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).]

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