SELECTION AND VAIUABILITY 



homozygotes could it be freed, and even then not ininiediately 

 (Fig. 70). The first effect of crossing must be to produce hetero- 

 zygotcsin which tlie variabihty is still potential. Only in the second, 

 segregating, generation derived from these heterozygotes will the 



VARIABILITY 



GENOTYPES PHENOTYPES 



HOMOZYCOTIC 



UJ 



O 



0.1 



AAbb AND aaBB 



INTERCROSSED 



HETEROZYCOTIC 



PARTLY FREE 



1 



AaBb 



INBRED 



Segregating 

 Family 



1 

 I 



Fig. 70. — Part of the potential polygenic variability of heterozygotes is released, 

 so that it shows as free variation in the phenotypes, by segregation in one generation. 

 The potential variability existing in the differences between homozygotes, which 

 are genetically unlike but having a similar balance and hence similar phenotypes, 

 camiot be released in one generation. It must first be converted into heterozygotic 

 potential by intercrossing. For the purpose of exposition in the diagram the genes 

 A-a and B-b are assumed to be alike and additive in their action, and also to show 

 no dominance. Thus AAbb, aaBB and AaBb have the same phenotypc (neglecting 

 non-heritable variation), but the segregating family includes five phenotypes, in 

 the frequencies shown also by Fig. 15 (after Mather, 1943). 



more divergent phenotypes be produced and the variability become 

 partly free — only partly free because the balanced homozygotes will 

 themselves reappear as part of the segregating generation. 



The heterozygotic potential state is therefore an essential inter- 

 mediate step in the freeing of homozygotic potential variability. But 

 on the other hand, the crossing of the phcnotypically extreme 

 AABB and aahh gives AciBh from which AAbb and nnBB must 



280 



