HEEEDITY 



produce heterozygous conditions. Under self- 

 pollination for 1 generation following a cross, 

 half the offspring become homozygous; after 

 2 generations, % of the offspring are homo- 

 zygous ; after 3 generations % are homozygous, 

 and so on. So if the closest inbreeding is 

 practiced there is a speedy return to homo- 

 zygous, pure racial conditions. We know fur- 

 ther that in some cases at least heterozygotes 

 are more vigorous than homozygotes. The 

 heterozygous yellow mouse is a vigorous lively 

 animal; the homozygous yellow mouse is so 

 feeble that it perishes as soon as produced, 

 never attaining maturity. Cross-breeding has, 

 then, the same advantage over close-breeding 

 that fertilization has over parthenogenesis. It 

 brings together differentiated gametes, which, 

 reacting on each other, produce greater meta- 

 bolic activity. 



Inbreeding, also, by its tendency to secure 

 homozygous combinations, tends to bring to 

 the surface latent or hidden recessive charac- 

 ters. If these are in nature defects or weak- 

 nesses of the organism, such as albinism and 

 feeble-mindedness in man, then inbreeding is 



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