crossing the F_1 moths with the
_lacticolor_ variety. And first we will take the cross _lacticolor_ female
x F_1 male. The gametes produced by the lacticolor female we have already
seen to be Fg and fg, while those produced by the F_1 male are fG and fg.
The bringing together of these two series of gametes must result in equal
numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, _i.e._ of
female _grossulariata_ and _lacticolor_, and of male _grossulariata_ and
_lacticolor_ in equal numbers. Here, again, the calculated results accord
with those of experiment. Lastly, we may examine what should happen when
the F_1 female is crossed with the _lacticolor_ {104} male. The F_1 female,
owing to the repulsion between F and G, produces only the two kinds of ova
Fg and fG, and produces them in equal numbers. Since the _lacticolor_ male
can contain neither F nor G, all of its spermatozoa must be fg. The results
of such a cross, therefore, should be to produce equal numbers of the two
kinds of zygote Ffgg and ffGg, _i.e._ of _lacticolor_ females and of
_grossulariata_ males. And this, as we have already seen, is the actual
result of such a cross.

Before leaving the currant moth we may allude to an interesting discovery
which arose out of these experiments. The _lacticolor_ variety in Great
Britain is a southern form and is not known to occur in Scotland. Matings
were made between wild Scotch females and _lacticolor_ males. The families
resulting from such matings were precisely the same as those from
_lacticolor_ males and F_1 females, viz. _grossulariata_ males and
_lacticolor_ females only. We are, therefore, forced to regard the
constitution of the wild _grossulariata_ female as identical with that of
the F_1 female, _i.e._ as heterozygous for the _grossulariata_ factor as
well as for the factor for femaleness. Though from a region where
_lacticolor_ is unknown, the "pure" wild _grossulariata_ female is
nevertheless a permanent mongrel, but it can never reveal its true colours
unless it is mated with a male which is either heterozygous for G or pure
_lacticolor_. And as all the wild northern males are {105} pure for the
_grossulariata_ character this can never happen in a state of nature.

[Illustration: FIG. 19.

Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn
cock. Black sex signs denote deeply pigmented birds, and light sex signs
those without pigmentation. The light signs with a blac