Homozygotes with two sickle-cell alleles are selected against because of mortality. however, heterozygotes for sickle-cell allele experience little effects of sickle allele and are more likely to survive malaria. the net effect of this exposure produced evolutionary change in areas where malaria is endemic by

Increased frequency of the allele for sickle cell anemia

Explanation:

Sickle cell anemia is fatal in homozygous genotype. The homozygous genotype having two copies of the allele for sickle cell anemia is not favored by natural selection due to its reduced survival success.

However, the heterozygous genotype for sickle cell anemia with one allele for the disease exhibit malaria resistant. The presence of one allele is not fatal for the organisms as the presence of one normal allele ensures the formation of some normal hemoglobin protein and RBCs to support the oxygen delivery to the body parts.

The increased survival success of heterozygous genotype in malaria-prone regions allowed their natural selection. This increased the frequency of allele for the sickle cell anemia in the gene pool of the local population and increased the frequency of heterozygous dominant genotype.

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