Charles Benedict Davenport

“ Actually we get all tailed in some cases; in others 25 per cent with no tail. Which hypothesis best fits the facts? Which is the more probable—that the 25 per cent recessive no-tail should produce a tail (as it were, out of nothing) or that the 25 per cent dominant tail inhibitor should be ineffective, permitting the development of a tail? It is clear that the ontogenetic failure of an inhibitor is easier to understand than the development of a character that is not represented at all in the germ-plasm. ”

“ In the last few decades the view has been widespread that characters can be built up from perhaps nothing at all by selecting in each generation the merely quantitative variation that goes farthest in the desired direction. ”

“ As to the inheritance of type, my statistics are not extensive enough to give a final answer, but if all types be grouped into those with straight and those with curved toes, then in crosses of straight-toed syndactyl and normal 33 per cent of the offspring were of the curved type, whereas in crosses of curved-toed syndactyls and normal 45 per cent were of the curved type. These averages depend on 22 and 15 individuals, respectively. They lead us to look for an inheritance of type when more extensive data shall be available. ”

“ The prize-winning birds—those which conform to the breeder's ideals—are only a small proportion of each family of offspring. For instance, the Ancona type of plumage, which is black, each feather tipped with white, has to be carefully sought for in the progeny of each Ancona pen. The same is true of the Silver Spangled and Golden Spangled Hamburgs. There is little true spangling in the first plumage; the darker chicks prove the best; that is, there is the same tendency to grow whiter with age that I have noted above. And, finally, only a few birds in any flock are even fairly good show birds. ”

“ An inspection of Table 33, which gives the distribution of grades of boot in the offspring constituting the first hybrid generation, might well lead to the conclusion that inheritance is here of a blending nature, or that, if either condition is dominant, it is the booted one, as suggested in my 47 report of 1906. On this hypothesis the offspring with no boot illustrate imperfection of dominance, and one would say that, in booting, dominance is very imperfect. ”

“ The definition of dominance on the ground of results meets at the outset with a difficulty the germ of which is observable in Mendel's cautious statement "ganz oder fast unverändert." Even Mendel observed that the hybrids between white-flowered and purple-red flowered peas have flowers less intensely colored than the darker parent. The experiments of the last seven years have shown that the "dominant" character is often very greatly changed—indeed, in extreme cases a blending of characters may occur—in the first generation. ”

“ Accepting, then, the general principle of the greater variability of the dominant character, we have demonstrated conclusively that high nostril, or rather the factor that determines high nostril, is dominant.
Comparing tables 45 to 54, we see that recessive parents are characterized by a low grade of nostril and they, of course, tend to produce offspring with a low grade. Similarly, dominants have a high grade and tend to produce offspring of the same sort, while heterozygous parents are of intermediate grade and their children have nostril grades that are, on the average, intermediate. ”

“ A positive character has a real ontogeny. But, as we have seen, the development of any character may be interrupted at any stage. Most aberrations among organisms are due to a retardation or failure of normal development. In human affairs we recognize this tendency in the terms "degenerates" and "defectives" (constituting from 2 to 4 per cent of the population) . Indeed, there are few persons who are not defective in some physical or psychical character. ”

“ If a heterozygous bird be mated to a recessive the variability of the offspring is much increased, owing to the occurrence in the progeny of both DR and RR individuals (table 40) . The offspring do not, to be sure, fall into two distinct and well-defined types, as in typical Mendelian cases; but one part of the range of variation agrees fairly with that of pure RR's, i. e., Brahmas, and the remainder with that of heterozygotes. ”

“ Silkies, that differences in the degree of development of the extra-toes are inherited. But the average condition of the toes in the offspring of second or later generation hybrids can not be used as evidence of inheritance of the degree of parental development of the toes, since these are dependent on the same basal cause, namely, the hidden gametic constitution of the parents. Despite the obscuration of imperfect dominance, polydactylism in poultry proves itself to be a unit-character that segregates. ”

“ Even in a pure strain a determiner does not always develop fully, and this is an important cause of individual variability. But in a heterozygote potency is usually more or less reduced. When the reduction is slight dominance is nearly complete; but when the reduction is great dominance is more or less incomplete and, in the extreme case, may be absent altogether. The series of cases of varying perfection of dominance described in this work illustrate at the same time varying potency. The extreme case is that of the rumpless fowl. The character in this case is an inhibitor of tail development. ”

“ Taking the Brahma crosses, which are by far the most numerous, we see, from tables 31, B and 33, that those F2 individuals that have a boot of grade 6 or higher are almost certainly extracted recessives (which are equivalent to pure-bred Dark Brahmas) . Those with a grade of 3 or even 4 and lower to 2 or even 1 are probably heterozygotes, while those with grade 0 and some of those with grade 1 are extracted dominants. In cases of doubt the distribution of grades in the offspring will give the deciding vote. ”

“ The reply is elaborated to the effect that, since dominance is due to the presence of a character and recessiveness to its absence, dominance may fail to develop, but recessiveness never can do so. Consequently two extracted recessives mated inter se can not throw the dominant condition; but two imperfect dominants, even though indistinguishable from recessives, will throw dominants. On the other hand, owing to the very fact that the dominant condition often fails of development, two extracted "pure" dominants will, probably always, throw some apparent "recessives." ”

“ The Black Minorca and White-faced Black Spanish have the gametic formula CJNwx. Owing to the absence of the graying factor and the presence of the color factor these appear as pigmented birds, but the supermelanic coat, N, obscures the Jungle coloration, so that the bird appears entirely black. Nevertheless the black is not of uniform quality, but just those parts of the feathers of the wing, back, hackle, saddle, and breast that are red in the Jungle fowl are of an iridescent black, while the portion that is not red in the Jungle is of a dead black. ”

“ In the last year or two the presence-and-absence theory has gained wide acceptance, but I still think the cases where there is dominance of the advanced condition over the less advanced—of the quantitatively well-developed over the quantitatively less well-developed—have not been sufficiently considered. In human hair-color any other hypothesis demands that there are many units in the higher grades of pigmentation and fewer in the lower grades and that the presence of the surplus factor in any other higher grade dominates over its absence in the next lower grade ”

“ Many black fowl gain white feathers as they grow older, and every fancier knows that birds with complex white-and-black patterns can usually be "exhibited" only once, on account of loss of "standard" coloration late in life. In these cases the advanced condition in the series of melanic colors appears only late in ontogeny. [13] Similarly Lang (1908, p. 54) finds that in snail hybrids often the young shells have the recessive yellow color, only later in life showing the dominant red color. This is, of course, no reversal of dominance in ontogeny, but mere ontogenesis of pigmentation. ”

“ The inhibition of a process is quite as positive a function of heredity as its initiation. The hair of a young rabbit grows until it attains a certain length and then the growth 87 ceases. The growing character is a youthful, embryonic one; the new character is the stoppage of growth. Similarly the young feathers of birds grow continuously until something intervenes that stops the growth and dries up the sheath. Now, in Angora rabbits and long-tailed fowl the epidermal organ continues its embryonic growth indefinitely; the something that intervenes to stop growth is absent. ”

“ In all recent birds the tail is a distinct but much reduced organ—the uropygium—which contains several vertebræ in a degenerate condition. The uropygium supports the tail feathers, which are of much use in directing the bird in flight, but in ground birds, such as the grouse and poultry, seem to function only for display in the male and, in the female, to facilitate copulation. ”

“ So well-nigh universal is heredity that it is justifiable to entertain a doubt whether any character may fail of inheritance. So far as my experience goes, non-inheritable characters are such as are weak in ontogeny, so that they may readily fail of development even when conditions are propitious; or else they are so complex—so far removed from simple unit-characters—that their heritability in accordance with established canons is obscured. The first case is apparently illustrated by the rumpless cock (No. 117) and the wingless fowl ”

“ The reason for the precocious development of black pigment over the belly and primaries of the hybrid chicks is probably the presence of an extension factor (cf. Castle, 1909) 73 derived from the Silkie. Certain it is that the ordinary Jungle pattern develops pigment on the belly and on the wings, as well as on other parts of the plumage. The hybrid chicks may be said to have the extended pigmentation dominant over interrupted pigmentation. In the adult hybrids a difference appears between the coloration of the male and female, even as Darwin pointed out. ”

“ Owing to the extreme imperfection of dominance it is rarely possible to say with certainty from inspection whether a given bird has germ-cells dominant or recessive, or 52 both, with reference to booting; only breeding enables us to make a decision. There is an exception, however, in the case of pure extracted recessives. They are distinguished by heavy booting and produce only booted offspring. I propose to give, in detail, the matings of these later generations and their progeny, the families being arranged in decreasing order of average grade of booting (table 38) . ”

“ The whites obtained in F2 are of 3 types, but in all alike the color factor is missing. Hence it can not reappear in the offspring, and, consequently, no colored offspring are to be expected. But, first, it must be stated that the extracted whites of the F2 generation are not always of a pure white. Indeed, the parent Silkies are in some cases not perfectly white, but show traces of "smoke." There are different degrees of albinism; the coloring enzyme may be absent to small traces. This variability in degree of albinism is familiar to all students of albinism in man. ”

“ Although the offspring resulting from a cross between any two of the forms employed are usually indistinguishable from the type which is dominant as regards the particular character crossed, yet in other cases the intensity of a dominant character may be more or less diminished either in particular individuals or in particular parts of one individual. ”

“ I bred a cock derived from the last year's pen, a bird that resembled much the male Dark Brahma (except that it was somewhat darker) , to sundry hens, hybrids between the Dark Brahma and Minorca—some of the first and some of a later hybrid generation, but all black except that some of the 1906 birds had a little buff on the breast and the primaries. The F1 (black) × F2 (Dark Brahma) gave 51 per cent black offspring, 27 per cent with a black-and-red Game pattern, and 22 per cent with the Dark Brahma pattern devoid of red. ”

“ It is probable, therefore, that syndactylism, under the conditions of the poultry-yard, has little life and death significance, but is one of those neutral characters whose existence Darwin clearly recognized. ”

“ Suffice it to say that white, buff, black, and Game-colored chicks all appeared in the F2 generation, as well as some with a mixture of colors, as called for by the hypothesis. White, due to the powerful graying factor, was the commonest color, buff and black were about equally common, and each about one-third as abundant as white, while Games, due to the hypostatic J factor, were about one-third as common as buff. ”

“ Plumage color, like hair color, varies greatly among domesticated animals. This diversity is, no doubt, in part due to the striking nature of color variations, but chiefly to the fact that the requisite variations are afforded in abundance. The principal color varieties, in poultry as in other domesticated animals, are melanism, xanthism, and albinism. ”

“ The facts presented by the authors support the idea that syndactylism is dominant rather than recessive, but they deny the application of Mendelian principles to this case. Actually, the foot deformities described by Lewis and Embleton are inherited much like syndactylism in poultry. No extracted normal (recessive) extremity produces the abnormal condition. Heterozygotes show much variation, from very abnormal to slightly abnormal (possibly perfectly normal?) appendages. Dominance is, indeed, much more potent than in poultry. ”