Heterozygote superiority is the term used to describe the phenomenon when the heterozygous genotype is at more of an evolutionary advantage than either homozygous genotype. This is one mechanism by which genetic diversity is preserved. It generally applies when the homozygous recessive genotype is lethal, and ensures that the recessive allele does not disappear from the gene pool. Also, heterozygote superiority is responsible for heterosis, also known as hybrid vigor.

An example of heterozygote superiority can be found by studying human populations with alleles that, in a homozygous recessive form, cause sickle cell anemia. Under normal circumstances, this recessive allele would be selected against, and would quickly become quite rare. However, in some African tribes, as much as 45 percent of the tribe may be heterozygous for the gene that causes blood sickling. This is because the heterozygous genotype protects against malaria, which causes large numbers of illnesses and death in many regions of Africa. So the heterozygous genotype is actually selected for, and the recessive allele does not disappear.

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