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 F_1 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 {110} inhibitory factor owing to the repulsion between these factors. The nature of the F_2 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 F_1 males and females when crossed with the Brown Leghorn. And, first, the cross of Brown Leghorn female by F_1 male. The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi and fpI. The F_1 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. [Illustration: FIG. 24. Diagram showing the nature of the offspring from a Brown Leghorn hen and an F_1 cock bred from Silky hen x Brown Leghorn cock, or _vice versa_.] On the other hand, the Brown Leghorn cock is on our hypothesis ffppII. All his gametes consequently contain the inhibitor factor, and when he is mated with an F_1 {111} hen all the zygotes produced must contain I. None of his offspring, therefore, can be fully pigmented, for this condition only occurs in the absence of the inhibitor factor among zygotes which are either homozygous or heterozygous for P. [Illustration: FIG. 25. Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen. For explanation see text.]