Blackwell Publishing

The theory of natural selection (part 2) - Why does sickle cell anemia persist at high frequencies?


The high frequency of sickle cell anemia in some human populations is the classic example of heterozygous advantage.

Sickle shaped blood cells (pictured opposite) are a near fatal genetic condition which is still maintained at a high frequency in certain populations. The concept of heterozygous advantage explains why.

Heterozygous advantage describes the case in which the heterozygote is fitter than both. The fitnesses can be written:


When heterozygotes are fitter than the homozygotes, therefore, natural selection will maintain a polymorphism. The result was first proved by Fisher in 1922 and independently by Haldane.

Heterozygous advantage is one of several possible explanations for the existence of genetic variability in natural populations.

Previous Next