Why is the sickle-cell allele common among people who live in malarial regions?
The sickle cell allele is common among people who live in malarial regions because sickle cell trait helps to protect people against malaria. When people have the sickle cell trait (as opposed to full-blown sickle cell disease), they are more resistant to malaria. The shape of their red blood cells, which have hemoglobin that is affected by the sickle cell trait, makes it easier for the malaria parasite to be eliminated from the blood stream.
Sickle cell trait, then, confers some degree of protection from malaria. This means that natural selection will select for the sickle cell allele to some degree in malarial regions.