Question

The allele (a) for sickle cell anemia is recessive to the allele (A) that produces functional hemoglobin molecules. When an individual receives two copies of the sickle cell allele (aa), that individual produces misshapen red blood cells due to a mutation. The individual often has problems with oxygen circulation, filtering of the red blood cells in the spleen, and can suffer from chronic pain. Individuals with sickle cell anemia can die from this condition. What explains how the allele for sickle cell anemia is deleterious but can persist with certain populations of humans?
Responses
A The allele is decreasing in the population but has not fully been eradicated from the gene pool.The allele is decreasing in the population but has not fully been eradicated from the gene pool.
B The mutation is dominant in males and recessive in females, so it is carried by females without causing them to display any symptoms.The mutation is dominant in males and recessive in females, so it is carried by females without causing them to display any symptoms.
C The allele has vanished from the human population's gene pool only to reappear as an identical random mutation occurred with the same allele.The allele has vanished from the human population's gene pool only to reappear as an identical random mutation occurred with the same allele.
D The heterozygote (Aa) has functional hemoglobin within the red blood cells but is resistant to a deadly blood parasite that causes malaria. This is known as heterozygote advantage.

Answer 1

D. The heterozygote (Aa) has near-normal hemoglobin but partial resistance to malaria; this heterozygote advantage (a form of balancing selection) maintains the sickle-cell allele at appreciable frequencies in malaria-endemic populations despite the deleterious effects in aa homozygotes.