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 the 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.