is daughters.
Such birds bred together gave an F_2 generation containing chicks with the
full deep pigment, chicks without pigment, and chicks with various grades
of pigmentation, all the different kinds in both sexes.

[Illustration: FIG. 21.

Scheme to illustrate the result of crossing F_1 birds (_e.g._ Brown Leghorn
x Silky) with the pure Brown Leghorn.]

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