In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β haemoglobin alleles in the human gene pool.
In the United States, the parasite that causes malaria is not present, but it is present in African-Americans whose ancestors were from equatorial Africa. What should be happening to the sickle-cell allele in the United States, and what should be happening to it in equatorial Africa?
A) stabilising selection; disruptive selection
B) disruptive selection; stabilising selection
C) directional selection; disruptive selection
D) directional selection; stabilising selection

Question

Quizplus · Accepted Answer

In the United States, where malaria is not present, directional selection should decrease the frequency of the sickle-cell allele because it is disadvantageous without the presence of malaria. In equatorial Africa, stabilising selection maintains the sickle-cell allele at a higher frequency because it provides a heterozygote advantage against malaria.

In Those Parts of Equatorial Africa Where the Malaria Parasite