Abraxas grossulariata, the common currant moth, and (on the right) its paler lacticolor variety.
In their simplest expression the phenomena exhibited by Mendelian characters are sharp and clean cut. Clean cut and sharp also are the phenomena of sex. It was natural, therefore, that a comparison should have been early instituted between these two sets of phenomena. As a general 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
(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 F1 × F1, and also the expected ratio of equality when the heterozygote is crossed with the recessive.
(2) The female is heterozygous for a dominant factor (F) which is lacking in the male. The constitution of a female is consequently Ff, and of a male ff. This assumption is in harmony with the fact that the sexes are produced in approximately equal numbers.
(3) There exists repulsion between the factors G and F in a zygote which is heterozygous for them both. Such zygotes (FfGg) must always be females, and on this assumption will produce gametes Fg and fG in equal numbers.
