Phenomena of hilieritan 87 



in this case the mixed dominant-recessives P(P) arc always 

 distinguishable from the pure dominants DD. 



Results in Later Generations. — In the F 2 generation and in all 

 subsequent ones the pure dominants and the pure recessives al- 

 ways breed true when self- fertilized, whereas the mixed domi- 

 nant-recessives continue to split up in each successive generation 

 into pure dominants, mixed dominant-recessives and pure reces 

 sive in the proportion 1:2:1. The result of this is that with 

 continued self-fertilization the relative number of dominants and 

 recessives increases in successive generations, whereas the relative 

 number of mixed dominant-recessives decreases, and in a few gen- 

 erations 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 in- 

 crease and of recessives to decrease in successive generations ; an 

 equal number of pure dominants and pure recessives is pro- 

 duced in each generation (Fig. 27). 



"Purity" of Genu Cells. — 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-flowered 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 hy- 

 brid. But the fact that one-quarter of the progeny of this hy- 

 brid 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 

 parental generation. 



