riment also showed that the pea comb (Fig. 4, A),
a form with a low central and two well-developed lateral ridges, such as is
found in Indian game, behaves as a simple dominant to the single comb. The
interesting question arose as to what would happen when the rose and the
pea, two forms each dominant to the same third form, were mated together.
It seemed reasonable to suppose that things which were alternative to the
same thing would be alternative to one another--that either rose or pea
would dominate in the hybrids, and that the F_2 generation would consist of
dominants and recessives in the ratio 3 : 1. The result of the experiment
was, however, very different. The cross rose x pea led to the production of
a comb quite unlike either of them. This, the so-called walnut comb (Fig.
4, D), {34} from its resemblance to the half of a walnut, is a type of comb
which is normally characteristic of the Malay fowl. Moreover, when these
F_1 birds were bred together, a further unlooked-for result was obtained.
As was expected, there appeared in the F_2 generation the three forms
walnut, rose, and pea. But there also appeared a definite proportion of
single-combed birds, and among many hundreds of chickens bred in this way
the proportions in which the four forms walnut, rose, pea, and single
appeared was 9 : 3 : 3 : 1.

Rose x Pea
|
+----+----+
Walnut x Walnut
|
+--------+---+---+--------+
Walnut Rose Pea Single
(9) (3) (3) (1)

Now this, as Mendel showed, is the ratio found in an F_2 generation when
the original parents differ in two pairs of alternative characters, and
from the proportions in which the different forms of comb occur we must
infer that the walnut contains both dominants, the rose and the pea one
dominant each, while the single is pure for both recessive characters. This
accorded with subsequent breeding experiments, for the singles bred
perfectly true as soon as they had once made their appearance. So far the
case is clear. The difficulty comes when we attempt to define these two
pairs of characters. How are we to express the fact that while single
behaves as a simple recessive to either pure rose, or to pure pea, it can
yet appear in F_2 from a cross {35} between these two pure forms, though
neither of them should, on Mendel's view, contain the single? An
explanation which covers the facts in a simple way is t