Previous Chapter : I. Introductory. Mendel's discovery
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† Lang, A., Vierteljahrs. d. nat. Ges. Zürich, 1896, and 168, p. 42, together with information kindly sent in a letter. < 049 > styled individuals. Yet the long-styled are always pure. Moreover, all the short-styled plants hitherto tested have proved to be simple heterozygotes, giving equality of longs and shorts when bred with longs. Hitherto no pure DD, viz. short-styled plant, has been found in the case of the pure Primrose, but no difficulty has been met with in raising short-styled plants of Primula Sinensis. Besides this example of Mendelian heredity manifested by a wild type several of the examples of colour-inheritance in insects relate to wild species. The circumstance that a character has not been previously bred pure does not, so far as is known, in any way influence the mode of transmission of that character. For instance, in the breeding of thoroughbred race-horses the heredity of chestnut colour is that of an ordinary recessive*, though the various colours, bay, brown, and chestnut have been indiscriminately united together in the breed. No difference is manifested between colour-inheritance of chestnuts which have had many chestnut ancestors in recent generations, and those that have no chestnut progenitor in the nearer degrees. The same is true for some of the colour-cases seen in Lepidoptera which had not been the subject of any previous selection. A remarkable example of an obviously Mendelian inheritance in a wholly wild form is that of the eye-colour of the Owl-Athene noctua†. Abundant examples of characters breeding true, though newly-constituted, will be provided by those cases in which a novelty of structure is brought suddenly into existence by the occurrence of fresh combinations. In spite of their recent origin, such new combinations have just the same genetic properties and powers of transmission that are possessed by the types of long-selected breeds. The suggestion hazarded by several writers that a distinction may be drawn between inter-racial and intra-racial heredity has no foundation in fact. * Mr Hurst, who first elucidated the colour-inheritance of race-horses, found that according to the records, chestnuts of various ancestries have exclusively chestnut offspring with about 1% of exceptions, which are very possibly due to error in the returns (see later). † Giglioli, Ibis, 1903, p. 1. (See later, p. 110.) < 050 > Dominance : the Heterozygote Character. The character of the heterozygote, the “hybrid character” of Mendel, gives no indication as to the system by which the parental characters are transmitted. The expressions, “blended” inheritance, “particulate” inheritance and so on, terms formerly devised by Galton for describing the zygotic appearances, are now seen to be descriptive not so much of the mode of transmission as of the consequences of certain groupings of special allelomorphs ; and as it is obviously preferable in all possible cases to use the ultimate descriptions reduced to terms of gametic composition, such terms are now seldom requisite. Dominance must be discussed more fully when other facts have been set forth, and in this preliminary notice of the more salient features of the phenomena it will be enough to point out that dominance is no inseparable attribute of Mendelian inheritance. The essential phenomenon is segregation. The occurrence of dominance is often an assistance to the investigator and may greatly simplify the analysis of the various generations. Seldom however is dominance uniformly complete, and in certain cases, as those of the combs in poultry, where dominance is quite definite, it is still possible for an observer thoroughly familiar with the material to distinguish the homozygous dominants from the heterozygous with fair certainty. Provided the recessives as a class can be identified the application of Mendelian analysis is almost equally easy whether the heterozygotes show definite dominance or some intermediate condition. The statement made by de Vries that dominance is an attribute of the phylogenetically older character has not been borne out by more extended investigation. In the lists given above many examples to the contrary occur. No one, for instance, can doubt that the various types of dominant comb (rose, pea, &c.) in fowls and the colour called “Brown-breasted” have arisen since domestication. This colour-example is illustrated by Fig. 11, where the distinction between the striped Black-red type and the almost unicolorous Brown-red, or “Brown-breasted” type is shown. The striped type is practically that of the wild Gallus bankiva, but the unicolorous type of down-colour is a com- [complete] < 051 > plete dominant. Males raised as F1 between the two pure breeds are in their adult plumage almost intermediate, but the F1 hens are indistinguishable from the pure Brown-red hens. Fig. 11. Two newly-hatched chickens in F2 generation from the cross Brown-red Game Bantam x Black-red. A is the Black-red type having dark stripes on a light ground. B is the Brown-red (or “Brown-breasted”) type, a dark, almost unicolorous, blackish brown. A is the recessive and B is the dominant. The whole F2 family consisted of 58 like B and 18 like A. [Figure not reproduced in this version] As an example in which the heterozygotes are intermediate the inheritance of colour in the Andalusian fowl may be taken. Andalusians are in general colour what fanciers call blue-namely a diluted black. In the cocks the hackles and saddle-feathers are full black, and the feathers of the breast are edged or “laced” with black. The hens are blue, laced with black more or less, all over. This breed is recognized by the fanciers as never breeding true to colour. When blue is bred with blue three colours are produced, blacks, blues, and a peculiar white* splashed with grey. Experimenting with this breed we have found * These splashed whites are quite distinct from actual whites. They are in reality coloured birds as regards composition, and their down-colour is a faint bluish, very like that of the White Rosecomb bantam. < 052 > that the numbers from blue x blue average about 1 black : 2 blues : 1 splashed white. Both the blacks and the whites extracted from blues breed true to their respective types, black x black giving all blacks ; white x white giving all whites again. When however black is bred with one of these whites the offspring are all blues. There is thus no doubt that the blue is the heterozygous form, while the gametes bear either the blackness, or the whiteness. Obviously in such a case, continued selection of blues will not make them breed true. This can only come to pass if it shall be found possible to get a blue bird in the gametes of which the blue or intermediate character is carried as a definite factor. These results may be represented in tabular form thus : [Table non reproduced in this version] Such a case as this shows well what Mendel meant by the “hybrid-character.” It is that character, or appearance, or quality, which is produced by the meeting of the opposite allelomorphs of the same pair in one zygote or individual. The hybrid-character is a thing apart, which must always be separately determined by experiment. Sometimes it is indistinguishable from the dominant, sometimes, as here, it is an appearance recognizably distinct from that of either dominant or recessive. Prof. J. Wilson (311) has shown that a similar rule probably holds in the case of shorthorn colours, where red- [red-roan] < 053 > roan is a heterozygous character, caused by the meeting of the factors for red and white*. The blue-roan so often seen in the cross between black Aberdeen-Angus cattle and white shorthorns is presumably the corresponding heterozygote form for black and white. With further knowledge of the details and closer examination of material probably many such cases will be found. Darbishire, for example, has lately shown that though it is usually impossible in the case of peas to tell pure round seed from the heterozygous rounds, by external appearances, yet on microscopical examination the two classes can be distinguished at once by the different structure of the starch grains (94). Many such cases where dominance is imperfect are now known. This phenomenon has no bearing on the more important question of the degree of perfection with which segregation is accomplished. The supposition that dominance was an essential phenomenon of Mendelism was of course a delusion. Imperfection of dominance does not even obscure the application of Mendelian analysis. The cases in which difficulty does arise are those in which dominance is irregular and the recessive class cannot be distinguished with certainty. In the fowl, for instance, the extra toe is usually a dominant, but in some strains there is irregularity, and birds without the extra toe may nevertheless transmit it. So also the blue colour of maize seeds, though usually a dominant, may sometimes be carried on by seeds which appear white (Lock, 174). Even in these examples, however, there is no reason to think that such irregularities are indications of imperfect segregation. It is not impossible that they may be ascribed to interference caused by the presence of other factors in various combinations, and sometimes, no doubt, to disturbance by external conditions. All observations point to a conclusion of great importance, namely that a dominant character is the condition * Further information based on a long series of observations of the Sittytown herd of Shorthorns has since been published by Mr Robert Bruce in Breeder's Gazette, 25 Nov. 1908. Full statistics are given, affording with rare exceptions evidence strongly confirmatory of Prof. Wilson's views. I am obliged to Mr Alexander Bruce for a copy of this paper. < 054 > due to the presence of a definite factor, while the corresponding recessive owes its condition to the absence of the same factor. This generalisation, which so far as we yet see, is applicable throughout the whole range of Mendelian phenomena, renders invaluable assistance in the interpretation of the phenomena of Heredity. The green pea, for instance, owes its recessive greenness to the absence of the factor which, if present, would turn the colouring matter yellow, and so forth. With the examination of further evidence the significance of this principle will become readily apparent. Mendel's System distinguished from that of Galton. From the outline of the evidence now set forth the essential aims and methods of Mendelian inquiry will have been understood. By this method we reach reality and concrete fact among phenomena that had become almost proverbial for their irregularity. The key to the problems of genetics and, as we confidently believe, to that of Species also, lies in the recognition of the character-units, or factors as we often call them. Their allelomorphism is a phenomenon of gametogenesis, and is a consequence of those attractions and repulsions by which the germinal cell-divisions are effected. Discontinuity in variation -to use the word variation in its old, comprehensive sense- results from the existence of these units. We recognize therefore that this discontinuity -Galton's “Organic Stability”- is ultimately dependent on the physiology of gametogenesis, and not as we formerly supposed on some feature in the physiology of zygotes. How this simple conclusion was missed we may in vain surmise. The discovery at one stroke replaces all previous disquisitions regarding the laws of inheritance. The magnitude of the discovery and the novelty of its consequences have indeed delayed general recognition of its truth. To this may have been due the curious fact that the famous Nägeli failed altogether to realise the importance of Mendel's work. Nägeli was of course especially devoted to the study of heredity, and even made it the subject of elaborate mathematical treatment. As we now know, he was in correspondence with Mendel, < 055 > from whom he received a considerable series of letters and illustrative specimens (197). These must have utterly failed to arouse his interest, for when in 1884, the year of Mendel's death, he published his great treatise on heredity, no reference was made to Mendel or his work. That this neglect was due to want of comprehension is evident from a passage where he describes an experiment or observation on cats, which as it happens, gave a simple Mendelian result. The Angora character (recessive) disappeared in a cross with a certain common cat whose hair-character is, as we now know, dominant. The cross-breds were mated together and the Angora character reappeared in one individual among a litter of common cats*. This typically Mendelian fact was thus actually under Nägeli's own observation, but from the discussion which he devotes to the occurrence it is clear that Mendel's work must have wholly passed from his memory, having probably been dismissed as something too fanciful for serious consideration. It may be useful to specify the distinctive features of Mendelian inheritance which differentiate the cases exhibiting it from those to which Galton's system of calculation -or any other systems based on ancestral composition- can apply. (1) In Mendelian cases, in which the characters behaves as units, the types of individuals considered with respect to any pair of allelomorphic characters are three only, two being homozygous and one heterozygous ; while according to such a system as Galton's the number of possible types is regarded as indefinite. (2) The Mendelian system recognizes that purity of type may be absolute, and that it may arise in individuals of the F2 or any later generation bred from heterozygotes. The views based on ancestry regard purity of type as relative, and arising by the continued selection of numbers of individuals. (3) In Galton's system no account is taken of dominance, a phenomenon which plays so large a part in the practical application of any true scheme of heredity. These distinctions are so definite and striking that at first sight it seemed likely that the two methods might be * C. Nägeli, Mechanisch-physiologische Theorie der Abstammungslehre, 1884, p. 199. < 056 > applicable to two physiologically distinct classes of phenomena. It was anticipated that some characters, or possibly even some forms of life, might follow the one system and others the other. The results of further researches make this supposition increasingly improbable ; and though undoubtedly there are cases which cannot yet be subjected to Mendelian analysis, it is fairly certain that there is no large group of facts in heredity to which the Galtonian system or any modification of it exclusively applies. There are however numerous examples where the arithmetical results predicable by elther system are nearly or quite the same, though further breeding would of course reveal that even in these cases the applicability of the Galtonian method was only superficial. The first aim of genetics must now be to determine the magnitude, number and ultimately the nature of those units which together make up the visible fact we call heredity ; and so to discover the consequences of their several combinations in zygosis or fertilisation. For the power thus to formulate our purpose and for the development of a method by which it may be successfully pursued we are beholden to Mendel's genius. The difficulty which some feel in realising the significance of Mendelism arises from the habit of looking on the bodies of animals and plants as single structures. So soon as the mind becomes thoroughly accustomed to the fact that all individuals, at least those of the higher and more familiar types, are double, it becomes easy to think in Mendelian terms, and the world of gametes, whose pairings have brought into existence the individuals we see, comes naturally and persistently before the mind. Henceforth we have to penetrate behind the visible appearances of the individual, and endeavour to reconstruct first those processes of cell-division which produced the germ-cells or gametes, distributing the characters or factors among them according to definite systems ; and then the subsequent process of union of those gametes pair by pair, in fertilisation to form zygotes, each developing and manifesting in its development those properties of structure, instinct and conduct conferred upon it by that particular complement of factors which its two original gametes contained. Next Chapter : III. NUMERICAL CONSEQUENCES AND RECOMBINATIONS : page 57. TABLE OF CONTENTS |