Final answer:
Heterozygosity for SCD offers resistance to malaria by creating red blood cells that are less hospitable to the disease's parasites, providing a survival advantage in regions where malaria is prevalent, hence keeping the sickle cell allele prevalent in the gene pool.
Step-by-step explanation:
Benefit of Heterozygosity for SCD in Malaria-Prevalent Regions
Heterozygosity for sickle cell disease (SCD), characterized by the presence of one normal hemoglobin allele (βA) and one sickle cell allele (βS), offers a selective advantage in regions afflicted by malaria. This is because the presence of the sickle cell trait confers some resistance to malaria. Individuals with just one sickle cell allele have a modified red blood cell shape that is less hospitable to the Plasmodium parasites responsible for malaria, thus reducing the severity or likelihood of infection.
While individuals with two sickle cell alleles (homozygous individuals) suffer from severe health issues and gain little protection against malaria, heterozygous individuals carry only one allele for the sickle cell trait and generally maintain enough normal hemoglobin to function without severe sickle cell disease symptoms. Their modified red blood cells still allow for sufficient oxygen transport but are also inhospitable to the malaria parasite, which diminishes the parasite's ability to survive and replicate within the host. This modification ironically provides a survival advantage in malarial regions, thereby maintaining the presence of the sickle cell allele within the local gene pool.