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Natural Selection: Balanced Polymorphism
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Sickle cell anemia has not been eliminated from the African population because -Sickle cell anaemia is caused by a recessive allele $Hb^S$; individuals homozygous for it ($Hb^SHb^S$) suffer from the severe disease.
Heterozygous individuals ($Hb^AHb^S$) are carriers; they are almost normal.
The malarial parasite Plasmodium cannot multiply well in the red blood cells of the carriers, because the infected cells sickle and are destroyed.
So in the malaria-affected regions of Africa the heterozygotes survive better than both the normal homozygotes (who die of malaria) and the sickle cell homozygotes (who die of anaemia).
Because of this advantage of the heterozygotes, natural selection maintains the sickle cell allele in the population; this is called balanced polymorphism.
So sickle cell anaemia has not been eliminated because it provides immunity (resistance) against malaria.