The frequency of the sickle-cell allele is higher is malaria prone areas. How can this be explained?

Answer 1

The sickle cell allele is developed in these parts of the world. If you or your ancestors are never exposed to malaria, there's no reason to have an immunity to it.

Answer 2

People who have one allele for sickle cell (causing sickle cell trait) have some defense against malaria. Those unfortunate to have two alleles for sickle cell have sickle cell anemia or sickle cell disease and may die from that. Those with no alleles for sickle cell can get very serious cases of malaria and die from the malaria. Those with one allele for sickle cell are more likely to survive, and that allele has a 50% chance of being passed to any offspring.