Sickle-cell heterozygotes are generally asymptomatic under ordinary conditions and have increased resistance to severe malaria. The persistence of the sickle-cell allele in malaria-endemic regions best illustrates

Core Concept

In malaria-endemic regions, heterozygotes can have higher relative fitness than either homozygote: they receive protection from severe malaria without usually developing sickle-cell disease. This heterozygote advantage maintains both alleles in the population as a balanced polymorphism.