and the two
homozygous forms appear renders the interpretation obvious. But the nature
of the F_2 generation may be much more complex, and, where we are dealing
with factors which interact upon one another, may even present the
appearance of a series of intermediate forms grading from the condition
found in one of the original parents to that which occurred in the other.
As an illustration we may consider the cross between the Brown Leghorn and
Silky fowls which we have already dealt with in connection with the
inheritance of sex. The offspring of a Silky hen mated with a Brown Leghorn
are in both sexes birds with but a trace of the Silky pigmentation. But
when such birds are bred together they produce a generation consisting of
chicks as deeply pigmented as the original Silky parent, chicks devoid of
pigment like the Brown Leghorn, and chicks in which the pigmentation shows
itself in a variety of intermediate stages. Indeed from a hundred chicks
bred in this way it would be possible to pick out a number of individuals
and arrange them in an apparently continuous series of gradually increasing
pigmentation, with the completely unpigmented at one end and the most
deeply pigmented at the other. Nevertheless, the case is one in which
complete segregation of the different factors takes {127}
[Illustration]place, place, and the apparently continuous series of
intermediates is the result of the interaction of the different factors
upon one another. The constitution of the F_1 [male] is a ffPpIi, and such
a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI,
fpi. The constitution of the F_1 [female] in this case is FfPpIi. Owing to
the repulsion between F and I she produces the four kinds of gametes FPi,
Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing
two such series of gametes together is shown in Fig. 28. Out of the sixteen
types of zygote formed one (FfPPii) is homozygous for the pigmentation
factor, and does not contain the inhibitor factor. Such a bird is as deeply
pigmented as the pure Silky parent. Two, again, contain a single dose of P
in the absence of I. These are nearly as dark as the pure Silky. Four
zygotes are destitute of P, though they may or may not contain I. These
birds are completely devoid of pigment like the Brown Leghorn. The
remaining nine zygotes show {128} various combinations of the two factors P
and I, being either PPIi, PPII, PpII, or PpIi, and in each of