Question

Sickle cell anaemia is favoured by nature in a malaria-prone area. Which of the following category will be favoured and what type of selection is it?

• A. Normal individual, balancing selection
• B. Homozygous individual sickle cell, disruptive selection
• C. Heterozygous female, balancing selection
• D. Homozygous male, normalising selection

Answer 1

Answer: (C)

Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies above that of gene mutation.
Sickle cell anaemia is an abnormality of haemoglobin first reported by an American physician, James B. Herrick (1949) in Africa. In sufferers, RBCs become crescentic under oxygen deficiency conditions and clog blood capillaries causing death. The incidence of sickle cell anaemia is more widespread in regions malaria caused by Plasmodium falciparum and earlier it was believed that it was the only well-established case of balanced polymorphism. When forests were cleared to provide land for agriculture in Africa, it produced more breeding areas for mosquito, Anopheles gambiae, and caused spread of malaria. In this changed situation heterozygotes having both normal and sickle genes got selective advantage as their sickled RBCs could not support schizogony of malarial parasite but still carried enough oxygen under hypoxia conditions through normal RBCs. Dominant homozygotes suffered from malaria and died while recessive homozygotes carrying sickle gene died before reaching sexual maturity because of the inability of blood to carry enough oxygen and clogging of blood capillaries by sickled cells. Therefore, about 25% superiority of heterozygotes prevented elimination of the sickle gene and its frequency increased in malaria-prone areas.