BALANCE IN HOMOZYCOTES AND IIETEROZ YC OTES 



Balance in Homozygotes and Heterozygotes 



While inbreeding can maintain a state of uniform homozygosity 

 apart from the effects of mutation, outbreeding, as we have seen' 

 cannot of itself maintain a state of uniform and high heterozygosity, 

 except for such genes, or super-genes, or differential segments, as 

 control the breeding system itself The maximum proportion of 

 heterozygotes for any one gene with two allelomorphs will not 

 exceed one half in a large population. Yet, when all the genes 

 carried by a zygote are taken into account, clearly the chance of its 

 being completely homozygous is remote. Each genotype will be 

 a mixture of the heterozygous and the homozygous. The average 

 proportions of the mixture will depend on three factors, the breeding 

 system, the number of allelomorphs of each gene, and the relative 

 frequencies of these allelomorphs. 



This dual condition of the genotype has two consequences. At 

 any given time some of the combinations of genes are being exposed 

 to the test of natural selection in the homozygous condition. 

 Linkage wiU therefore be favoured in the way that we have already 

 seen. The whole set of genes, on the other hand, even those of one 

 chromosome, will virtually never be homozygous simultaneously. 

 The combinations of genes will therefore be adjusted or balanced to 

 perform their task always in a partly heterozygous condition. 



Now where more than one gene is involved, and each gene 

 shows dominance, the phenotype of a heterozygote will show no 

 predictable relation to those of the corresponding homozygotes. 

 Even with only two genes, the phenotype o(AaBh may fall between 

 those of AAhb and aaBB or may transgress their range in either 

 direction, according to the directions and strengths of the dominance 

 properties of A-a and B-h. Thus, where a polygenic system is 

 concerned, the favouring and balancing of partly heterozygous 

 genotypes by natural selection can offer no guarantee that the 

 corresponding homozygotes wiU be similarly balanced. Indeed in 

 view of the multipHcity of genotypes, all more homozygous than 

 the heterozygote from which they arise by segregation and 

 recombination, there is no reason why any one of them should 

 be automatically balanced. Smaller gene combinations forming 

 parts of the genotype may themselves be balanced as a result of 



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