Dictionary of Human Evolution and Biology

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Sickle Cell Trait

Relatively minor blood disorder caused by the possession of the hemoglobin S allele, a mutant form of a critical metabolic protein possessed by some individuals in one copy or dose; a heterozygote. The trait enhances the fitness in heterozygous carriers of the trait (AS) in an environment in which malaria is endemic, but reduces the fitness of homozygotes (SS), who have sickle cell disease. In carriers, 20%-40% of total Hb is HbS, but carriers should avoid hypoxia and under certain conditions can experience a sickling crisis.

See genotype.

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