Heterozygous (Hb^AHb^S) individuals for sickle cell anaemia

The sickle cell trait provides a survival advantage against malaria fatality over people with normal hemoglobin in regions where malaria is endemic. The trait is known to cause significantly fewer deaths due to malaria, especially when Plasmodium falciparum is the causative organism. Lifecycle of Plasmodium involves entering biconcave RBCs. Since they are sickle shaped in these heterozygotes, it confers resistance to them.