ygous for the factor A, but heterozygous for B. All of its
gametes will contain A, but only one-half of them will contain B, _i.e._ it
produces equal numbers of gametes AB and Ab. Two such series of gametes
coming together must give a generation consisting of x AABB, 2x AABb, and x
AAbb, that is, reds and whites in the ratio 3 : 1. Lastly the red zygotes
of the constitution AaBb have the same constitution as the original red
made from the two whites, and must therefore when bred from give reds and
whites in the ratio 9 : 7. The existence of all these three sorts of reds
was demonstrated by experiment, and the proportions in which they were met
with tallied with the theoretical explanation.

The theory was further tested by an examination into the properties of the
various F_2 whites which come from a coloured plant that has itself been
produced by the mating of two whites. As Fig. 7 shows, these are, in
respect of their constitution, of five different kinds, viz. AAbb, Aabb,
aaBB, aaBb, and aabb. Since none of them produce anything but whites on
self-fertilisation it was found necessary to test their properties in
another way, and the method adopted was that of crossing them together. It
is obvious that when this is done we should expect different results in
different cases. Thus the cross between two whites of the constitution AAbb
and aaBB should give nothing but coloured plants; for these two whites are
of {48} the same constitution as the original two whites from which the
experiment started. On the other hand, the cross between a white of the
constitution aabb and any other white can never give anything but whites.
For no white contains both A and B, or it would not be white, and a plant
of the constitution aabb cannot supply the complementary factor necessary
for the production of colour. Again, two whites of the constitution Aabb
and aaBb when crossed should give both coloured and white flowers, the
latter being three times as numerous as the former. Without going into
further detail it may be stated that the results of a long series of
crosses between the various F_2 whites accorded closely with the
theoretical explanation.

From the evidence afforded by this exhaustive set of experiments it is
impossible to resist the deduction that the appearance of colour in the
sweet pea depends upon the interaction of two factors which are
independently transmitted according to the ordinary scheme of Mendelian
inheritance.