Question

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

Answer 1

The sickle cell anemia is controlled by a single pair of allele; ${Hb}^A$ and ${Hb}^S.$
If both the parents are heterozygous $\left({Hb}^A{Hb}^S\right)$ then the offspring can suffer from this disease.
The offspring should be homozygous ( ${Hb}^S{Hb}^S$ ). Heterozygous individuals are carriers of this ofthis disease.
Heterozygous individuals are advantageous in terms of adaptation. Due to this, sickle cell anaemia is persisting in the human population.