Question

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

Answer 1

Sickle-cell anaemia still persists in the population despite being harmful because this mutation can also be beneficial in certain conditions.

The mutant Hb⁵ type haemoglobin is found at high frequencies (up to 20% and above) in the tropical Africa.

It is known that heterozygotes (Hb^s/Hb^A), having both types of haemoglobin show resistance to malarial infection because the body targets the Plasmodium falciparum-infected cell for destruction. In contrast, individuals homozygous for normal haemoglobin (Hb^A/Hb^A) suffer high mortality rates in early childhood due to malarial infection.

Thus, the allele for sickle cells has been maintained because heterozygotes have a higher reproductive success than either of the two possible homozygotes.