cessive.

(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. {102}

[Illustration: FIG. 18.

Scheme of inheritance in the F_1 and F_2 generations resulting from the
cross of _lacticolor_ female with _grossulariata_ male. The character of
each individual is represented by the sex signs in brackets, the black
being _grossulariata_ in appearance and the light ones _lacticolor_.]

We may now construct a scheme for comparison with that on page 100 to show
how these assumptions explain the experimental results. The original
parents were _lacticolor_ female and _grossulariata_ male, which on our
assumptions must be Ffgg and ffGG respectively in constitution. Since the
female is always heterozygous for F, her gametes must be of two kinds, viz.
Fg and fg, while those of the pure _grossulariata_ male must be all fG.
When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote,
FfGg, is heterozygous for both F and G, and in appearance is a female
_grossulariata_. The zygote resulting from the fertilisation of an ovum fg
by a spermatozoon fG is heterozygous for G, but does not contain F, and
therefore is a male _grossulariata_. Such a male being in constitution
{103} ffGg must produce gametes of two kinds, fG and fg, in equal numbers.
And since we are assuming repulsion between F and G, the F_1 female being
in constitution FfGg, must produce equal numbers of gametes Fg and fG. For
on our assumption F and G cannot enter into the same gamete. The series of
gametes produced by the F_1 moths, therefore, are fG, fg by the male and
Fg, fG by the female. The resulting F_2 generation consequently consists of
the four classes of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In
other words, the sexes are produced in equal numbers, the proportion of
normal grossulariata to _lacticolor_ is 3 : 1, and all of the _lacticolor_
are females; that is to say, the results worked out on our assumptions
accord with those actually produced by experiment. We may now turn to the
results which should be obtained by