PHENOMENA OF INHERITANCE 329 



separated out from mixed dominant-recessives, D (R). The parental 

 generation is indicated by the letter P, and the successive filial genera- 

 tions by F x , F 2 , F 3 , etc. 



In the case of the peas studied by Mendel the hybrids of the F t 

 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 x 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-reces- 

 sives 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 rela- 

 tive number of dominants and recessives increases in successive genera- 

 tions, whereas the relative number of mixed dominant-recessives de- 

 creases, 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. 



vol. lxxxiv. — 23. 



