Even though sickle cell anemia can be a lethal disease, it persists because individuals with one allele for sickle cell anemia are resistant to malaria.
This phenomenon is an example of balanced polymorphism, where the heterozygous condition (having one normal allele and one sickle cell allele) provides a survival advantage in malaria-endemic regions, thus maintaining the allele in the population despite its harmful effects in the homozygous condition (when an individual has two sickle cell alleles).
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