THE STATES OF VARIABILITY 



the complex hetero2ygote) segregation cannot be prevented. This 

 segregation will be repeated regularly, and since the homozygotcs 

 are eliminated as parents in each generation, the population will 

 have the same composition each time after the first selection. It will 

 always be ^ AA :\ Aa :\aa. Selection is not lowering the potential 

 variability. No variability is being fixed, and selection is therefore 

 ineffective in changing the average phenotype. 



With one gene difference in operation, as in the case we have 

 been considering, there are only two states of variability. The 

 heterozygotes contain all the potential variability, while the differ- 

 ence between the phenotypes of the homozygotes always expresses 

 the full action of the gene. There are, however, more possibihties 

 where, as in polygenic variation, the genes have similar and 

 supplementary effects on the phenotype. Even in the absence of 

 dominance, different genotypes can, in such a case, give phenotypes 

 differing only to the extent by which the genes do not correspond 

 in effect. With genes of equal effect the same phenotype can be 

 produced by a number of genotypes. In particular, genetically 

 dissimilar homozygotes may, as we saw in Chapter 3, show like 

 phenotypes. 



Thus with only two genes of equal effect, the homozygotes 

 AAbh and aaBB will be alike, and will be intermediate betw^een 

 AABB and aabh in their phenotype (Fig. 15). Yet these two inter- 

 mediate homozygotes contain between them all the genetical 

 material necessary for the production of the whole range of variation 

 to which the two genes can give rise. They contain a new kind of 

 variability, the potential variability of homozygotes, as opposed to that 

 of heterozygotes. This new potential variability depends for its 

 existence on the genes A and b, or a and B, balancing one another 

 in action. 



Unlike the potential variability of heterozygotes, which not only 

 can, but must, be partly freed by segregation in the next generation, 

 the homozygotic potential will remain as such so long as cross- 

 breeding is absent or at least restricted to like homozygotes. Thus, 

 with close inbreeding, a population could be maintained which 

 consisted of pure lines uniformly of the same phenotype, yet 

 genetically of several kinds. The potential variability would be, as 

 it were, frozen in such a population. Only by the crossing of unlike 



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