Heterozygote advantage

Natural selection increases or decreases allele frequencies, depending on their survival value (e.g., heterozygote advantage for the sickle cell mutation)... 

High Low Low Heterozygote advantage Sickle-cell anemia... 

It is presumably the ability to survive as a heterozygote, even with one or more highly deleterious mutations, that has led to the dominance of the diploid phase in higher plants and animals.76 However, to the biochemical geneticist organisms with a haploid phase offer experimental advantages because recessive mutants can be detected readily. 

Perhaps some genetic variants are maintained at high levels because of the hybrid vigor exhibited by heterozygotes. Other variants may be useful in different habitats or at different seasons. Still others may have a selective advantage when they are rare but become disadvantageous when they become too frequent (Powell and Dobzhansky, 1976). 

An interesting feature of the disease is that heterozygotes suffering from sickle cell trait are less severely affected by malaria than normal individuals, so that in malarial districts natural selection has favoured this condition since it confers a specific advantage in this particular environment. 

Up to date no such advantage can be demonstrated for the heterozygotes in Tay-Sachs disease, and no such explanation can be found in Niemann-Pick s disease. Specific studies have also failed to detect an increased fertility of heterozygotes. Under to-day s circumstances considering psychological and social factors an increased fertility could be expected to appear only in heterozygotes who were not aware of being carriers, in other words who had not produced sick children and had not experienced the disease in near relatives. 

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