportions in which they came showed the Himalayan character to be a simple recessive. A certain number of the coloured forms exhibited the Dutch marking to a 56 MENDELISM CHAP. greater or less extent, but as its inheritance in this set of experiments is complicated and has not yet been worked out, we may for the present neglect it and confine our attention to the coloured types and to the Himalayans. The proportion in which the four coloured types appeared in F2 was very nearly 9 agoutis, 3 blacks, 3 yellows, and i tortoiseshell. Evidently we are here dealing with two factors: (i) the grey factor (G\ which modifies black into agouti, or tortoiseshell into yellow ; and (2) an intensifying factor (/), which intensifies yellow into agouti and Yellow X Himalayan Agouti X Agouti r~ ~r~ Agouti Yellow Black Tortoise Himalayan Shell (27) (9) (9) (3) (16) tortoiseshell into black. It may be mentioned here that other experiments confirmed the view that the yellow rabbit is a dilute agouti, and the tortoiseshell a dilute black. The Himalayan pattern behaves as a recessive to self-colour. It is a self-coloured black rabbit lacking a factor that allows the colour to develop except in the points. That factor we may denote by X, and as far as it is concerned the Himalayan is constitutionally xx. The Himalayan contains the intensifying factor, for such pigment as it possesses in the points is full coloured. At the same time it is black, i.e. lacking in the factor G. With regard to these three factors, therefore, the con- stitution of the Himalayan hggllxx. The last char- vi REVERSION 57 acter which we have to consider in this cross is the Dutch character. This was found by Hurst to behave as a recessive to self-colour (S), and for our present purpose we will regard it as differing from a self-coloured rabbit in the lack of this factor.1 The Himalayan is really a self-coloured animal, which, however, is unable to show itself as a full black owing to its not possessing the factor X. The results of breeding experiments then suggest that we may denote the Himalayan by the formula ggllxxSS and the yellow Dutch by GGiiXXss. Each lacks two of the factors upon the full complement of which the agouti colour depends. By crossing them the com- plete series GIXS is brought into the same zygote, and the result is a reversion to the colour of the wild rabbit. Most of the instances of reversion yet worked out are those in which colour characters are concerned. The sweet -pea, however, supplies us with a good example of reversion in structural characters. A dwarf variety known as the " Cupid " has been exten- sively grown for some years. In these little plants the internodes are very short and the stems are few in number, and attain to a length of only 9-10 inches. In course of growth they diverge from one another, and come to lie prostrate on the ground (PL II., 2). Curiously enough, although the whole plant is dwarfed in other respects, this does not seem to affect the size of the flower, which is that of a normal sweet-pea. Another though less-known variety is the " Bush " sweet-pea. Its name is derived from 1 Hurst's original cross was between a Belgian hare and an albino Angora which turned out to be a masked Dutch. 58 MENDELISM CHAP. its habit of growth. The numerous stems do not diverge from one another, but all grow up side by side giving the plant the appearance of a compact bush (PI. II., i). Under ordinary conditions it attains a height of 3^-4 feet. A number of crosses were made between the Bush and Cupid varieties, with the somewhat unexpected result that in every instance the F plants showed complete reversion to the size and habit of the ordinary tall sweet- pea (PI. II., 3), which is the form of the wild plant as it occurs in Sicily to-day. The F2 generation from Bush X Cupid Tall FT Tall Bush Cupid Cupid F2 (procumbent) (erect) (3) (3) (i) these reversionary tails consisted of four different types, viz. tails, bushes, Cupids of the procumbent type like the original Cupid parent, and Cupids with the compact upright Bush habit (PI. II., 4). These four types appeared in the ratio 9:3:3: I, and this, of course, provided the clue to the nature of the case. The characters concerned are (i) long internode of stem between the leaves which is dominant to short internode, and (2) the creeping procumbent habit which is dominant to the erect bush-like habit. Of these characters length of inter- node was carried by the Bush, and the procumbent habit by the original Cupid parent. The bringing of them together by the cross resulted in a pro- PLATE II. i, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, F, reversionary Tall; 4. Erect Cupid Sweet7 Pea ; 5, Purple Invincible; 6. Painted Lady; •j, Duke of Westminster (hooded standard). vi REVERSION 59 cumbent plant with long internodes. This is the ordinary tall sweet-pea of the wild Sicilian type, reversion here, again, being due to the bringing together of two complementary factors which had somehow become separated in the course of evolution. To this interpretation it may be objected that the ordinary sweet-pea is a plant of upright habit. This, however, is not true. It only appears so because the conventional way of growing it is to train it up sticks. In reality it is of procumbent habit, with divergent stems like the ordinary Cupid, a fact which can easily be observed by any one who will watch them grow without the artificial aid of prepared supports. The cases of reversion with which we have so far dealt have been cases in which the reversion occurs as an immediate result of a cross, i.e. in the F: generation. This is perhaps the commonest mode of reversion, but instances are known in which the reversion that occurs when two pure types are crossed does not appear until the F2 generation. Such a case we have already met with in the fowls' combs. It will be remembered that the cross between pure pea and pure rose gave walnut combs in Fp while in the F2 generation a definite proportion, I in 16, of single combs appeared (cf. p. 30). Now the single comb is the form that is found in the wild jungle fowl, which is generally regarded as the ancestor of the domestic breeds. If this is so, we have a case of reversion in F2 ; and this in the absence of the two factors brought together by the rose-comb and pea-comb parents. Instead of the reversion being due to the 60 MENDELISM CHAP. bringing together of two complementary factors, we must regard it here as due to the association of two complementary absences. To this question, how- ever, we shall revert later in discussing the origin of domesticated varieties. There is one other instance of reversion to which Black Barb x White Fantail Black Barb x Spot l I I Dark x Dark I Among the offspring one very similar to the wild blue rock. we must allude. This is Darwin's famous case of the occasional appearance of pigeons reverting to the wild blue rock (Columba livid} when certain domesticated races are crossed together. As is well known, Darwin made use of this as an Black v White Barb , Fantail Black v Black (White Splashed) (White Splashed) Black Black Blue Blue White (White Splashed) (White Splashed) ~~5T~ ~~5T~ (4) argument for regarding all the domesticated varieties as having arisen from the same wild species. The original experiment is somewhat complicated, and is shown in the accompanying scheme. Essentially 1 This is an almost white bird, the colour being confined to the tail and the characteristic spot on the head. REVERSION 61 it lay in following the results flowing from crosses between blacks and whites. Experiments recently made by Staples-Browne have shown that this case of reversion also can be readily interpreted in Mendelian terms. In these experiments the cross was made between black barbs and white fantails. The Fl birds were all black with some white splashes, evidently due to a separate factor intro- duced by the fan- tail. On breeding these blacks they gether to- gave an F2 generation, consisting of blacks (with or without white splashes), blues (with or without white splashes), and whites in the ratio 9:3:4. The factors concerned are colour (C), in the absence of which a bird is white, and a black modifier (B\ in the absence of which a coloured bird is blue. The original black barb contained both of these factors, being in constitution CCBB. The fantail, however, contained neither, and was con- stitutionally abb. The F: birds produced by crossing were in constitution CcBb, and being heterozygous for two factors produced in equal numbers the four sorts of gametes CB, Cb, cB, cb. FIG. ii. Diagram to illustrate the appearance of the rever- sionary blue pigeon in F2 from the cross of black with white. 62 MENDELISM CHAP, vi The results of two such series of gametes being brought together are shown in the usual way in Fig. II. A blue is a bird containing the colour factor but lacking the black modifier, i.e. of the constitution CCbb, or Ccbb, and such birds as the figure shows appear in the F, generation on the average three times out of sixteen. Reversion here comes about in F2, when the redistribution of the factors leads to the formation of zygotes containing one of the two factors but not the other. CHAPTER VII DOMINANCE IN the cases which we have hitherto considered the presence of a factor produces its full effect whether it is introduced by both of the gametes which go to form the zygote, or by one of them alone. The heterozygous tall pea, or the heterozygous rose- combed fowl cannot be distinguished from the homozygous form by mere inspection, however close. Breeding tests alone can decide which is the heterozygous and which the homozygous form. Though this is true for the majority of characters yet investigated, there are cases known in which the heterozygous form differs in appearance from either parent. Among plants such a case has been met with in the primula. The ordinary Chinese primula (P. sinensis} (Fig. I 2) has large rather wavy petals much crenated at the edges. In the Star Primula (P. stellata] the flowers are much smaller, while the petals are flat and present only a terminal notch instead of the numerous crenations of P. sinensis. The heterozygote produced by crossing these forms is intermediate in size and appearance. When self- fertilised such plants behave in simple Mendelian 63 64 MENDELISM CHAP. fashion, giving a generation consisting of sinensis, intermediates, and stellata in the ratio 1:2:1. Subsequent breeding from these plants showed that both the sinensis and stellata which appeared in the Fz generation bred true, while the intermediates FIG. 12. Primula flowers to illustrate the intermediate nature of the Fj flower when sinensis is crossed with stellata. always gave all three forms again in the same proportion. But though there is no dominance of the character of either parent in such a case as this, the Mendelian principle of segregation could hardly have a better illustration. Among birds a case of similar nature is that of the Blue Andalusian fowl. Fanciers have long vii DOMINANCE 65 recognised the difficulty of getting this variety to breed true. Of a slaty blue colour itself with darker hackles and with black lacing on the Sinensis X Stellata Intermediate p I I I 1 Sinensis Inter. Inter. Stellata F2 Sinensis sin. Int. Int. stell. Stellata F3 Sinensis Stellata- - - -F4 feathers of the breast, it always throws " wasters " of two kinds, viz. blacks, and whites splashed with black. Careful breeding from the blues shows that the three sorts are always produced in the same Blue X Blue Black Blue X Blue White Black Black Blue Blue White White Black X White Blue (all) definite proportions, viz. one black, two blues, one splashed white. This at once suggests that the black and the splashed white are the two homozy- gous forms, and that the blues are heterozygous, i.e. F 66 MENDELISM CHAP. producing equal numbers of " black " and " white splashed " gametes. The view was tested by breed- ing the " wasters " together — black with black, and splashed white with splashed white — and it was found that each bred true to its respective type. But when the black and the splashed white were crossed they gave, as was expected, nothing but blues. In other words, we have the seeming paradox of the black and the splashed white producing twice as many blues as do the blues when bred together. The black and the splashed white " wasters " are in reality the pure breeds, while the " pure " Blue Andalusian is a mongrel which no amount of selec- tion will ever be able to fix. In such cases as this it is obvious that we cannot speak of dominance. And with the disappearance of this phenomenon we lose one criterion for deter- mining which of the two parent forms possesses the additional factor. Are we, for example, to regard the black Andalusian as a splashed white to which has been added a double dose of a colour-intensifying factor, or are we to consider the white splashed bird as a black which is unable to show its true pigmentation owing to the possession of some inhibiting factor which prevents the manifestation of the black. Either interpretation fits the facts equally well, and until further experiments have been devised and carried out it is not possible to decide which is the correct view. Besides these comparatively rare cases where the heterozygote cannot be said to bear a closer re- semblance to one parent more than to the other, there are cases in which it is often possible to draw vii DOMINANCE 67 a visible distinction between the heterozygote and the pure dominant. There are certain white breeds of poultry, notably the White Leghorn, in which the white behaves as a dominant to colour. But the heterozygous whites made by crossing the dominant white birds with a pure coloured form (such as the Brown Leghorn) almost invariably show a few coloured feathers or " ticks " in their plumage. The dominance of white is not quite complete, and renders it possible to distinguish the pure from the impure dominant without recourse to breeding experiments. This case of the dominant white fowl opens up another interesting problem in connection with dominance. By accepting the Presence and Absence hypothesis we are committed to the view that the dominant form possesses an extra factor as com- pared with the recessive. The natural way of looking at this case of the fowl is to regard white as the absence of colour. But were this so, colour should be dominant to white, which is not the case. We are therefore forced to suppose that the absence of colour in this instance is due to the presence of a factor whose property is to inhibit the production of colour in what would otherwise be a pure coloured bird. On this view the dominant white fowl is a coloured bird plus a factor which inhibits the de- velopment of the colour. The view can be put to the test of experiment. We have already seen that there are other white fowls in which white is reces- sive to colour, and that the whiteness of such birds is due to the fact that they lack a factor for the development of colour. If we denote this factor by C and our postulated inhibitor factor in the dominant 68 MENDELISM CI CI CI Ci CI cl CI ci white bird by 7, then we must write the constitution of the recessive white as ccii, and the dominant white as CCII. We may now work out the results we ought to obtain when a cross is made between these two pure white breeds. The constitution of the F bird must be Ccli. Such birds being heterozygous for the inhibitor factor, should be whites showing some coloured " ticks." Being heterozygous for both of the two factors C and /, they will produce in equal numbers the four different sorts of gametes CI, Ci, cl, ci. The result of bringing two such similar series of gametes together is shown in Fig. 13. Out of the sixteen squares, twelve contain /; these will be white birds either with or without a few i coloured ticks. Three contain C but not / ; these must be coloured birds. One con- tains neither C nor /; this must be a white. From such a mating we ought, therefore, to obtain both white and coloured birds in the ratio 13:3. The results thus theoretically de- duced were found to accord with the actual facts of experiment. The Fa birds were all " ticked " whites, and in the F2 generation came white Ci CI ci CI ci CI Ci Ci ci Ci ci Ci Ci ci ci ci Cl ci cl ci FIG. 13. Diagram to illustrate the nature of the F2 generation from the cross between dominant white and recessive white fowls. VII DOMINANCE 69 and coloured birds in the expected ratio. There seems, therefore, little reason to doubt that the dominant white is a coloured bird in which the absence of colour is due to the action of a colour-inhibiting factor, though as to the nature of that factor we can FIG. 14. Ears of beardless and bearded wheat. The beardless condition is dominant to the bearded. at present make no surmise. It is probable that other facts, which at first sight do not appear to be in agreement with the " Presence and Absence " hypothesis, will eventually be brought into line through the action of inhibitor factors. Such a o * 70 MENDELISM CHAP. case, for instance, is that of bearded and beardless wheats. Though the beard is obviously the addi- tional character, the bearded condition is recessive to the beardless. Probably we ought to regard the beardless as a bearded wheat in which there is an inhibitor that stops the beard from growing. It is not unlikely that as time goes on we shall find many more such cases of the action of inhibitor factors, and we must be prepared to find that the same visible effect may be produced either by the addition or by the omission of a factor. The dominant and recessive white poultry are indistin- guishable in appearance. Yet the one contains a factor more and the other a factor less than the coloured bird. A phenomenon sometimes termed irregularity of dominance has been investigated in a few cases. In certain breeds of poultry such as Dorkings there occurs an extra toe directed backwards like the hallux (cf. Fig. 15). In some families this character behaves as an ordinary dominant to the normal, giving the expected 3 : I ratio in F2. But in other families similarly bred the proportions of birds with and with- out the extra toe appear to be unusual. It has been shown that in such a family some of the birds without the extra toe may nevertheless transmit the peculiarity when mated with birds belonging to strains in which the extra toe never occurs. Though the external appearance of the bird generally affords some indication of the nature of the gametes which it is carrying, this is not always the case. Nevertheless we have reason to suppose that the character segregates in the gametes, though the nature of these cannot vii DOMINANCE 71 always be decided from the appearance of the bird which bears them. There are cases in which an apparent irregularity of dominance has been shown to depend upon another character, as in the experiments with sheep carried out by Professor Wood. In these experi- ments two breeds were crossed, of which one, the Dorset, is horned in both sexes, while the other, the Suffolk, is without horns in either sex. Which- FIG. 15. Fowls' feet. On the right a normal, and on the left one with an extra toe. ever way the cross was made the resulting Fx generation was similar ; the rams were horned, and the ewes were hornless. In the F2 generation raised from these F1 animals both horned and hornless types appeared in both sexes but in very different pro- portions. While the horned rams were about three times as numerous as the hornless, this relation was reversed among the females, in which the horned formed only about one-quarter of the total. The simplest explanation of this interesting case is to MENDELISM CHAP. VII suppose that the dominance of the horned character depends upon the sex of the animal — that it is dominant in the male, but recessive in the female. A pretty experiment was devised for putting this view to the test. If it is true, equal numbers of gametes with and without the horned factor must be produced by the Fj ewes, while the factor should be lacking in all the gametes of the hornless F2 rams. A hornless ram, Dorset Suffolk Ram Ewe .' x 9 Suffolk Dorset Ram Ewe f 0 x 9 (FJ $-xd i — r 9 9 FIG. 16. Scheme to illustrate the inheritance of horns in sheep. Heterozygous males shown dark with a white spot, heterozygous females light with a dark spot in the centre. therefore, put to a flock of FT ewes should give rise to equal numbers of zygotes which are heterozygous for the horned character, and of zygotes in which it is completely absent. And since the heterozygous male's are horned, while the heterozygous females are hornless, we should expect from this mating equal numbers of horned and hornless rams, but only hornless ewes. The result of the experiment con- firmed this expectation. Of the ram lambs 9 were horned and 8 were hornless, while all the i i ewe lambs were completely destitute of horns. CHAPTER VIII WILD FORMS AND DOMESTIC VARIETIES IN discussing the phenomena of reversion we have seen that in most cases such reversion occurs when the two varieties which are crossed each contain certain factors lacking in the other, of which the full complement is necessary for the production of the reversionary wild form. This at once suggests the idea that the various domestic forms of animals and plants have arisen by the omission from time to time of this factor or of that. In some cases we have clear evidence that this is the most natural interpretation of the relation between the cultivated and the wild forms. Probably the species in which it is most evident is the sweet-pea (Lathyrus odoratus]. We have already seen reason to suppose that as regards certain structural features the Bush variety is a wild lacking the factor for the pro- cumbent habit, that the Cupid is a wild without the factor for the long internode, and that the Bush Cupid is a wild minus both these factors. Nor is the evidence less clear for the many colour varieties. In illustration we may consider in more detail a case in which the cross between two whites 73 74 MENDELISM CHAP. resulted in a complete reversion to the purple colour characteristic of the wild Sicilian form (PI. IV.). In this particular instance subsequent breeding from the purples resulted in the production of six different colour forms in addition to whites. The proportion of the coloured forms to the whites was 9 : 7 (cf. p. 41), but it is with the relation of the six coloured forms that we are concerned here. Of these six forms, three were purples and three were reds. The three purple forms were (i) the wild bicolor purple with blue wings known in cultivation as the Purple Invincible (PI. IV., 4) ; (2) a deep purple with purple wings (PI. IV., 5) ; and (3) a very dilute purple known as the Picotee (PI. IV., 6). Cor- responding to these three purple forms were three reds : (1) a bicolor red known as Painted Lady (PL IV., 7) ; (2) a deep red with red wings known as Miss Hunt (PI. IV., 8) ; and (3) a very pale red which we have termed Tinged White1 (PL IV., 9). In the F2 generation the total number of purples bore to the total number of reds the ratio 3:1, and this ratio was maintained for each of the corresponding classes. Purple, therefore, is dominant to red, and each of the three classes of red differs from its corresponding purple in not possessing the blue factor (/?) which turns it into purple. Again, the proportion in which the three classes of purples appeared was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, con- cerned here with the operation of two factors : (i) a light wing factor, which renders the bicolor 1 The reader who searches florists' catalogues for these varieties will probably experience disappointment. The sweet-pea has been much " improved " in the past few years, and it is unlikely that the modern seedsman would list such unfashionable forms. ^» ^^^9 i, 2, Emily Henderson; 3, F^ reversionary Purple; 4-10, Various FL, forms : 4, Purple ; 5, Deep Purple ; 6, Picotee ; 7. Painted Lady ; 8, Miss Hunt; g, Tinged White; 10, White. vin WILD AND DOMESTIC VARIETIES 75 dominant to the dark winged form ; and (2) a factor for intense colour, which occurs in the bicolor and in the deep purple, but is lacking in the dilute picotee. And here it should be mentioned that these con- clusions rest upon an exhaustive set of experiments involving the breeding of many thousands of plants. In this cross, therefore, we are concerned with the presence or absence of five factors, which we may denote as follows : — A colour base, R. A colour developer, C. A purple factor, B. A light wing factor, L A factor for intense colour, I. On this notation our six coloured forms are : — (1) Purple bicolor . . . CRBLL1 (2) Deep purple . . . CRBII. (3) Picotee .... CRBLi or CRBli. (4) Red bicolor ( = Painted Lady) CRbLI. (5) Deep red ( = Miss Hunt) ' . CRbll. (6) Tinged white . . . CRbLi or CRbli. It will be noticed in this series that the various coloured forms can be expressed by the omission of one or more factors from the purple bicolor of the wild type. With the complete omission of each factor a new colour type results, and it is difficult to resist the inference that the various cultivated forms of the sweet -pea have arisen from the wild by some process of this kind. Such a view tallies with what we know of the behaviour of the wild 1 It is to be understood that wherever a given factor is present the plant may be homozygous or heterozygous for it without alteration in its colour. 76 MENDELISM CHAP. form when crossed by any of the garden varieties. Wherever such crossing has been made the form of the hybrid has been that of the wild, thus supporting the view that the wild contains a complete set of all the differentiating factors which are to be found in the sweet-pea. Moreover, this view is in harmony with such historical evidence as is to be gleaned from botanical literature, and from old seedsmen's catalogues. The wild sweet-pea first reached this country in 1699, having been sent from Sicily by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the county of Middlesex. Somewhat later we hear of two new varieties, the red bicolor, or Painted Lady, and the white, each of which may be regarded as having " sported " from the wild purple by the omission of the purple factor, or of one of the two colour factors. In 1793 we find a seedsman offering also what he called black and scarlet varieties. It is probable that these were our deep purple and Miss Hunt varieties, and that somewhere about this time the factor for the light wing (L) was dropped out in certain plants. In 1860 we have evidence that the pale purple or Picotee, and with it doubtless the Tinged White, had come into existence. This time it was the factor for intense colour which had dropped out. And so the story goes on until the present day, and it is now possible to express by the same simple method the relation of the modern shades, of purples and reds, of blues and pinks, of hooded and wavy standards, to one another and to the original wild form. The constitution of many of these has now been worked out, and to-day it vin WILD AND DOMESTIC VARIETIES 77 would be a simple though perhaps tedious task to denote all the different varieties by a series of letters indicating the factors which they contain, instead of by the present system of calling them after kings and queens, and famous generals, and ladies more or less well known. From what we know of the history of the various strains of sweet -peas one thing stands out clearly. The new character does not arise from a pre-existing variety by any process of gradual selection, conscious or otherwise. It turns up suddenly complete in Ji itself, and thereafter it can be associated by crossing * with other existing characters to produce a gamut of new varieties. If, for example, the character of hooding in the standard (cf. PI. II., 7) suddenly turned up in such a family as that shown on Plate IV., we should be able to get a hooded form corre- sponding to each of the forms with the erect stan- dard ; in other words, the arrival of the new form would give us the possibility of fourteen varieties instead of seven. As we know, the hooded char- acter already exists. It is recessive to the erect standard, and we have reason to suppose that it arose as a sudden sport by the omission of the factor in whose presence the standard assumes the erect shape characteristic of the wild flower. It is largely by keeping his eyes open and seizing upon such sports for crossing purposes that the horticulturist " improves " the plants with which he deals. How these sports or mutations come about we can now surmise. They must owe their origin -to a disturbance in the processes of cell division through which the gametes originate. At some stage or 78 MENDELISM CHAP. other the normal equal distribution of the various factors is upset, and some of the gametes receive a factor less than others. From the union of two such gametes, provided that they are still capable of fertilisation, comes the zygote which in course of growth develops the new character. Why these mutations arise : what leads to the surmised unequal division of the gametes : of this we know practically nothing. Nor until we can induce the production of mutations at will are we likely to understand the conditions which govern their formation. Nevertheless there are already hints scattered about the recent literature of experi- mental biology which lead us to hope that we may know more of these matters in the future. In respect of the evolution of its now multi- tudinous varieties, the story of the sweet -pea is clear and straightforward. These have all arisen from the wild by a process of continuous loss. Everything was there in the beginning, and as the wild plant parted with factor after factor there came into being the long series of derived forms. Exquisite as are the results of civilisation, it is by the degrada- tion of the wild that they have been brought about. How far are we justified in regarding this as a picture of the manner in which evolution works ? There are certainly other species in which we must suppose that this is the way that the various domesticated forms have arisen. Such, for example, is the case in the rabbit, where most of the colour varieties are recessive to the wild agouti form. Such also is the case in the rat, where the black and albino varieties and the various pattern forms are also reces- vni WILD AND DOMESTIC VARIETIES 79 sive to the wild agouti type. And with the excep- tion of a certain yellow variety to which we shall refer later, such is also the case with the many fancy varieties of mice. Nevertheless there are other cases in which we must suppose evolution to have proceeded by the interpolation of characters. In discussing reversion on crossing, we have already seen that this may not, occur until the F2 generation, as, for example, in the "' instance of the fowls' combs (cp. p. 59). The rever- sion to the single comb occurred as tnVresult of the removal of the two factors for rose and pea. These two domesticated varieties must be regarded as each possessing an additional factor in comparison with the wild single-combed bird. During the evolution of the fowl, these two factors must be conceived of as having been interpolated in some way. And the same holds good for the inhibitory factor on which, as we have seen, the dominant white character of certain poultry depends. In pigeons, too, if we regard the blue rock as the ancestor of the domesti- cated breeds, we must suppose that an additional melanic factor has arisen at some stage. For we have already seen that black is dominant to blue, and the characters of Fv together with the greater number of blacks than blues in F2, negatives the possibility that we are here dealing with an inhibitory factor. The hornless or polled condition of cattle, again, is dominant to the horned condition, and if, as seems reasonable, we regard the original ancestors of domestic cattle as having been horned, we have here again the interpolation of an inhibitory factor somewhere in the course of evolution. 80 MENDELISM . CHAP, vm On the whole, therefore, we must be prepared to admit that the evolution of domestic varieties may come about by a process of addition of factors in some cases and of subtraction in others. It may be that what we term additional factors fall into distinct categories from the rest. So far, experiment seems to show that they are either of the nature of melanic factors, or of inhibitory factors, or of reduplication factors as in the case of the fowls' combs. But while the data remain so scanty, speculation in these matters is too hazardous to be profitable. CHAPTER IX REPULSION AND COUPLING OF FACTORS ALTHOUGH different factors may act together to produce specific results in the zygote through their interaction, yet in all the cases we have hitherto considered the heredity of each of the different factors is entirely independent. The interaction of the factors affects the characters of the zygote, but makes no difference to the distribution of the separate factors, which is always in strict accordance with the ordinary Mendelian scheme. Each factor in ., this respect behaves as though the other were not present. A few cases have been worked out in which the distribution of the different factors to the gametes is affected by their simultaneous presence in the zygote. And the influence which they are able to exert upon one another in such cases is of two kinds. They may repel one another, refusing, as it were, to enter into the same aygete, or they may attract one another, and, becoming linked together, pass into the same gamete, as it were, by preference. For the moment we may consider these two sets of pheno- mena apart. 81 G 82 MENDELISM CHAP. One of the best illustrations of repulsion between factors occurs in the sweet-pea. We have already seen that the loss of the blue or purple factor (B) from the wild bicolor results in the formation of the red bicolor known as Painted Lady (PI. IV., 7). Further, we have seen that the hooded standard is recessive to the ordinary erect standard. The omission of the factor for the erect standard (E) from the purple bicolor (PI. II., 5) results in a hooded purple known as Duke of Westminster (PI. II., 7). And here it should be mentioned that in the corresponding hooded forms the difference in colour between the wings and standard is not nearly so marked as in the forms with the erect standard, but the difference in structure appears to affect the colour, which becomes nearly uniform. This may be readily seen by comparing the picture of the purple bicolor on Plate II. with that of the Duke of Westminster flower. Now when a Duke of Westminster is mated with a Painted Lady the factor for erect standard (E} is brought in by the red, and that for blue (B} by the Duke, and the offspring are consequently all purple bicolors. Purples so formed are all heterozygous for these two factors, and were the case a simple one, such as those which have already been discussed, we should expect the F2 generation to consist of the four forms erect purple, hooded purple, erect red, and hooded red in the ratio 9:3:3:1. Such, how- ever, is not the case. The F,, generation actually consists of only three forms, viz. erect red, erect purple, and hooded purple, and the ratio in which these three forms occur is 1:2:1. No hooded red ix REPULSION AND COUPLING 83 has been known to occur in such a family. More- over, further breeding shows that while the erect reds and the hooded purples always breed true, the erect purples in such families never breed true, but Painted Lady > (erect red) C Duke of Westminster (hooded purple) Purple I (erect nvincible purple) 1 1 Painted Purple Invincible Duke of Lady Westminster (I) (2) (I) always behave like the original F: plant, giving the three forms again in the ratio i : 2 : I. Yet we know that there is no difficulty in getting purple bicolors to breed true from other families ; and we know also that hooded red sweet-peas exist in other strains. On the assumption that there exists a repulsion h between the factors for erect standard and blue in a plant which is heterozygous for both, this peculiar case receives a simple explanation. The constitutions of the erect red and the hooded purple are EEbb and eeBB respectively, and that of the Fa erect purple is EeBb. Now let us suppose that in such a zygote there exists a repulsion between E and B, such that when the plant forms gametes these two factors will not go into the same gamete. On this view it can only form two kinds of gametes, viz. Eb and £/?/and these, of course, will be formed in equal numbers. Such a plant on self- fertilisation must give the zygotic series EEbb + 2 EeBb + eeBB, i.e. 84 MENDELISM CHAP. i erect red, 2 erect purples, and i hooded purple. And because the erect reds and the hooded purples are respectively homozygous for E and B, they must thenceforward breed true. The erect purples, on the other hand, being always formed by the union of a gamete Eb with a gamete eB, are always heterozygous for both of these factors. They can, consequently, never breed true, but must always give erect reds, erect purples, and hooded purples in the EEbb eeBB Parents Eb Eb eB eB gametes EeBb E F2 generation ratio i : 2 : i. The experimental facts are readily explained on the assumption of repulsion between the two factors B and E during the formation of the gametes in a plant which is heterozygous for both. Other similar cases of factorial repulsion have been demonstrated in the sweet -pea, and two of these are also concerned with the two factors with which we have just been dealing. Two distinct varieties of pollen grains occur in this species, viz. the ordinary oblong form and a rather smaller rounded grain. The former is dominant to the ix REPULSION AND COUPLING 85 latter.1 When a cross is made between a purple with round pollen and a red with long pollen the Fx plant is a long pollened purple. But the F2 generation consists of purples with round pollen, purples with long pollen, and reds with long pollen in the ratio I : 2 : i. No red with round pollen appears in F2 owing to repulsion between the factors for purple (B) and for long pollen (Z,). Similarly plants produced by crossing a red hooded long with a red round having an erect standard give in Fx long pollened reds with an erect standard, and these in F9 produce the three types round pollened erect, long pollened erect, and long pollened hooded in the ratio I : 2 : I. The repulsion here is between the long pollen factor (£) and the factor for the erect standard (£). Yet another similar case is known in which we are concerned with quite different factors. In some sweet-peas the axils whence the leaves and flower- stalks spring from the main stem are of a deep red colour. In others they are green. The dark pigmented axil is dominant* to the light one. Again, in some sweet-peas the anthers are sterile, setting no pollen, and this condition is recessive to the ordinary fertile condition. When a sterile plant with a dark axil is crossed by a fertile plant with a light axil, the Fj plants are all fertile with dark axils. But such plants in F2 give fertiles with light axils, fertiles with dark axils, and steriles with dark axils in the ratio i : 2 : i. No light axilled steriles appear from 1 It should be mentioned that as the shape of the pollen coat, like that of the seed coat, is a maternal character, all the grains of any given plant are either long or 'else round. The two kinds do not occur together on the same plant^. 86 MENDELISM CHAP such a cross owing to the repulsion between the factor for dark axil (Z?) and that for the fertile anther (F). These four cases have already been found in the sweet -pea, and similar phenomena have been met with by Gregory in primulas. To certain seemingly analogous cases in animals where sex is concerned we shall refer later. Now all of these four cases present a common feature which probably has not escaped the attention of the reader. In all of them the original cross zvas such as to introduce one of the repelling factors with each of the two parents. If we denote our two factors by A and B, the crosses have always been of the nature A Abb x aaBB. Let us now consider what happens when both of the factors, which in these cases repel one another, are introduced by one of the parents, and neither by the other parent. And in particular we will take the case in which we are concerned with purple and red flower colour, and with long and round pollen, i.e. with the factors B and L. When a purpie long (BBLL) is crossed with a red round (bbll] the FT (BbLF) is a purple with long pollen, identical in appearance with that produced by crossing the long pollened red with the round pollened purple. But the nature of the F9 generation is in some respects very different. The ratio of purples to reds and of longs to rounds is in each case 3:1, as before. But instead of an association between the red and the long pollen characters the reverse is the case. The long pollen character is now associated with purple and the round pollen with red. The association, however, is not quite ix REPULSION AND COUPLING 87 complete, and the examination of a large quantity of similarly bred material shows that the purple longs are about twelve times as numerous as the purple rounds, while the red rounds are rather more than three times as many as the red longs. Now this peculiar result could be brought about if the gametic series produced by the Fl plant consisted of 7 BL+ i Bl+ i bL+ 7 bl out of every 16 gametes. Fertilisation between two such similar series of 16 gametes would result in 256 plants, of which 177 would be purple longs, 1 5 purple rounds, 1 5 red longs, and 49 red rounds — a proportion of the four different kinds very close to that actually found by experiment. It will be noticed that in the whole family the purples are to the reds as 3:1, and the longs are also three times as numerous as the rounds. The peculiarity of the case lies in the distribution of these two characters with regard to one another. In some way or other the factors for blue and for long pollen become linked together in the cell divisions that give rise to the gametes, but the linking is not complete. This holds good for all the four cases in which repulsion between the factors occurs when one of the two factors is intro- duced by each of the parents. When both of the factors are brought into the cross by the same parent we get coupling between them instead of repulsion. The phenomena of repulsion and coupling between separate factors are intimately related, though hitherto we have not been able to suggest why this should be so. Nor for the present can we suggest why certain factors should be linked together in the peculiar 88 MENDELISM CHAP. way that we have reason to suppose that they are during the process of the formation of the gametes. Nevertheless the phenomena are very definite, and it is not unlikely that a further study of them may throw important light on the architecture of the living cell. APPENDIX TO CHAPTER IX As it is possible that some readers may care, in spite of its complexity, to enter rather more fully into the peculiar phenomenon of the coupling of characters, I have brought together some further data in this Appendix. In the case we have already considered, where the factors for blue colour and long pollen are concerned, we have been led to suppose that the gametes produced by the heterozygous plant are of the nature 7 BL : i Bl : i bL : 7 bl. Such a series of ovules fertilised by a similar series of pollen grains will give a generation of the following com- position : — 49 BBLL + 7 BBLl+7 BbLL + 49 BbLl + 7 BBLI+ 7 BbLL + BbLl + BbLl + 49 BbLl « -- -- , -- 177 purple, long + BBII+ 7 Bf>ll+ bbLL + 7 bbLl+w bbll + 7 Bbll + 7 bbLl 15 purple, 15 red, 49 red, round long round and as this theoretical result fits closely with the actual figures obtained by experiment we have reason for supposing that the heterozygous plant produces a series of gametes in which the factors are coupled in this way. The intensity of the coupling, however, varies in different cases. Where we are dealing with another, viz. fertility (f) and ix REPULSION AND COUPLING 89 the dark axil (D] the experimental numbers accord with the view that the gametic series is here 1 5 FD : i Fd : i fD : 1 5 fd. The coupling is in this instance more intense. In the case of the erect standard (E) and blue- ness (£} the coupling is even more intense, and the experimental evidence available at present points to the gametic series here being 63 EB : i Eb : i eB 163 eb. There is evidence also for supposing that the intensity of the coupling may vary in different families for the same pair of factors. The coupling between blue and long pollen is generally on the 7:1:1:7 basis, but in some cases it may be on the 15 : i : i : 15 basis. But though the intensity of the coupling may vary it varies in an orderly way. If A and B are the two factors concerned, the results obtained in F2 are explicable on the assumption that the ratio of the four sorts of gametes produced is a term of the series — 3 AB + Ab + aB + 3 ab 7 AB + Ab + aB + 7 ab 15 AB + Ab + aB+ 15 ab, etc., etc. In such a series the number of gametes containing A is equal to the number lacking A, and the same is true for B. Consequently the number of zygotes form-ed contain- ing A is three times as great as the number of zygotes which do not contain A ; and similarly for B. The proportion of dominants to recessives in each case is 3 : i. It is only in the distribution of the characters with relation to one another that these cases differ from a simple Mendelian case. As the study of these series presents another feature of some interest, we may consider it in a little more detail. In the accompanying table are set out the results produced by these different series of gametes. The series marked by an asterisk have already been demonstrated experi- mentally. The first term in the series, in which all the four kinds of gametes are produced in equal numbers, is, of course, that of a simple Mendelian case where no coupling occurs. MENDELISM Factors in Gametic d|| Form of F2 Generation. i2" * c Series. ^N 2 a, 4 AB. A.6. aB. ab. I I 16 933 i 8 3 3 64 If-/-**" 77 9 16 7 7 256 177 IS 15 49* 32 15 15 1024 737 3i 31 =25* 64 31 31 4096 3009 63 63 961 128 63 63 16384 12161 127 127 3969 * 2« (« - i) : i : («- i) 4«* 3«2— (zn - i) (zn - i) (zn - i) w2 - (2« - i) Now, as the table shows, it is possible to express the gametic series by a general formula j^t \*PfAB + Ab + aB + ( (« - i) ab, where 2n is the total number of the gametes in the series. A plant producing such a series of gametes gives rise to a family of zygotes in which 3«2 - (zn - i) show both of the dominant characters and «2-(2«- i) show both of the recessive characters, while the number of the two classes which each show one of the two dominants is (zn - i). When in such a series the coupling becomes closer the value of ;/ increases, but in comparison with n2 its value becomes less and less. The larger n becomes the more negligible is its value relatively to n2. If, there- fore, the coupling were very close, the series 3«2 - (2n - i) : (2 « - i) : (zn - i) : n2 - (zn - i) would approximate more and more to the series 3«2 : «2, i.e. to a simple 3 : i ratio. Though the point is probably of more theoretical than practical interest, it is not impossible that some of the cases which have hitherto been regarded as following a simple 3 : i ratio will turn out on further analysis to belong to this more complicated scheme. CHAPTER X SEX IN their simplest expression the phenomena ex- hibited by Mendelian characters are sharp and clean cut. Clean cut and sharp also are the phenomena of sex. It was natural, therefore, that a comparison should have been early instituted between these two sets of phenomena. As a general rule, the cross between a male and a female results in the produc- tion of the two sexes in approximately equal numbers. The cross between a heterozygous domin- ant and a recessive also leads to equal numbers of recessives and of heterozygous dominants. Is it not, therefore, possible that one of the sexes is heterozygous for a factor which is lacking in the other, and that the presence or absence of this factor determines the sex of the zygote ? The results of some recent experiments would appear to justify this interpretation, at any rate in particular cases. Of these, the simplest is that of the common currant moth (Abraxas grossulariata), of which there exists a pale variety (Fig. 17) known as lacticolor. The experiments of Doncaster and Raynor showed that the variety behaved as a simple recessive to the 91 92 MENDELISM CHAP. normal form. But the distribution of the dominants and recessives with regard to the sexes was peculiar. The original cross was between a lacticolor female and a normal male. All the Fa moths of both sexes FIG. 17. Abraxas grossulariata, the common currant moth, and (on the right) its paler lacticolor variety. were of the normal grossulariata type. The Fa insects were then paired together and gave a generation consisting of 3 normal : I lacticolor. But all the lacticolor were females, and all the males Lacticolor X Grossulariata . 9 Lactd* X Gr.9 X Gr.d* X Lact. 9 Lact.9 Gr.dGr.C? Qr.9 Lact.9 Gr.d Lact.C? Gr.9 Lact.9 were of the normal pattern,. It was, however, found possible to obtain the lacticolor male by mating a lacticolor female with the FX male. The family result- ing from this cross consisted of normal males and normal females, lacticolor males and lacticolor females, SEX 93 and the four sorts were produced in approximately equal numbers. In such a family there was no special association of either of the two colour varieties with one sex rather than the other. But the reverse cross, Ft female by lacticolor male, gave a very different result. As in the previous cross, such families contained equal- numbers of the normal form and of the recessive variety. But all of the normal grossulariata were males, while all the lacti- color were females. Now this seemingly complex collection of facts is readily explained if we make the following three assumptions : — (1) The grossulariata character (£) is dominant to the lacticolor character (g]. This is obviously justified by the experiments, for, leaving the sex distribution out of account, we get the expected 3 : i ratio from FX x Fx, and also the expected ratio of equality when the heterozygote is crossed with the recessive. (2) The female is heterozygous for a dominant factor (F] which is lacking in the male. The con- stitution of a female is consequently Ff, and of a male ff. This assumption is in harmony with the fact that the sexes are produced in approximately equal numbers. (3) There exists repulsion between the factors G and F in a zygote which is heterozygous for them both. Such zy gates'- (FfGg) must always be females, and on this assumption will produce gametes Fg and fG in equal numbers. We may now construct a scheme for com- parison with that on page 92 to show how these assumptions explain the experimental results. The 94 MENDELISM CHAP. original parents were lacticolor female and grossu- lariata male, which on our assumptions must be Ffgg and ffGG respectively in constitution. Since the female is always heterozygous for F, her gametes must be of two kinds, viz. Fg and fg, while those of the pure grossulariata male must be all fG. When an ovum Fg is fertilised by a spermatozoon fG, the gametes Ffgg [9] -j ^ V \/C W] fifeS FfGg[f] ffGgW] Ffgg [9] fa\ ( Fg\ I/G\ V {*& fS}— f— \fo}— x-Ur U Ffgg ffGg Ffgg FfGg ffGg ffGG Ff Gg Ffgg ffGg ffgg [9] UJ [9] [f ] W] Wl [f] [9] FIG. 18. Scheme of inheritance in the Ft and F2 generations resulting from the cross of lacticolor female with grossulariata male. The character of each individual is represented by the sex signs in brackets, the black being grossulariata in appearance and the light ones lacticolor. resulting zygote, FfGg, is heterozygous for both F and G, and in appearance is a female grossulariata. The zygote resulting from the fertilisation of an ovum fg by a spermatozoon fG is heterozygous for G, but does not contain F, and therefore is a male grossulariata. Such a male being in constitution ffGg must produce gametes of two kinds, fG and fg, in equal numbers. And since we are assuming repulsion between F and G, the FT female being in constitution, FfGg must produce equal numbers of SEX 95 gametes Fg and fG. For on our assumption F and G cannot enter into the same gamete. The series of gametes produced by the F1 moths, therefore, are fG, fg by the male and Fg, fG by the female. The resulting F2 generation consequently consists of the four classes of zygotes Ffgg, FfGg, ffGg, and f/GG in equal numbers. In other words, the sexes are produced in equal numbers, the proportion of normal grossulariata to lacticolor is 3:1, and all of the lacticolor are females ; that is to say, the re- sults worked out on our assumptions accord with those actually produced by experiment. We may now turn to the results which should be obtained by crossing the Fx moths with the lacticolor variety. And first we will take the cross lacticolor female x F male. The gametes produced by the lacticolor female we have already seen to be Fg and fg, while those produced by the Fj male are fG and fg. The bringing together of these two series of gametes must result in equal numbers of the four kinds of zygotes FfGg, Ffgg,ffGg, zn&ffgg, i.e. of female grossulariata and lacticolor, and of male grossulariata and lacticolor in equal numbers. Here, again, the calculated results accord with those' of experiment. Lastly, we may examine what should happen when the FX female is crossed with the lacticolor male. The Fx female, owing to the repulsion between F and G, produces only the two kinds of ova Fg and fG, and produces them in equal numbers. Since the lacticolor male can contain neither F nor G, all of its spermatozoa must \)Qfg. The results of such a cross, therefore, should be to produce equal numbers of the two kinds of zygote Ffgg and ffGg, i.e. of lacticolor 96 MENDELISM CHAP. females and of grossulariata males. And this, as we have already seen, is the actual result of such a cross. Before leaving the currant moth we may allude to an interesting discovery which arose out of these experiments. The lacticolor variety in Great Britain is a southern form and is not known to occur in Scotland. Matings were made between wild Scotch females and lacticolor males. The families resulting from such matings were precisely the same as those from lacticolor males and FI females, viz. grossulariata males and lacticolor females only. We are, therefore, forced to regard the constitution of the wild grossu- lariata female as identical with that of the FI female, i.e. as heterozygous for the grossulariata factor as well as for the factor for femaleness. Though from a region where lacticolor is unknown, the " pure " wild grossulariata female is nevertheless a permanent mongrel, but it can never reveal its true colours unless it is mated with a male which is either heterozygous for G or pure lacticolor. And as all the wild northern males are pure for the grossu- lariata character this can never happen in a state of nature. An essential feature of the case of the currant moth lies in the different results given by reciprocal crosses. Lacticolor female X grossulariata male gives grossulariata alone of both sexes. But grossulariata female x lacticolor male gives only grossulariata males and lacticolor females. Such a difference between reciprocal crosses has also been found in other animals, and the experimental results, though some- times more complicated, are explicable on the same lines. An interesting case in which three factors PLATE V. i, 2, F, Cock and Hen, ex Brown Leghorn Hen x Silky Cock; 3, Silky Cock ; 4, Hen ex Silky Hen x Brown Leghorn Cock. x SEX 97 are concerned has been recently worked out in poultry. The Silky breed of fowls is characterised among other peculiarities by a remarkable abundance of melanic pigment. The skin is dull black, while the comb and wattles are of a deep purple colour contrasting sharply with the white plumage (PI. V., 3). Dissection shows that this black pigment is widely spread throughout the body, being especially marked in such membranes as the mesenteries, the periosteum, and the pia mater surrounding the brain. It also occurs in the connective tissues among the muscles. In the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal crosses between these two breeds gave a remarkable differ- ence in result. A cross between the Silky hen and the Brown Leghorn Silky Brown Leghorn cock produced Fa birds, * x * in which both sexes exhibited only traces of the pigment. On X G? F, casual observation they might have passed for • • • i 1 A A F unpigmented birds, for • with the exception of FlG- I9> i n 1 r Scheme illustrating the result of crossing a an OCCaSlOnal tleCk OI silky hen with a Brown Leghorn cock. , , • i • Black sex signs denote deeply pigmented pigment tneir SKin, birds, and light sex signs those without , , , , , pigmentation. The light signs with a COmb, and Wattles Were y^k dot in the centre denote birds with i • ,1 -n _ a small amount of pigment. as clear as m the Brown Leghorn (PI. V., i and 4). Dissection revealed the presence of a slight amount of internal pigment. Such birds bred together gave some offspring with the full pigmentation of the Silky, some without any pigment, and others showing different degrees of H 98 MENDELISM CHAP. pigment. None of the F2 male birds, however, showed the full deep pigmentation of the Silky. When, however, the cross was made the other way, viz. Brown Leg- Brown Leghorn Silky ^ hen x snky I cock, the result i 1 1 was different. <$- --Fi While the Fx male birds were almost destitute of pig- ii <$ <$ <5 t ? ? 9""F2 ment as in the FIG. 20. previous cross, the Scheme illustrating the result of crossing a Brown F henS, On the Leghorn hen with a Silky cock (cf. Fig. 19). other hand, were nearly as deeply pigmented as the pure Silky (PI. V., 2). The male Silky transmitted the pigmentation, but only to his daughters. Such birds bred together gave an F2 generation containing chicks with the full deep pigment, chicks without pigment, and chicks with various grades of pigmentation, all the different kinds in both sexes. In analysing this complicated case many other different crosses were made, but for the present it will be sufficient to mention but one of these, viz. that between the Fx birds and the pure Brown Leghorn. The cross between the Ft hen and the Brown Leghorn cock produced only birds with a slight amount of pigment and birds without pigment. And this was true for both the deeply pigmented and the slightly pigmented types of FI hen. But when the Fl cock was mated to a Brown Leghorn hen, a definite proportion of the chicks, one in eight, were deeply pigmented, and these deeply pigmented SEX 99 birds zvere always females (cf. Fig. 21). And in this respect all the Fa males behaved alike, whether they were from the Silky hen or from the Silky cock. We have, therefore, the paradox that the F hen, though herself deeply pigmented, cannot trans- mit this condition to any of her offspring when she is mated to the unpigmented Brown Leghorn, but that, when similarly mated, the FX cock can transmit this pigmented condition to a quarter of his female (Brown Leghorn) Silky (BrownLegh.)^ X ? X 9 (Brown Leghorn) 9 9 T 1 1 1 FIG. 21. Scheme to illustrate the result of crossing Ft birds (e.g. Brown Leghorn X Silky) with the pure Brown Leghorn. offspring though he himself is almost devoid of pigment. Now all these apparently complicated results, as well as many others to which we have not alluded, can be expressed by the following simple scheme. There are three factors affecting pigment, viz. (i) a pigmentation factor (/*) ; (2) a factor which inhibits the production of pigment (/) ; and (3) a factor for femaleness (F), for which the female birds are heterozygous, but which is not present in the males. Further, we make the assumptions (a) that there is repulsion between F and / in the female zygote (Fflt), and (#) that the male Brown Leghorn IOO MENDELISM is homozygous for the inhibitor factor (/), but that the hen Brown Leghorn is always heterozygous for this factor just in the same way as the female of the currant moth is always heterozygous for the grossu- lariata factor. We may now proceed to show how this explanation fits the experimental facts which we have given. The Silky is pure for the pigmentation factor, but does not contain the inhibitor factor. The Brown Leghorn, on the ffppllfo] other hand, contains the inhibitor factor, but not the pigmentation factor. In crossing a Silky hen with a Brown Leghorn cock we are mating two birds of the constitution FfPPii and ffppll, and all the [f ] FfPPii gives gives gametes gametes FPij fPi / ( (*! I fpl i FfPpli ffPpli FIG. 22. Scheme to illustrate the nature of the Fj generation from the Silky hen and Brown Leghorn cock (cf. Fig. 23). FI birds are conse- quently heterozygous for both P and /. In such birds the pigment is almost but not completely suppressed, and as both sexes are of the same constitution with regard to these two factors they are both of similar appearance. In the reciprocal cross, on the other hand, we are mating a Silky male (ffPPit) with a Brown Leghorn hen which on our assumption is heterozygous for the inhibitor factor (/), and in constitution therefore is Ffppli. Owing to the repulsion between F and / the gametes produced by such a bird are Fpi and fpl in equal numbers. All the gametes produced by the Silky cock are fPi. Hence the constitution of SEX 101 the Fj male birds produced by this cross is ffPpli as before, but the female birds must be all of the constitution FfPpii, The Silky cock transmits the fully pigmented condition to his daughters, because the gametes of the Brown Leghorn hen which con- tain the factor for femaleness do not contain the inhibitory factor owing to the repulsion between these factors. The nature of the F2 genera- 1 91 FfPPI{ ffppii tion in each case is gives &ives gametes gametes in harmony with the p . .. , . . above scheme. As, fpj j x however, it serves to illustrate certain points i — i i in connection with in- FfPpii ffPpli termediate forms we [?] [(•?] shall postpone further Scheme to illustrate the nature of the Ft Consideration Of it till generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22). we discuss these matters, and for the present shall limit ourselves to the explanation of the different behaviour of the Fx males and females when crossed with the Brown Leghorn. And, first, the cross of Brown Leghorn female by Fl male. The Brown Leghorn hen is on our hypothesis Ffppli, and produces gametes Fpi and fpL The F! cock is on our hypothesis ffPpli, and produces in equal numbers the four kinds of gametes fPJ, fPi, fpl, fpi. The result of the meeting of these two series of gametes is given in Fig. 24. Of the eight different kinds of zygote formed only one contains P in the absence of /, and this is a female. The result, as we have already seen, is in accordance with the experimental facts. IO2 MENDELISM Fpi fPI 9 Fpi fPi * Fpi fpl 9 Fpi . fpi 9 fpl fPI I fpl fPi 4 fpi fpi 6 fpi fpi d FIG. 24. Diagram showing the nature of the offspring from a Brown Leghorn hen and an Fi cock bred from Silky hen x Brown Leghorn cock, or vice versa. On the other hand, the Brown Leghorn cock is on our hypothesis ffppll. All his gametes conse- quently contain the inhibitor factor, and when he is mated with an F! hen all the zygotes produced must contain 7. None of his off- spring, therefore, can be fully pig- men ted, for this condition only sence of the in- hibitor factor among zygotes which are either homo- zygous or heterozygous for P. The interpretation of this case turns upon the constitution of the Brown Leghorn hen, upon her heterozygous condition with regard to the two factors F and 7, and upon the repulsion that occurs between them when the gametes are formed. Through an independent set of experiments this view of the nature of the Brown Leghorn hen has been con- firmed in an interesting way. There are fowls which possess neither the factor for pigment nor the in- hibitory factor, which are in constitution ppii. Such birds when crossed with the Silky give dark pig- mented birds of both sexes in Fx, and the F2 genera- tion consists of pigmented and unpigmented in the ratio 3:1. Now a cock of such a strain crossed with a Brown Leghorn hen should give only com- pletely unpigmented birds. But if, as we have supposed, the Brown Leghorn hen is producing SEX 103 gametes Fpi and fpf, the male birds produced by such a cross should be heterozygous for /, i.e. in constitution ffpplt, while the hen birds, though identical in appearance so far as absence of pig- mentation goes, should not contain this factor but should be constitutionally Ffppii. Crossed with the pure Silky, the Fl birds of opposite sexes should give an entirely different result. For while the hens C9] Ffppii gives gametes Fpi \ gives gametes ffPPii [ 9 ] Ffppii ^/fppli[<5 ] FfPPii [ f ] gives gametes fPi\ fPi/ gives gametes f Fpi I fpi gives gametes gives gametes ( I FPi fPp FfPpii ffPpii FfPpli FfPpii ffPpli ffPpii w] m [f] E^] w] FIG. 25. Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen. For explanation see text. should give only deeply pigmented birds of both sexes, the cocks should give equal numbers of deeply pigmented and slightly pigmented birds (cf. Fig. 25). These were the results which the experiment actually gave, thus affording strong confirmation of the view which we have been led to take of the Brown Leghorn hen. Essentially the poultry case is that of the currant moth. It differs in that the 104 MENDELISM CHAP. factor which repels femaleness produces no visible effect, and its presence or absence can only be deter- mined by the introduction of a third factor, that for pigmentation. This conception of the nature of the Brown Leghorn hen leads to a curious paradox. We have stated that the Silky cock transmits the pigmented condition, but transmits it to his daughters only. Apparently the case is one of unequal transmission by the father. Actually, as our analysis has shown, it is one of unequal transmission by the mother, the father's contribution to the offspring being identical for each sex. The mother transmits to the daughters her dominant quality of femaleness, but to balance this, as it were, she transmits to her sons another quality which her daughters do not receive. It is a matter of common experience among human families that in respect to particular qualities the sons tend to resemble their mothers more than the daughters do, and it is not improbable that such observations have a real foundation for which the clue may be provided by the Brown Leghorn hen. Nor is this the only reflection that the Brown Leghorn suggests. Owing to the repulsion between the factors for femaleness and for pigment inhibition, it is impossible by any form of mating to make a hen which is homozygous for the inhibitor factor. She has bartered away for femaleness the possibility of ever receiving a double dose of this factor. We know that in some cases, as, for example, that of the blue Andalusian fowl, the qualities of the individual are markedly different according as to whether he or she has received a single or a double dose of a x SEX 105 given factor. It is not inconceivable that some of the qualities in which a man differs from a woman are founded upon a distinction of this nature. Certain qualities of intellect, for example, may depend upon the existence in the individual of a double dose of some factor which is repelled by femaleness. If this is so, and if woman is bent upon achieving the results which such qualities of intellect imply, it is not education or training that will help her. Her problem is to get the factor on which the quality depends into an ovum that carries also the factor for female- ness. CHAPTER XI SEX (continued} THE cases which we have considered in the last chapter belong to a group in which the peculiarities of inheritance are most easily explained by supposing that the female is heterozygous for some factor that is not found in the male. Femaleness is an addi- tional character superposed upon a basis of maleness, and as we imagine that there is a separate factor for each the full constitutional formula for a female is FfMM, and for a male ffMM. Both sexes are homozygous for the male element, and the difference between them is due to the presence or absence of the female element F. There are, however, other cases for which the explanation will not suffice, but can be best inter- preted on the view that the male is heterozygous for a factor which is not found in the female. Such a case is that recently described by Morgan in America for the pomace fly (Drosophila ampelophild). Normally this little insect has a red eye, but white- eyed individuals are known to occur as rare sports. Red eye is dominant to white. In their relation to sex the eye colours of the pomace fly are inherited 1 06 CHAP, xi SEX 107 on the same lines as the grossularidta and lacticolor patterns of the currant moth, but with one essential difference. The factor which repels the red-eye factor is in this case to be found in the male, and here consequently it is the male which must be regarded as heterozygous for a sex factor that is lacking in the female. In order to bring these cases and others into line an interesting suggestion has recently been put forward by Bateson. On this suggestion each sex is heterozygous for its own sex factor only, and does not contain the factor proper to the opposite sex. The male is of the constitution Mmff and the Mmff Ffmm JJ gives gives female Ffmm. Each Sex gametes gametes produces two SOrtS of Mf >-fm\ productive gametes, Mf and mf in '«/ ^/^/fertilisations ,1 r j_i i 'i Mf — Ftn \ unproductive the case of the male, and mf y>,t j fertilisations Fm, fm in that of the female. But on this view a further supposition is necessary. If each of the two kinds of spermatozoa were capable of fertilising each of the two kinds of ova, we should get individuals of the constitution MmFf and mmff, as well as the normal males and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction afford us no grounds for assuming the existence of these two classes of indi- viduals, whatever they may be, we must suppose that fertilisation is productive only between the sperma- tozoa carrying M and the ova without F, or between the spermatozoa without M and the ova containing F. In other words, we must on this view suppose that fertilisations between certain forms of gametes, even loS MENDELISM CHAP. if they can occur, are incapable of giving rise to zygotes with the capacity for further development. If we admit this supposition, the scheme just given will cover such cases as those of the currant moth and the fowl, equally as well as that of the pomace fly. In the former there is repulsion between either the grossulariata factor and F, or else between the pigment inhibitor factor and F, while in the latter there is repulsion between the factor for red eye and M. Whatever the merits or demerits of such a scheme it certainly does offer an explanation of a peculiar form of sex limited inheritance in man. It has long been a matter of common knowledge that colour-blind- ness is much more common among men than among scheme to illustrate the probable women.and also that unaffected mode of inheritance of colour- blindness. The dark signs re- present affected individuals. ' A black dot in the centre de- notes an unaffected female who • n_f. nnliW fViaf r>f fVif* is capable of transmitting the 1S nOt UnllKC tHat . OI trie sheep, where the horned char- acter is apparently dominant in the male but recessive in the female. The hypothesis that the colour-blind condition is due to the presence of an extra factor as compared with the normal, and that a single dose of it will produce colour-blindness in the male but not in the female, will cover a good many of the observed facts (cf. Fig. 26). Moreover, it serves to explain the remarkable fact that all the sons of colour-blind women are also colour-blind. For a woman cannot be colour-blind unless she is •n,r.mf^n ~Qrl francmi'f if f/-> women can transmit it to crvrlo A f. f;rcr cirrVif tVif* S>OnS. At HrSt SlgHt HlC xi SEX 109 homozygous for the colour-blind factor, in which case all her children must get a single dose of it even if she marries a normal male. And this is sufficient to produce colour-blindness in the male though not in the female. But there is one notable difference in this case as compared with that of the sheep. When crossed with pure hornless ewes the heterozygous horned ram transmits the horned character to half his male offspring (cf. p. 71). But the heterozygous colour-blind man does not behave altogether like a sheep, for he apparently does not transmit the colour- blind condition to any of his male offspring. If, however, we suppose that the colour-blind factor is repelled by the factor for maleness, the amended scheme will cover the observed facts. For, denoting the colour-blind factor by X, the gametes produced by the colour-blind male are of two sorts only, viz. Mfx and mfX. If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova Jmx to give normal males, while the spermatozoa mfX unite with ova Fmx to give females which are heterozygous for the colour - blind factor. These daughters are themselves normal, but transmit the condition to about half their sons. The attempt to discover a simple explanation of the nature of sex has led us to assume that certain combinations between gametes are incapable of giving rise to zygotes which can develop further. In the various cases hitherto considered there is no reason to suppose that anything of the sort occurs, or that the different gametes are otherwise than completely fertile one with another. One peculiar no MENDELISM CHAP. case, however, has been known for several years in which some of the gametes are apparently incapable of uniting to produce offspring. Yellow in the mouse is dominant to agouti, but hitherto a homozygous yellow has never been met with. The yellows from families where only yellows and agoutis occur pro- duce, when bred together, yellows and agoutis in the ratio 2:1. If it were an ordinary Mendelian case the ratio should be 3:1, and one out of every three yellows so bred should be homozygous and give only yellows when crossed with agouti. But Cue"not and others have shown that all of the yellows are heterozygous, and when crossed with agoutis give both yellows and agoutis. We are led, there- fore, to suppose that an ovum carrying the yellow factor is unproductive if fertilised by a spermatozoon which also bears this factor. In this way alone does it seem possible to explain the deficiency of yellows and the absence of homozygous ones in the families arising from the mating of yellows together. At present, however, it remains the only definite instance among animals in which we have grounds for assuming that anything in the nature of unpro- ductive fertilisation takes place.1 If we turn from animals to plants we find a more complicated state of affairs. Generally speaking, the higher plants are hermaphrodite, both ovules and pollen grains occurring on the same flower. Some plants, however, like most animals, are of separate sexes, a single plant bearing only male or female flowers. In other plants the separate flowers are 1 For the most recent discussion of this peculiar case the reader is referred to Professor Castle's paper in Science, December 16, 1910. SEX 1 1 1 either male or female, though both are borne on the same individual. In others, again, the conditions are even more complex, for the same plant may bear flowers of three kinds, viz. male, female, and herma- phrodite. Or it may be that these three forms occur in the same species but in different individuals — female and hermaphrodites in one species ; males, females, and hermaphrodites in another. One case, however, must be mentioned as it suggests a possi- bility which we have not hitherto encountered. In the common English bryony (J3ryonia dioica) the sexes are separate, some plants having only male and others only female flowers. In another Euro- pean species, B. alba, both male and female flowers occur on the same plant. Correns crossed these two species reciprocally, and also fertilised B. dioica by its own male with the following results : — dioica ? x dioica $ gave $ ? and 6 3 x alba (J „ $ $ only alba ? x dioica $ „ $ ? and c? <£. The point of chief interest lies in the striking differ- ence shown by the reciprocal crosses between dioica and alba. Males appear when alba is used as the female parent but not when the female dioica is crossed by male alba. It is possible to suggest more than one scheme to cover these facts, but we may confine ourselves here to that which seems most in accord with the general trend of other cases. We will suppose that in dioica femaleness is dominant to maleness, and that the female is heterozygous for this additional factor. In this species, then, the female produces equal numbers of H2 MENDELISM CHAP. ovules with and without the female factor, while this factor is absent in all the pollen grains. Alba ? X dioica <3 gives the same result as dioica 9 x dioica c? , and we must therefore suppose that alba produces male and female ovules in equal numbers. Alba $ x dioica 9 , however, gives nothing but females. Unless, therefore, we assume that there is selective fertilisation we must suppose that all the pollen grains of alba carry the female factor — in other words, that so far as the sex factors are concerned there is a difference between the ovules and pollen grains borne by the same plant. Unfortunately further investigation of this case is rendered im- possible owing to the complete sterility of the F plants. That the possibility of a difference between the ovules and pollen grains of the same individual must be taken into account in future work there is evidence from quite a different source. The double stock is an old horticultural favourite, and for centuries it has been known that of itself it sets no seed, but must be raised from special strains of the single variety. " You must understand withall," wrote John Parkinson of his gilloflowers,1 "that those plants that beare double flowers, doe beare no seed at all ... but the onely way to have double flowers any yeare is to save the seedes of those plants of this kinde that beare single flowers, for from that seede will rise, some that will beare single, and some double flowers." With regard to the nature of these double -throwing strains of singles, Miss Saunders has recently brought out some interesting facts. She 1 Paradisiis Terrestris, London, 1629, p. 261. xi SEX II3 crossed the double - throwing singles with pure singles belonging to strains in which doubles never occur. The cross was made both ways, and in both cases all the Fl plants were single. A distinc- tion, however, appeared when a further generation was raised from the Fl plants. All the F plants from the pollen of the double throwing single behaved FIG. 27. Single and double stocks raised from the same single parent. like double throwing singles, but of the FI plants from the ovules of the double throwers some behaved as double throwers, and some as pure singles. We are led to infer, therefore, that the ovules and pollen grains of the double throwers, though both produced by the same plant, differ in their relation to the factor (or factors) for doubleness. Doubleness is apparently carried by all the pollen grains of such I MENDELISM CHAP. plants, but only by some of the ovules. Though the nature of doubleness in stocks is not yet clearly understood, the facts discovered by Miss Saunders suggest strongly that the ovules and pollen grains of the same plant may differ in their transmitting properties, probably owing to some process of segregation in the growing plant which leads to an unequal distribution of some or other factors to the cells which give rise to the ovules as compared Single Single Double Pollen of pure single X Ovule Pollen Single Single Single Single Double Single Single Double Ovule of x pure single Single Single Double Single Double with those from which the pollen grains eventually spring. Whether this may turn out to be the true account or not, the possibility must not be over- looked in future work. From all this it is clear enough that there is much to be done before the problem of sex is solved even so far as the biologist can ever expect to solve it. The possibilities are many, and many a fresh set of facts is needed before we can hope to decide among them. Yet the occasional glimpses of clear- cut and orderly phenomena, which Mendelian xi SEX 115 spectacles have already enabled us to catch, offer a fair hope that some day they may all be brought into focus, and assigned their proper places in a general scheme which shall embrace them all. Then, though not till then, will the problem of the nature of sex pass from the hands of the biologist into those of the physicist and the chemist. CHAPTER XII INTERMEDIATES So far as we have gone we have found it possible to express the various characters of animals and plants in terms of definite factors which are carried by the gametes, and are distributed according to a definite scheme. Whatever may be the nature of these factors it is possible for purposes of analysis to treat them as indivisible entities which may or may not be present in any given gamete. When the factor is present it is present as a whole. The visible properties developed by a zygote in the course of its growth depend upon the nature and variety of the factors carried in by the two gametes which went to its making, and to a less degree upon whether each factor was brought in by both gametes or by one only. If the given factor is brought in by one gamete only, the resulting heterozygote may be more or less intermediate between the homozygous form with a double dose of the factor and the homozygous form which is entirely destitute of the factor. Cases in point are those of the primula flowers and the Andalusian fowls. Nevertheless these intermediates produce only pure gametes as is 116 CHAP, xii INTERMEDIATES 117 shown by the fact that the pure parental types appear in a certain proportion of their offspring. In such cases as these there is but a single type of intermediate, and the simple ratio in which this and the two homozygous forms appear renders the interpretation obvious. But the nature of the F0 generation may be much more complex and, where we are dealing with factors which interact upon one another, may even present the appearance of a series of intermediate forms grading from the condition found in one of the original parents to that which occurred in the other. As an illustration we may consider the cross between the Brown Leghorn and Silky fowls which we have already dealt with in connection with the inheritance of sex. The offspring of a Silky hen mated with a Brown Leghorn are in both sexes birds with but a trace of the Silky pigmentation. But when such birds are bred together they produce a generation consisting of chicks as deeply pigmented as the original Silky parent, chicks devoid of pigment like the Brown Leghorn, and chicks in which the pigmentation shows itself in a variety of intermediate stages. Indeed from a hundred chicks bred in this way it would be possible to pick out a number of indi- viduals and arrange them in an apparently continuous series of gradually increasing pigmentation, with the completely unpigmented at one end and the most deeply pigmented at the other. Nevertheless, the case is one in which complete segregation of the different factors takes place, and the apparently continuous series of intermediates is the result of the interaction of the different factors upon one another. The con- n8 MENDELISM CHAP. stitution of the Fl£ is a ffPpli, and such a bird produces in equal numbers the four sorts of gametes fPf, fPi, fpl, fpi. The constitution of the Fl ? in this case is FfPpIi. Owing to the repulsion between F and / she produces the four kinds of gametes FPi, Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing two such series of gametes together is shown in Fig. 28. Out of the sixteen types of zygote formed one (FfPPit) is homo- zygous for the pig- mentation factor, and does not con- tain the inhibitor factor. Such a bird is as deeply pure Silky parent. Two, again, contain a single dose of P in the absence of /. These are nearly as dark as the pure Silky. Four zygotes are destitute of P though they may or may not contain /. These birds are completely devoid of pigment like the Brown Leghorn. The remaining nine zygotes show various combinations of the two factors P and /, being either PPIi, PPII, PpII, or Ppli, and in each of these cases the pigment is more or less intense according to the constitution of the bird. Thus a bird of the constitution PPIi FPi fPI 9 FPi fPi f FPi fpl | FPi fpi * Fpi fPI 9 Fpi fPi * Fpi fpl 9 Fpi fpi 9 fPI fPI rf fPI fPi because the pollen tubes of the former are not long enough to penetrate down to the ovules of the latter. Hybrids can nevertheless be obtained from the reciprocal cross. Nor should we expect offspring from a St. Bernard and a toy terrier without recourse to artificial fertilisation. Or sterility may be due to pathological causes which prevent the gametes from meeting one another in a healthy state. But in most cases it is probable that the sterility is due to some other cause. It is not inconceivable that definite differences in chemical composition render the protoplasm of one species toxic to the gametes of the other, and if this is so it is not impossible that we may some day be able to express these differences in terms of Mendelian factors. The very nature of the case makes it one of extreme difficulty for experimental investigation. At any rate, we realise more clearly than before that the problem of species is not one that can be resolved by the study of. morphology or of systematics. It is a problem in physiology. CHAPTER XIV ECONOMICAL SINCE heredity lies at the basis of the breeder's work, it is evident that any contribution to a more exact knowledge of this subject must prove of service to him, and there is no doubt that he will be able to profit by Mendelian knowledge in the conduct of his operations. Indeed, as we shall see later, these ideas have already led to striking results in the raising of new and more profitable varieties. In the first place, heredity is a question of individuals. Identity of appearance is no sure guide to repro- ductive qualities. Two individuals similarly bred and indistinguishable in outward form may never- theless behave entirely differently when bred from. Take, for instance, the family of sweet peas shown on Plate IV. The F2 generation here con- sists of seven distinct types, three sorts of purples, three sorts of reds, and whites. Let us suppose that our object is to obtain a true breeding strain of the pale purple picotee form. Now from the pro- portions in which they come we know that the dilute colour is due to the absence of the factor which intensifies the colour. Consequently the 141 142 MENDELISM en picotee cannot throw the two deeper shades of red or purple. But it may be heterozygous for the purpling factor, when it will throw the dilute red (Tinged white), or it may be heterozygous for either or both of the two colour factors (cf. p. 41), in which case it will throw whites. Of the picotees which come in such a family, therefore, some will give picotees, tinged whites, and whites, others will give picotees and tinged whites only, others will give picotees and whites only, while others, .again, and these the least numerous, will give nothing but picotees. The new variety is already fixed in a certain definite proportion of the plants ; in this particular instance in i out of every 27. All that remains to be done is to pick out these plants. Since all the picotees look alike, whatever their breeding capacity, the only way to do this is to save the seed from a number of such plants individually, and to raise a further generation. Some of them will be found to breed true. The variety is then established, and may at once be put on the market with full confidence that it will hereafter throw none of the other forms. The all-important thing is to save and sow the seed of separate individuals separately. However alike they look, the seed from different individuals must on no account be mixed. Provided that due care is taken in this respect no long and tedious process of selection is required for the fixation of any given variety. Every possible variety arising from a cross appears in the F2 generation if only a sufficient number is raised, and of all these different varieties a certain ^proportion of each is already fixed. Heredity is a question of xiv ECONOMICAL 143 individuals, and the recognition of this will save the breeder much labour, and enable him to fix his varieties in the shortest possible time. Such cases as these of the sweet pea throw a fresh light upon another of the breeder's conceptions, that of purity of type. Hitherto the criterion of a " pure-bred " thing, whether plant or animal, has been its pedigree, and the individual was regarded as more or less pure bred for a given quality accord- ing as it could show a longer or shorter list of ancestors possessing this quality. To-day we realise that this is not essential. The pure-bred picotee appears in our F2 family though its parent was a purple bicolor, and its remoter ancestors whites for generations. So also from the cross between pure strains of black and albino rabbits we may obtain in the F2 generation animals of the wild agouti colour which breed as true to type as the pure wild rabbit of irreproachable pedigree. The true test of the pure breeding thing lies not in its ancestry but in the nature of the gametes which have gone to its making. Whenever two similarly constituted gametes unite, whatever the nature of the parents from which they arose, the resulting individual is homozygous in all respects and must consequently breed true. In deciding questions of purity it is to the gamete, and not to ancestry, that our appeal must hence- forth be made. Improvement is after all the keynote to the breeder's operations. He is aiming at the produc- tion of a strain which shall combine the greatest number of desirable properties with the least number of undesirable ones. This good quality he must 144 MENDELISM CHAP. take from one strain, that from another, and that, again, from a third, while at the same time avoiding all the poor qualities that these different strains possess. It is evident that the Mendelian concep- tion of characters based upon definite factors which are transmitted on a definite scheme must prove of the greatest service to him. For once these factors have been determined their distribution is brought under control, and they can be associated together or dissociated at the breeder's will. The chief labour involved is that necessary for the determina- tion of the factors upon which the various characters depend. For it often happens that what appears to be a simple character turns out when analysed to depend upon the simultaneous presence of several distinct factors. Thus the Malay fowl breeds true to the walnut comb, as does also the Leghorn to the single comb, and when pure strains are crossed all the offspring have walnut combs. At first sight it would be not unnatural to regard the difference as dependent upon the presence or absence of a single factor. Yet, as we have already seen, two other types of comb, the pea and the rose, make their appearance in the F2 generation. Analysis shows that the difference between the walnut and the single is a difference of two factors, and it is not until this has been determined that we can proceed with certainty to transfer the walnut character to a single- combed breed. Moreover, in his process of analysis the breeder must be prepared to encounter the various phenomena that we have described under the headings of interaction of factors, coupling, and repulsion, and the recognition of these phenomena xiv ECONOMICAL 145 will naturally influence his procedure. Or, again, his experiments may show him that one of the characters he wants, like the blue of the Andalusian fowl, is dependent upon the heterozygous nature of the individual which exhibits it, and if such is the case he will be wise to refrain from any futile attempt at fixing it. If it is essential it must be built up again in each generation, and he will recognise that the most economical way of doing this is to cross the two pure strains so that all the offspring may possess the desired character. The labour of analysis is often an intricate and tedious business. But once done it is done once for all. As soon as the various factors are determined upon which the various characters of the individual depend, as soon as the material to be made use of has been properly analysed, the production and fixation of the required combinations becomes a matter of simple detail. An excellent example of the practical application of Mendelian principles is afforded by the experi- ments which Professor Biffen has recently carried out in Cambridge. Taken as a whole English wheats compare favourably with foreign ones in respect of their cropping power. On the other hand, they have two serious defects. They are liable to suffer from the attacks of the fungus which causes rust, and they do not bake into a good loaf. This last property depends upon the amount of gluten present, and it is the greater proportion of this which gives to the "hard" foreign wheat its quality of causing the loaf to rise well when baked. For some time it was held that " hard " wheat with L 146 . MENDELISM CHAP. a high glutinous content could not be grown in the English climate, and undoubtedly most of the hard varieties imported for trial deteriorated greatly in a very short time. Professor Biffen managed to obtain a hard wheat which kept its qualities when, grown in England. But in spite of the superior quality of its grain from the baker's point of view, its cropping capacity was too low for it to be grown profitably in competition with English wheats. Like the latter, it was also subject to rust. Among the many varieties which Professor Biffen collected and grew for observation he managed to find one which was completely immune to the attacks of the rust fungus, though in other respects it had no desirable quality to recommend it. Now as the result of an elaborate series of investigations, he was able to show that the qualities of heavy cropping capacity, " hardness " of grain, and immunity to rust can all be expressed in terms of Mendelian factors. Having once analysed his material the rest was comparatively simple, and in a few years he has been able to build up a strain of wheat which com- bines the cropping capacity of the best English varieties with the hardness of the foreign kinds, and at the same time is completely immune to rust. This wheat has already been shown to keep its qualities unchanged for several years, and there is little doubt that when it comes to be grown in quantity it will exert an appreciable influence on wheat-growing in Great Britain. It may be objected that it is often with small differences rather than with the larger and more striking ones that the breeder is mainly concerned. ECONOMICAL 147 It does not matter much to him whether the colour of a pea flower is purple or pink or white. But it does matter whether the plant bears rather larger seeds than usual, or rather more of them. Even a small difference when multiplied by the size of the crop will effect a considerable difference in the profit. It is the general experience of seedsmen and others that differences of this nature are often capable of being developed up to a certain point by a process •3400 I • 0) u.300 o |200 I 2 ioo / s s ***^ \ t \ \ 1 / , / / \ \\ / / / / // 3 V s / s. \ \\ Xy 4 6 8 xo 12 14 1 6 18 20 Weight of individual seeds FIG. 30. Curves to illustrate the influence of selection. I of careful selection each generation. At first sight this appears to be something very like the gradual accumulation of minute variations through the con- tinuous application of a selective process. Some recent experiments by Professor Johannsen of Copen- hagen set the matter in a different light. One of his investigations deals with the inheritance of the weight of beans, but as an account of these experi- ments would involve us in the consideration of a large amount of detail we may take a simple imaginary case to illustrate the nature of the con- 148 MENDELISM CHAP. elusions at which he arrived. If we weigh a number of seeds collected from a patch of plants such as Johannsen's beans we should find that they varied considerably in size. The majority would probably not diverge very greatly from the general average, and as we approached the high or low extreme we should find a constantly decreasing number of individuals with these weights. Let us suppose that the weight of our seed varied between 4 and 20 grains, that the greatest number of seeds were of the mean weight, viz. 1 2 grains, and that as we passed to either extreme at 4 and 20 the number became regularly less. The weight relation of such a collection of seeds can be expressed by the accompanying curve (Fig. 30). Now if we select for sowing only that seed which weighs over 1 2 grains, we shall find that in the next generation the average weight of the seed is raised and the curve becomes somewhat shifted to the right as in the dotted line of Fig. 30. By con- tinually selecting we can shift our curve a little more to the right, i.e. we can increase the average weight of the seeds until at last we come to a limit be- yond which further selection has no effect. This phenomenon has been long known, and it was customary to regard these variations as of a con- tinuous nature, i.e. as all chance fluctuations in a homogeneous mass, and the effect of selection was supposed to afford evidence that small continuous variations could be increased by this process. But Johannsen's results point to^ another interpretation. Instead of our material being homogeneous it is probably a mixture of several strains each with its XIV ECONOMICAL 149 own average weight about which the varying con- ditions of the environment cause it to fluctuate. Each of these strains is termed a pure line. If we imagine that there are three such pure lines in our imaginary case, with average weights 10, 12, 14 grains respectively, and if the range of fluctuation of each of these pure lines is 1 2 grains, then our 4 6 8 10 12 14 16 18 20 Weight of individual seeds FIG. 31. Curves to illustrate the conception of pure lines in a population. curve must be represented as made up of the three components A fluctuating between 4 and 16 with a mean of 10 B „ „ 6 „ 18 „ „ 12 C „ „ 8 „ 20 „ „ 14 as is shown in Fig. 31. A seed that weighs 1 2 grains may belong to any of these three strains. It may be an average seed of B, or a rather large seed of A, or a rather small seed of C. If it belongs to B its offspring will average 12 grains, if to A they will average 10 grains, and if to C they will average 14 grains. Seeds of similar weight 150 MENDELISM CHAP. may give a different result because they happen to be fluctuations of different pure lines. But within the pure line any seed, large or small, produces the average result for that line. Thus a seed of line C which weighs 20 grains will give practically the same result as one that weighs 10 grains. On this view we can understand why selection of the largest seed raises the average weight in the next generation. We are picking out more of C 'and less of A and B, and as this process is repeated the proportion of C gradually increases and we get the appearance of selection acting on a continuously varying homogeneous material and producing a permanent effect. This is because the interval between the average weight of the different pure lines is small compared with the environmental fluctuations. None the less it is there, and the secret of separating and fixing any of these pure lines is again to breed from the individual separately. As soon as the pure line is separated further selection becomes superfluous. Since the publication of Darwin's famous work upon the effects of cross- and self-fertilisation, it has been generally accepted that the effect of a cross is commonly, though not always, to introduce fresh vigour into the offspring, though why this should be so we are quite at a loss to explain. Continued close inbreeding, on the contrary, eventually leads to deterioration, though, as in many self- fertilised plants, a considerable number of generations may elapse before it shows itself in any marked degree. The fine quality of many of the seedsman's choice varieties of vegetables probably depends upon the xiv ECONOMICAL 151 fact that they have resulted from a cross but a few generations back, and it is possible that they often oust the older kinds not because they started as something intrinsically better, but because the latter had gradually deteriorated through continuous self- fertilisation. Most breeders are fully alive to the beneficial results of a cross so far as vigour is con- cerned, but they often hesitate to embark upon it owing to what was held to be the inevitably lengthy and laborious business of recovering the original variety and refixing it, even if in the process it was not altogether lost. That danger Mendelism has removed, and we now know that by working on these lines it is possible in three or four generations to recover the original variety in a fixed state with all the superadded vigour that follows from a cross. Nor is the problem one that concerns self-fertilised plants only. Plants that are reproduced asexually often appear to deteriorate after a few generations unless a sexual generation is introduced. New varieties of potato, for example, are frequently put upon the market, and their excellent qualities give them a considerable vogue. Much is expected of them, but time after time they deteriorate in a dis- appointing way and are lost to sight. It is not improbable that we are here concerned with a case in which the plants lose their vigour after a few asexual generations of reproduction from tubers, and can only recover it with the stimulus that results from the interpolation of a sexual generation. Un- fortunately this generally means that the variety is lost, for owing to the haphazard way in which new kinds of potatoes are reproduced it is probable that 152 MENDELISM CHAP. most cultivated varieties are complex heterozygotes. Were the potato plant subjected to careful analysis and the various factors determined upon which its variations depend, we should be in a position to remake continually any good potato without running the risk of losing it altogether, as is now so often the case. The application of Mendelian principles is likely to prove of more immediate service for plants than animals, for owing to the large numbers which can be rapidly raised from a single individual and the prevalence of self-fertilisation, the process of analysis is greatly simplified. .Even apart from the circum- stance that the two sexes may sometimes differ in their powers of transmission, the mere fact of their separation renders the analysis of their properties more difficult. And as the constitution of the indi- vidual is determined by the nature and quality of its offspring, it is not easy to obtain this knowledge where the offspring, as in most animals, are relatively few. Still, as has been abundantly shown, the same principles hold good here also, and there is no reason why the process of analysis, though more trouble- some, should not be effectively carried out At the same time, it affords the breeder a rational basis for some familiar but puzzling phenomena. The fact, for instance, that certain characters often "skip a generation " is simply the "effect of dominance in F^ and the reappearance of the recessive character in the following generation. " Reversion " and " atavism," again, are phenomena which are no longer mysterious, but can be simply expressed in Mendelian terms as we have already suggested in Chap. VI. The xiv ECONOMICAL 153 occasional appearance of a sport in a supposedly pure strain is often due to the reappearance of a recessive character. Thus even in the most highly pedigreed strains of polled cattle such as the Aber- deen-Angus, occasional individuals with horns appear. The polled character is dominant to the horned, and the occasional reappearance of the horned animal is due to the fact that some of the polled herd are heterozygous in this character. When two such indi- viduals are mated, the chances are I in 4 that the offspring will be horned. Though the heterozygous individuals may be indistinguishable in appearance from the pure dominant, they can be readily separated by the breeding test. For when crossed by the recessive, in this case horned animals, the pure dominant gives only polled beasts, while the hetero- zygous individual gives equal numbers of polled and horned ones. In this particular instance it would probably be impracticable to test all the cows by crossing with a horned bull. For in each case it would be necessary to have several polled calves from each before they could with reasonable certainty be regarded as pure dominants. But to ensure that no horned calves should come, it is enough to use a bull which is pure for that character. This can easily be tested by crossing him with a dozen or so horned cows. If he gets no horned calves out of these he may be regarded as a pure dominant and thenceforward put to his own cows, whether horned or polled, with the certainty that all his calves will be polled. Or, again, suppose that a breeder has a chestnut mare and wishes to make certain of a bay foal from 154 MENDELISM CHAP. her. We know that bay is dominant to chestnut, and that if a homozygous bay stallion is used a bay foal must result. In his choice of a sire, therefore, the breeder must be guided by the previous record of the animal, and select one that has never given anything but bays when put to either bay or chest- nut mares. In this way he will assure himself of a bay foal from his chestnut mare, whereas if the record of the sire shows that he has given chestnuts he will be heterozygous, and the chances of his getting a bay or a chestnut out of a chestnut mare are equal. It is not impossible that the breeder may be unwilling to test his animals by crossing them with a different breed through fear that their purity may be thereby impaired, and that the influence of the previous cross may show itself in succeeding genera- tions. He might hesitate, for instance, to test his polled cows by crossing them with a horned bull for fear of getting horned calves when the cows were afterwards put to a polled bull of their own breed. The belief in the power of a sire to influence sub- sequent generations, or telegony as it is sometimes called, is not uncommon even to-day. Nevertheless, carefully conducted experiments by more than one competent observer have failed to elicit a single shred of unequivocal evidence in favour of the view. Until we have evidence based upon experiments which are capable of repetition, we may safely ignore telegony as a factor in heredity. Heterozygous forms play a greater part in the breeding of animals than of plants, for many of the qualities sought after by the breeder are of this nature. Such is the blue of the Andalusian fowl, ECONOMICAL 155 and, according to Professor Wilson, the roan of the Shorthorn is similar, being the heterozygous form produced by mating red with white. The characters of certain breeds of canaries and pigeons again appear to depend upon their heterozygous nature. Such forms cannot, of course, ever be bred true, and where several factors are concerned they may when bred together produce but a small proportion of offspring like themselves. As soon, however, as their constitution has been analysed and expressed in terms of Mendelian factors, pure strains can be built up which when crossed will give nothing but offspring of the desired heterozygous form. The points with which the breeder is concerned are often fine ones, not very evident except to the practised eye. Between an ordinary Dutch rabbit and a winner, or between the comb of a Hamburgh that is fit to show and one that is not, the differences are not very apparent to the uninitiated. Whether Mendelism will assist the breeder in the production of these finer points is at present doubtful. It may be that these small differences are heritable, such as those that form the basis of Johannsen's pure lines. In this case the breeder's outlook is hopeful. But it may be that the variations which he seeks to per- petuate are of the nature of fluctuations, dependent upon the earlier life conditions of the individual, and not upon the constitution of the gametes by which it was formed. If such is the case, he will get no help from the science of heredity, for we know of no evidence which might lead us to suppose that variations of this sort can ever become fixed and heritable. CHAPTER XV MAN THOUGH the interest attaching to heredity in man is more widespread than in other animals, it is far more difficult to obtain evidence that is both com- plete and accurate. The species is one in which the differentiating characters separating individual from individual are very numerous, while the number of the offspring is comparatively few, and the gener- ations are far between. For these reasons, even if it were possible, direct experimental work with man would be likely to prove both tedious and expensive. There is, however, another method besides the direct one from which something can be learned. This consists in collecting all the evidence possible, ar- ranging it in the form of pedigrees, and comparing it with standard cases already worked out in animals and plants. In this way it has been possible to demonstrate in man the existence of several char- acters showing simple Mendelian inheritance. As few besides medical men have hitherto been con- cerned practically with heredity, such records as exist are, for the most part, records of deformity or of disease. So it happens that most of the pedigrees at present available deal with characters which are 156 CHAP. XV MAN 157 usually classed as abnormal. In some of these the inheritance is clearly Mendelian. One of the cases which has been most fully worked out is that of a FIG. 32. Normal and brachydactylous hands placed together for comparison. (From Drinkwater.) deformity known as brachydactyly. In brachy- dactylous people the whole of the body is much stunted, and the fingers and toes appear to have two joints only instead of three (cf. Figs. 32 and 33). The inheritance of this peculiarity has been carefully 158 MENDELISM CHAP. investigated by Dr. Drinkwater, who collected all the data he was able to find among the members of a large family in which it occurred. The result is the pedigree shown on p. 159. It is assumed that all who are recorded as having offspring were married to normals. Examination of the pedigree brings out the facts (i) that all affected individuals have an affected parent ; (2) that none of the unaffected in- FIG. 33. Radiograph of a brachydactylous hand. dividuals, though sprung from the affected, ever have descendants who are affected ; and (3) that in families where both affected and unaffected occur, the numbers of the two classes are, on the average, equal. (The sum of ^uch families in the complete pedigree is thirty-nine affected and thirty-six normals.) It is obvious that these are the conditions which are fulfilled in a simple Mendelian case, and there is nothing in this pedigree to contradict the assertion that brachydactyly, whatever it may be due to, MAN 159 behaves as a simple dominant to the normal form, i.e. that it depends upon a factor which the normal does not contain. The recessive normals cannot transmit the affected condition whatever their ancestry. Once free they are always free, and can marry other normals with full confidence that none of their chil- dren will show the deformity. The evidence available from pedi- grees has revealed the simplest form of Mecfelian in- heritance in several human defects and diseases, among which may be men- tioned presenile cataract of the eyes, an abnormal form of skin thickening in the palms of the •*—*•- L**_»+ 160 MENDELISM CHAP. hands and soles of the feet, known as tylosis, and epidermolysis bullosa, a disease in which the skin rises up into numerous bursting blisters. Among the most interesting of all human pedi- grees is one recently built up by Mr. Nettleship from the records of a night-blind family living near Montpellier in the south of France. In night- blind people the retina is insensitive to light which falls below a certain intensity, and such people are consequently blind in failing daylight or in moonlight. As the Montpellier case had excited interest for some time, the records are unusually complete. They commence with a certain Jean Nougaret, who was born in 1637, and suffered from night-blindness, and they end for the present with children who are to-day but a few years of age. Particulars are known of over 2000 of the descend- ants of Jean Nougaret. Through ten generations and nearly three centuries the affection has behaved as a Mendelian dominant, and there is no sign that long-cpntinued marriage with folk of normal vision has produced any amelioration of the night -blind state. Besides cases such as these where a simple form of Mendelian inheritance is obviously indicated, there are others which are more difficult to read. Of some it may be said that on the whole the peculiarity behaves as though it were an ordinary dominant ; but that exceptions occur in which affected children are born to unaffected parents. It is not impossible that the condition may, like colour in the sweet-pea, depend upon the presence or absence of more than one factor. In none of these cases, however, are MAN I6-, the data sufficient for determining with certainty whether this is so or not. A group of cases of exceptional interest is that in which the incidence of disease is largely, if not absolutely, restricted to one sex, and so far as is hitherto known the burden is invariably borne by the male. In the inheritance of colour-blindness (p. 1 08) we have already discussed an instance in which the defect is rare, though not unknown, in the female. Sex-limited inheritance of a similar nature 1 9 I i 9 9 9 i 1 i 49 9 9 rh 9 rVr-i i i i r^r^f 1 i iii 99 9 4 i ~i i 1 i i 1 i i Children (?) to lO