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How did malaria influence the distribution of the sickle cell trait?

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The distribution of the sickle cell trait is significantly influenced by the presence of malaria, particularly in tropical regions where the parasite Plasmodium falciparum is endemic.12

While the sickle cell anemia allele (HbS) is deleterious in its homozygous form—often leading to severe health complications and reduced survival—it persists in these populations due to a phenomenon known as overdominance or heterozygote advantage.12

Individuals who are heterozygous for the trait (HbAHbS) possess a survival advantage in malarial environments. The presence of sickle cell hemoglobin in their red blood cells is detrimental to the malaria parasite, providing these individuals with increased resistance to the disease. Consequently, in regions where malaria is prevalent, natural selection favors the maintenance of the HbS allele because the fitness of the heterozygote exceeds that of both the homozygous normal (HbAHbA) and homozygous affected (HbSHbS) individuals. This creates a stable genetic polymorphism where the trait remains common despite the risks associated with the disease.123