CHAPTER VII
DOMINANCE
Primula flowers to illustrate the intermediate nature of the F1 flower when sinensis is crossed with stellata.
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. 12) 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 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 F2 generation bred true, while the intermediates 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 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 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 definite
proportions, viz., one black, two blues, one splashed white. This at once suggests that the black and the splashed white are the two homozygous forms, and that the blues are heterozygous, i.e., producing equal numbers of "black" and "white splashed" gametes. The view was tested by breeding 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 selection 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 determining 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 resemblance to one parent more than to the other, there are cases in which it is often possible to draw 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.
Diagram to illustrate the nature of the F2 generation from the cross between dominant white and recessive white fowls.
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 compared 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 development 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 recessive 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 white bird by I, 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 F1 bird must be CcIi. 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 I, they will produce in equal numbers the four different sorts of gametes CI, Ci, cI, ci. The result of bringing two such similar series of gametes together is shown in Fig. 13. Out of the sixteen squares, twelve contain I; these will be white birds either with or without a few coloured ticks. Three contain C but not I: these must be coloured birds. One contains neither C nor I; 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
deduced were found to accord with the actual facts of experiment. The F1 birds were all "ticked" whites, and in the F2 generation came white 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 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 case, for instance, is that of bearded and beardless wheats. Though the beard is obviously the additional 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 indistinguishable in appearance. Yet the one contains a factor more and the other a factor less than the coloured bird.
Ears of beardless and bearded wheat. The beardless condition is dominant to the bearded.
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 : 1 ratio in F2. But in other families similarly bred the proportions of birds with and without 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 always be decided from the appearance of the bird which bears them.
Fowls' feet. On the right a normal and on the left one with an extra toe.
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.
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 experiments 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. Whichever way the cross was made the resulting F1 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 proportions. 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 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 F1 ewes, while the factor should be lacking in all the gametes of the hornless F2 rams. A
hornless ram, therefore, put to a flock of F1 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 males are horned, while the heterozgyous 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 confirmed this expectation. Of the ram lambs 9 were horned and 8 were hornless, while all the 11 ewe lambs were completely destitute of horns.
Sheep