Q. 15.  It is said, that the harmful alleles get eliminated from the population over a period of time, yet sickle-cell anaemia is persisting in the human population. Why?

Sickle-cell anaemia is an autosomal recessive disease caused by haemoglobins an oxygen-carrying protein in blood cells.

Despite the disease's lethal symptoms, it protects the carrier from malaria. Its allele are most common on the people of Africa descent (about 7% people of African descent carry an allele) and some other areas where malaria in prevalent.

It provides vital protection from malaria. The individual with HbAS heterozygotes tends to survive better than individuals with HbSS (homozygotes) as they are not exposed to the same severity of risk.