Scheme to illustrate the nature of the F1 generation from the Silky hen and Brown Leghorn cock (cf. Fig. 23).
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. (1) a pigmentation factor (P); (2) a factor which inhibits the production of pigment (I); 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 I in the female zygote (FfIi), and (b) that the male Brown Leghorn is homozygous for the inhibitor factor (I), 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 grossulariata 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 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 ffppII, and all the F1 birds are consequently heterozygous for both P and I. 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.
Scheme to illustrate the nature of the F1 generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22).
In the reciprocal cross, on the other hand, we are mating a Silky male (ffPPii) with a Brown Leghorn hen which on our assumption is heterozygous for the inhibitor factor (I), and in constitution therefore is FfppIi. Owing to the repulsion between F and I the gametes produced by such a bird are Fpi and fpI in equal numbers. All the gametes produced by the Silky cock are fPi. Hence the constitution of the F1 male birds produced by this cross is ffPpIi 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 contain the factor for femaleness do not contain the
inhibitory factor owing to the repulsion between these factors. The nature of the F2 generation in each case is in harmony with the above scheme. As, however, it serves to illustrate certain points in connection with intermediate forms we shall postpone further consideration of it till we discuss these matters, and for the present shall limit ourselves to the explanation of the different behaviour of the F1 males and females when crossed with the Brown Leghorn. And, first, the cross of Brown Leghorn female by F1 male. The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi and fpI. The F1 cock is on our hypothesis ffPpIi, and produces in equal numbers the four kinds of gametes fPI, fPi, fpI, 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 I, and this is a female. The result, as we have already seen, is in accordance with the experimental facts.
