er a factor less than the
coloured bird.

[Illustration: FIG. 14.

Ears of beardless and bearded wheat. The beardless condition is dominant to
the bearded.]

{76}

A phenomenon sometimes termed irregularity of dominance has been
investigated in a few cases. In certain breeds of poultry such as Dorkings
there occurs an extra toe directed backwards like the hallux (cf. Fig. 15).
In some families this character behaves as an ordinary dominant to the
normal, giving the expected 3 : 1 ratio in F_2. But in other families
similarly bred the proportions of birds with and without the extra toe
appear to be unusual. It has been shown that in such a family some of the
birds without the extra toe may nevertheless transmit the peculiarity when
mated with birds belonging to strains in which the extra toe never occurs.
Though the external appearance of the bird generally affords some
indication of the nature of the gametes which it is carrying, this is not
always the case. Nevertheless we have reason to suppose that the character
segregates in the gametes, though the nature of these cannot always be
decided from the appearance of the bird which bears them.

[Illustration: FIG. 15.

Fowls' feet. On the right a normal and on the left one with an extra toe.]

[Illustration: FIG. 16.

Scheme to illustrate the inheritance of horns in sheep. Heterozygous males
shown dark with a white spot, heterozygous females light with a dark spot
in the centre.]

There are cases in which an apparent irregularity of dominance has been
shown to depend upon another character, as in the experiments with sheep
carried out by Professor Wood. In these experiments two breeds were
crossed, of which one, the Dorset, is horned in both sexes, while the
other, the Suffolk, is without horns in either sex. Whichever way the cross
was made the resulting F_1 generation was similar; the rams were horned,
and {77} the ewes were hornless. In the F_2 generation raised from these
F_1 animals both horned and hornless types appeared in both sexes but in
very different proportions. While the horned rams were about three times as
numerous as the hornless, this relation was reversed among the females, in
which the horned formed only about one-quarter of the total. The simplest
explanation of this interesting case is to suppose that the dominance of
the horned character depends upon the sex of the animal--that it is
dominant in the male but recessive in the female. A pre