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.]

The interpretation of this case turns upon the constitution of the Brown
Leghorn hen, upon her heterozygous condition with regard to the two factors
F and I, and upon the repulsion that occurs between them when the gametes
are formed. Through an independent set of experiments this view of the
nature of the Brown Leghorn hen has been confirmed in an interesting way.
There are fowls which possess neither the factor for pigment nor the
inhibitory factor, which are in constitution ppii. Such birds when crossed
with the Silky give dark pigmented birds of both sexes in F_1, and the F_2
generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a
cock of such a strain crossed with a Brown Leghorn hen should give only
completely unpigmented birds. But if, as we have supposed, the Brown
Leghorn hen is producing gametes Fpi and fpI, the male birds produced by
such a cross should be heterozygous for I, {112} _i.e._ in constitution
ffppIi, while the hen birds, though identical in appearance so far as
absence of pigmentation goes, should not contain this factor but should be
constitutionally Ffppii. Crossed with the pure Silky, the F_1 birds of
opposite sexes should give an entirely different result. For while the hens
should give only deeply pigmented birds of both sexes, the cocks should
give equal numbers of deeply pigmented and slightly pigmented birds (cf.
Fig. 25). These were the results which the experiment actually gave, thus
affording strong confirmation of the view which we have been led to take of
the Brown Leghorn hen. Essentially the poultry case is that of the currant
moth. It di