eneral
rule, the cross between a male and a female results in the production of
the two sexes in approximately equal numbers. The cross between a
heterozygous dominant and a recessive also leads to equal numbers of
recessives and of heterozygous dominants. Is it not, therefore, possible
that one of the sexes is heterozygous for a factor which is lacking in the
other, and that the presence or absence of this factor determines the sex
of the zygote? The results of some recent experiments would appear to
justify this interpretation, at any rate in particular cases. Of these, the
simplest is that of the common currant moth (_Abraxas grossulariata_), of
which there exists a pale variety (Fig. 17) known as _lacticolor_. The
experiments of Doncaster and Raynor showed that the variety behaved as a
simple recessive to the normal form. But the distribution of the dominants
and {100} recessives [Illustration]with with regard to the sexes was
peculiar. The original cross was between a _lacticolor_ female and a normal
male. All the F_1 moths of both sexes were of the normal _grossulariata_
type. The F_1 insects were then paired together and gave a generation
consisting of 3 normals : 1 _lacticolor_. But all the _lacticolor_ were
females, and all the males were of the normal pattern. It was, however,
found possible to obtain the _lacticolor male_ by mating a _lacticolor_
female with the F_1 male. The family resulting from this cross consisted of
normal males and normal females, _lacticolor_ males and _lacticolor_
females, and the {101} four sorts were produced in approximately equal
numbers. In such a family there was no special association of either of the
two colour varieties with one sex rather than the other. But the reverse
cross, F_1 female by _lacticolor_ male, gave a very different result. As in
the previous cross such families contained equal numbers of the normal form
and of the recessive variety. But all of the normal _grossulariata_ were
males, while all the _lacticolor_ were females. Now this seemingly complex
collection of facts is readily explained if we make the following three
assumptions:--

[Illustration]

(1) The _grossulariata_ character (G) is dominant to the lacticolor
character (g). This is obviously justified by the experiments, for, leaving
the sex distribution out of account, we get the expected 3 : 1 ratio from
F_1 x F_1, and also the expected ratio of equality when the heterozygote is
crossed with the re