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Why can the sickle cell trait provide some protection against malaria?

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Individuals who carry the sickle cell trait—meaning they are heterozygous for the sickle cell gene (Hb^A Hb^S)—possess a survival advantage in regions where malaria is endemic.

When the malaria parasite, Plasmodium falciparum, infects the red blood cells of a person with the sickle cell trait, it induces a "sickling" effect in those cells. This process causes the infected cells to be destroyed, which effectively eliminates the parasites along with the cells.1

This phenomenon is a classic example of overdominance, or heterozygote superiority. In malaria-prone areas, individuals who are homozygous for the normal hemoglobin gene (Hb^A Hb^A) are more susceptible to severe or fatal malaria, while those who are homozygous for the sickle cell mutation (Hb^S Hb^S) suffer from severe sickle cell anemia. Consequently, the heterozygotes have a higher relative fitness because they are protected against the most severe effects of malaria while avoiding the life-threatening complications of sickle cell anemia.2