The harmful allele of sickle cell anemia has not been eliminated from human population. Such afflicted people derive some other benefit. Discuss.

Introduction to sickle cell anemia:

Sickle cell anaemia is characterised by the presence of defective haemoglobin molecules due to point mutation in the HbA allele.

The mutant allele, represented by HbS, results in altered amino acid (Glutamic acid by Valine at the sixth position) in the beta globin chain of haemoglobin.
Under low oxygen tension, the mutant haemoglobin molecule undergoes polymerisation resulting in the transformation of RBCs into rigid and sickle-shape. These RBCs become inefficient in carrying oxygen and moving through the narrow capillaries.

Selective advantage of HbS genes

Individuals with genotype HbSHbS suffer from sickle cell anaemia.

Although the allele HbS is harmful (can be lethal) in the homozygous condition but in the heterozygous condition, it provides resistance against malaria.

This is because the malarial parasite, Plasmodium sp., is an intra-erythrocytic parasite.

It is unable to multiply in sickle-shaped RBCs. The sickle cells have membranes which are stretched because of their unusual shape.

This membrane becomes porous and leaks nutrients that the parasites need to survive. The faulty cells get destroyed fast along with the parasites.
This makes the heterozygous sickle cell anaemic persons (carriers) resistant to malaria. Hence, the mutant allele is not eliminated from the population.