e F_1 plants must all be red. As
this case is of considerable importance for the proper understanding of
much that is to follow, and as it has been completely worked out, we shall
consider it in some detail. Denoting these two colour factors by A and B
respectively we may proceed to follow out the consequences of this cross.
Since all the F_1 plants were red the constitution of the parental whites
must have been AAbb and aaBB respectively, and their gametes consequently
Ab and aB. The constitution of the F_1 plants must, therefore, be AaBb.
Such a plant being heterozygous for two factors produces a series of
gametes of the four kinds AB, Ab, aB, ab, and produces them in equal
numbers (cf. p. 36). To obtain the various types of zygotes which are
produced when such {46} a series of pollen grains meets a similar series of
ovules we may make use of the same "chessboard" system which we have
already adopted in the case of the fowls' combs.

+------+------+------+------+
|AB....|AB....|AB....|AB....|
|AB....|Ab....|aB....|ab....|
|......|......|......|......|
+------+------+------+------+
|Ab....|Ab |Ab....|Ab |
|AB....|Ab |aB....|ab |
|......| |......| |
+------+------+------+------+
|aB....|aB....|aB |aB |
|AB....|Ab....|aB |ab |
|......|......| | |
+------+------+------+------+
|ab....|ab |ab |ab |
|AB....|Ab |aB |ab |
|......| | | |
+------+------+------+------+

FIG. 7.

Diagram to illustrate the nature of the F_2 generation from the two
white sweet peas which give a coloured F_1.

An examination of this figure (Fig. 7) shows that 9 out of the 16 squares
contain both A and B, while 7 contain either A or B alone, or neither. In
other words, on this view of the nature of the two white sweet peas we
should in the F_2 generation look for the appearance of coloured and white
flowers in the ratio 9 : 7. And this, as we have already seen, is what was
actually found by experiment. Further examination of the figure shows that
the coloured plants are not all of the same constitution, but are of four
kinds with respect to their zygotic constitution, viz. AABB, AABb, AaBB,
and AaBb. Since AABB is homozygous for both A and B, all the gametes which
it produces must contain both of these factors, and such a plant must
therefore breed true to the red colour. A plant of the {47} constitution
AABb is homoz