The incidence of the sickle-cell trait in indigenous East Africans from 35 different tribes is recorded. All the tribes coming from regions where malaria is hyperendemic have an incidence of more than 10 per cent. with the trait, whereas in the tribes coming from areas where malaria is epidemic or absent the incidence of the trait is less than 10 per cent. Other evidence is quoted which indicates that severity of malaria is a very important factor in determining the frequency of the sickle-cell gene. This fact can be correlated with the observation that persons with the sickle-cell trait have a considerable natural resistance to infection with Plasmodium falciparum. The incidence of the sickle-cell trait in any population appears to be the result of a balance between at least two factors: the severity of malaria, which will tend to increase the frequency of the trait, and the rate of elimination of sickle-cell genes in subjects dying of sickle-cell anaemia.

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