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.