48.

Explain natural selection on the basis of sickle-cell anaemia.

Sickle-cell disease is a hereditary disease affecting hemoglobin molecules in the blood. The disorder arises as a result of a single nucleotide change in the gene encoding B-hemoglobin, one of the key proteins used by red blood cells to transport oxygen. The sickle-cell mutation changes the sixth amino acid in the B-hemoglobin chain (position B6) from glutamic acid (very polar) to valine (nonpolar). The result is the deformed “sickle-shaped” red blood cell.

Persons homozygous for the sickle-cell genetic mutation in the b-hemoglobin gene frequently have a reduced life span. This is because the sickled form of hemoglobin does not carry oxygen atoms well, and red blood cells that are sickled do not flow smoothly through the tiny capillaries.
Heterozygous individuals, who have both a defective and a normal form of the gene, make enough functional hemoglobin to keep their red blood cells healthy.

In Africa the frequency of sickle-cell anaemia is quite high. this is because people who are heterozygous for the sickle-cell allele are much less susceptible to malaria, one of the leading causes of death in Central Africa.
Even though the population pays a high price—the many individuals in each generation who are homozygous for the sickle-cell allele die—the deaths are far fewer than would occur due to malaria if the heterozygous individuals were not malaria resistant.
Natural selection has favored the sickle-cell allele in Central Africa and other areas hit by malaria because the payoff in survival of heterozygotes more than makes up for the price in death of homozygotes. This phenomenon is an example of heterozygote advantage.