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 the human population. Why?

Answer 1

Introduction to sickle cell anaemia

Sickle-cell anaemia is an autosomal recessive disease carried and transmitted by both male and female individuals.
It is characterised by the presence of defective haemoglobin molecules due to point muation in the Hb${}^A$ allele, represented by Hb${}^S$, 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 Hb${}^S$ genes

Three genotypes possible in the population are Hb${}^A$Hb${}^A$ (normal, homozygous), Hb${}^A$Hb${}^S$ (normal, carrier) and Hb${}^S$Hb${}^S$ (diseased, dies before adulthood).
Althought the allele Hb${}^S$ 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.