Question
Most humans are homozygous for the A allele (β-hemoglobin locus), lacking the sickle-cell S allele. Populations in some regions of Africa and Asia also have appreciable frequencies of the S allele. So there are AA, SA and SS zygote combinations.
Individuals who are SS homozygotes suffer from a debilitating condition called sickle-cell anemia, which drastically decreases survival. But the S allele has not been eliminated by natural selection. Why?
Individuals who are SS homozygotes suffer from a debilitating condition called sickle-cell anemia, which drastically decreases survival. But the S allele has not been eliminated by natural selection. Why?
A Because there is an “overdominance” of the heterozygote SA carriers in central Africa where Malaria is common. It means in that area the heterozygote has better fitness than either homozygotes AA and SS (outside that area though it is better to be an unaffected homozygote AA).
B Because the S allele is reintroduced into the human gene pool by mosquitoes that have been encapsulated in amber and preserve the sickle cell anemia genes.
C Because Malaria is caused by the sickle cell anemia as long as mosquitoes are around. In areas where mosquitoes can’t survive, sickle cell anemia cannot be transmitted.
This question is taken from Bio 322 – Evolutionary Biology » Spring 2022 » Quiz 3 Genetics & Natural Selections
