Sickle-cell anemia

The importance of selection for heterozygotes for maintaining polymorphism is not currently apparent. However, it is almost certain that at least some alleles are maintained in the population in this way. The best known example of selection for heterozygotes is selection for persons with sickle-cell anemia in areas affected by the occurrence of malaria, a disease caused by parasitic protozoa of the Plasmodium genus.  Sickle-cell anemia is a hereditary disease that appears in individuals with allele s, i.e. with an allele coding an abnormal β-chain of haemoglobulin. While the normal β-chain of haemoglobulin, coded by allele S, has glutamic acid in position 6, allele s codes valine in this position. This single substitution means that the blood cells containing abnormal haemoglobulin are deformed into a sickle shape at sites with lower oxygen partial pressure, i.e. in the capillaries, so that they are used up more rapidly and removed from circulation.  This effect is manifested drastically in s/s homozygotes, so that these individuals do not generally survive to reproductive age. s/S heterozygotes are also somewhat handicapped, but the reduction in their fitness compared to S/S homozygotes is not so significant.

The frequency of the occurrence of s alleles is highly positively correlated with the occurrence of malaria. It has been found that this is not a random correlation. Heterozygous s/S individuals are much more resistant against malaria than S/S homozygous individuals. The mechanism of this resistance is not exactly defined. As the protozoa develop in the red blood cells, it can be assumed that blood cells attacked by the parasite are deformed more readily than cells that are not attacked. Thus, they are more rapidly removed from circulation in the spleen, together with the parasite. Because malaria is a very serious disease that currently affects about 500 million people and that kills 1-2 million people every year, resistance against malaria provides a substantial selection advantage to heterozygotes. Thus, individuals with the s/S heterozygous genotype have the greatest fitness in areas with increased occurrence of malaria and selection for heterozygotes permanently maintains the presence of both alleles in the population. Malaria similarly maintains polymorphism in some other genes (Ruwende et al. 1995).See also Origin of Rh-blood group polymorphism.

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