+------+------+------+------+ |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 homozygous 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.