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 F_1 birds and the pure Brown Leghorn. The cross between the F_1 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 F_1 hen. But when the F_1 cock was mated to a Brown Leghorn hen, a definite proportion of the chicks, one in eight, was deeply pigmented, and _these deeply pigmented birds were always females_ (cf. Fig. 21). And in this respect all the F_1 males behaved alike, whether they were from the Silky hen or from the Silky cock. We have, therefore, the paradox that the F_1 hen, though herself deeply pigmented, cannot transmit this condition to any of her offspring when she is mated to the unpigmented Brown Leghorn, but that, when similarly mated, the F_1 cock can transmit this pigmented condition to a quarter of his female offspring though he himself is almost devoid of pigment. [Illustration: PLATE V. 1, 2, F_1 Cock and Hen, ex Brown Leghorn Hen x Silky Cock; 3, Silky Cock; 4, Hen ex Silky Hen x Brown Leghorn Cock.] {108} [Illustration: FIG. 22. Scheme to illustrate the nature of the F_1 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 {109} 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 F_1 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. [Illustration: FIG. 23. Scheme to illustrate the nature of the F_1 generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22).]