ur, are incapable of giving
rise to zygotes with the capacity for further development. If we admit this
supposition, the scheme just given will cover such cases as those of the
currant moth and the fowl, equally as well as that of the pomace fly. In
the former there is repulsion between either the _grossulariata_ factor and
F, or else between the pigment inhibitor factor and F, while in the latter
there is repulsion between the factor for red eye and M.

[Illustration: FIG. 26.

Scheme to illustrate the probable mode of inheritance of colour-blindness.
The dark signs represent affected individuals. A black dot in the centre
denotes an unaffected female who is capable of transmitting the condition
to her sons.]

Whatever the merits or demerits of such a scheme it certainly does offer an
explanation of a peculiar form of sex limited inheritance in man. It has
long been a matter of common knowledge that colour-blindness is much more
common among men than among women, and also that unaffected women can
transmit it to their sons. At first sight the case is not unlike that of
the sheep, where the horned character is apparently dominant in the male
but recessive in the female. The hypothesis that the colour-blind condition
is due to the presence of an extra factor as compared with the normal, and
that a single dose of it will produce {118} colour-blindness in the male
but not in the female, will cover a good many of the observed facts (cf.
Fig. 26). Moreover, it serves to explain the remarkable fact that _all_ the
sons of colour-blind women are also colour-blind. For a woman cannot be
colour-blind unless she is homozygous for the colour-blind factor, in which
case all her children must get a single dose of it even if she marries a
normal male. And this is sufficient to produce colour-blindness in the
male, though not in the female.

But there is one notable difference in this case as compared with that of
the sheep. When crossed with pure hornless ewes the heterozygous horned ram
transmits the horned character to half his male offspring (cf. p. 71). But
the heterozygous colour-blind man does not behave altogether like a sheep,
for he apparently does not transmit the colour-blind condition to any of
his male offspring. If, however, we suppose that the colour-blind factor is
repelled by the factor for maleness, the amended scheme will cover the
observed facts. For, denoting the colour-blind factor by X, the gametes
produced b