88 Heredity and Environment 



ered and red-flowered forms in the proportion of I white : 2 

 pink: I red, as shown in Fig. 28. This is a better illustration of 

 Mendel's principle of splitting than is offered by the peas, since 

 in this case the heterozygotes D(R) are 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 heterozygotes 

 continue to split up in each successive generation into pure domi- 

 nants, heterozygotes and pure recessive in the proportion of 

 1:2: i. The result of this is that with continued self-fertiliza- 

 tion the relative number of dominants and recessives increases 

 in successive generations, whereas the relative number of hetero- 

 zygotes decreases, and in a few generations a hybrid race will 

 revert in large part to its parental types if continued hybridiza- 

 tion 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 

 (Fig. 27). 



"Pwity" of Germ 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 



