One would think that deleterious genes would be eliminated by natural selection, yet we have two human genetic disorders with a high carrier frequency, sickle-cell anemia and cystic fibrosis. Why has this high frequency for these conditions remained in the population?

Question

What will be an ideal response?

Answer 1

The distribution of the sickle cell allele matches the geographic distribution of malaria that is transmitted to humans by infected mosquitoes. Those who carry one or two copies of the sickle cell allele are resistant to malaria, and experiments on human volunteers have confirmed this. In heterozygotes and recessive homozygotes, the plasma membrane of red blood cells is resistant to infection by the malarial parasite. This resistance gives heterozygotes a better chance at survival and the opportunity to leave more offspring than those with a homozygous dominant genotype. Those with a homozygous recessive genotype are resistant to malaria but also have sickle cell anemia and have a much lower survival rate. In this case, natural selection favors the survival and differential reproduction of heterozygotes at the expense of both the recessive homozygous genotype and homozygous dominant genotype. Because of their resistance to malaria and the ability of heterozygotes to leave more offspring, the sickle cell allele is spread through the population and maintained at a high frequency.