Heterozygote advantage.
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   The  recessive  sickle-cell  allele  produces  hemoglobin with reduced
   capacity to carry oxygen.
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   This mutation also confers malaria resistance in heterozygotes.
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   This  heterozygote  advantage  leads  to  a  larger  proportion of the
   recessive allele than usual in areas where malaria is widespread.
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   These populations exhibit balanced polymorphism between the mutant and
   wild-type alleles.