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?

It is said, that the harmful alleles get eliminated from population ov

1.	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» Sickle-cell anaemia still persists in the population despite being harmful because this mutation can also be beneficial in certain conditions. The mutant `Hb^(5)` 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.