In parts of equatorial Africa, where the malaria parasite is most common, the sickle- cell allele constitutes 20% of the B-hemoglobin alleles in the human gene pool. The
sickle cell trait provides an advantage against malaria compared to people with
normal hemoglobin. In the United States, the parasite that causes malaria is not
present, but African Americans whose ancestors were from equatorial Africa have
the sickle-cell B-hemoglobin allele. These differences in traits illustrate