Page:Popular Science Monthly Volume 85.djvu/333

Rh separated out from mixed dominant-recessives, D (R). The parental generation is indicated by the letter P, and the successive filial generations by F$1$, F$2$, F$3$, etc.

In the case of the peas studied by Mendel the hybrids of the F$1$ generation show only the dominant character, the contrasted recessive character being present but not expressed. However in certain cases it has been found that the hybrids differ from either parent and in successive generations split up into both parental types and into the hybrid type; thus Correns found that when a white flowered variety of Mirabilis, the four o'clock, was crossed with a red flowered variety all of the hybrids in the F$1$ generation had pink flowers and from those in the F$2$ generation there came white-flowered, pink-flowered and red-flowered forms in the proportion of 1 white:2 pink:1 red, as shown in Fig. 56. This is a better illustration of Mendel's principle of splitting than is offered by the peas, since in this case the mixed dominant-recessives D(R) are always distinguishable from the pure dominants DD.

In the F$2$ generation and in all subsequent ones the pure dominants, and the pure recessives always breed true when self-fertilized, whereas the mixed dominant-recessives continue to split up in each successive generation into pure dominants, mixed dominant-recessives and pure recessives in the proportion 1:2:1. The result of this is that the relative number of dominants and recessives increases in successive generations, whereas the relative number of mixed dominant-recessives decreases, and in a few generations a hybrid race will revert in large part to its parental types if continued hybridization is prevented. On the other hand there is no tendency for the relative number of dominants to increase and of recessives to decrease in successive generations; an equal number of pure dominants and pure recessives is produced in each generation.

With remarkable insight Mendel recognized that the real explanation of the splitting of pure recessives and pure dominants from hybrid parents must be found in the composition of the male and female sex cells. Since such extracted dominants and recessives breed true, just as pure species do, it must be that their germ cells are pure. In the cross between pure races of white and red-flowered Mirabilis the germ cells which unite in fertilization must be pure with respect to white and red, though the individual which develops from this cross is a pink hybrid. But the fact that one quarter of the progeny of this hybrid are pure white, and another quarter pure red, and that these thereafter breed true, proves that the hybrid produces germ cells which are pure with respect to red and white. Furthermore the fact that one half the progeny of this hybrid are themselves hybrid may be explained by assuming that they were produced by the union of germ cells carrying pure white and pure red, as in the first cross in the parental generation.