e
different to that produced by either when present alone. Thus we are led to
the conception of characters which depend for their manifestation on more
than one factor in the zygote, and in the present chapter we may consider a
few of the {44} phenomena which result from such interaction between
separate and distinct factors.

White x White
|
Red -------------- F_1
|
+----------+
Red White ------- F_2
(9) (7)

One of the most interesting and instructive cases in which the interaction
between separate factors has been demonstrated is a case in the sweet pea.
All white sweet peas breed true to whiteness. And generally speaking the
result of crossing different whites is to produce nothing but whites,
whether in F_1 or in succeeding generations. But there are certain strains
of white sweet peas which when crossed together produce only coloured
flowers. The colour may be different in different cases, though for our
present purpose we may take a case in which the colour is red. When such
reds are allowed to self-fertilise themselves in the normal way and the
seeds sown, the resulting F_2 generation consists of reds and whites, the
former being rather more numerous than the latter in the proportion of
9 : 7. The raising of a further generation from the seeds of these F_2
plants shows that the whites always breed true to whiteness, but that
different reds may behave differently. Some breed true, others give reds
and whites in the ratio 3 : 1, while others, again, give reds and whites in
the ratio 9 : 7. As in the case of the fowls' combs, this case may be
interpreted in terms of the presence and absence of two factors. {45}

White White
AAbb aaBB
/ \ / \
/ \ / \
Ab Ab aB aB gametes of parents
`-------'
Red F_1
AaBb
/ \
/ \
AB AB
Female gametes Ab Ab Male gametes
of F_1 aB aB of F_1
ab ab

Red in the sweet pea results from the interaction of two factors, and
unless these are both present the red colour cannot appear. Each of the
white parents carried one of the two factors whose interaction is necessary
for the production of the red colour, and as a cross between them brings
these two complementary factors together th