Why are sickle cell anemia and thalassemia carrier tests associated with particular ancestral backgrounds?

The association between sickle cell anemia, thalassemia, and specific ancestral backgrounds is rooted in the evolutionary pressure exerted by malaria.123

In regions where malaria is or was endemic, individuals who carry one copy of the gene for these blood disorders (heterozygotes) possess a selective advantage. The presence of these variant hemoglobins provides a measure of protection against the lethal effects of the malaria parasite, which allows carriers to survive and reproduce more effectively than those without the trait. Consequently, these genes have been retained and are more frequent in populations with ancestors from areas where malaria has historically been prevalent.132

• Sickle cell anemia: The gene for sickle cell anemia is most common among people of African descent, as it provides protection against falciparum malaria.14
• Thalassemia: These disorders are most frequently found among people with ancestors from the Mediterranean Sea region, the Middle East, and southern Asia, where the gene has persisted due to similar evolutionary advantages against malaria.234