Why do you think the sickle cell allele is still common in tropical regions?

Sickle-cell disease is common in tropical and sub-tropical regions because being a carrier, carrying a single sickle-cell mutation (sickle cell trait), affords some resistance to malaria

Explanation:

Sickle cell disease is caused by a recessive allele of the haemoglobin gene. ... Additionally, the single copy confers resistance to malaria - a disease most common in tropical regions (e.g. sub-Saharan Africa). So in these regions, people with sickle cell trait are more likely to survive and reproduce than those without. Malaria is prevalent in the lowlands of East Africa, and so the sickle cell allele became common because it provided protection against malaria. As a result, sickle cell disease (individuals h.o.m.ozygous for the sickle cell allele) also became common.

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