Individuals, who have one normal allele and one sickle cell allele, are able to resist malaria. This gives them a better chance of reaching reproductive age. This is known as impure breed advantage.
Heterotic balance (impure breed advantage ) polymorphisms expand whilst the health ofimpure breed is better than the health of each pure breeds in a given population. A traditional case of balanced polymorphism in human populations is sickle cell anemia.
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