In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β hemoglobin alleles in the human gene pool.
Considering the overall human population of the U. S. mainland at the time when the slave trade brought large numbers of people from equatorial Africa, what was primarily acting to change the frequency of the sickle-cell allele in the overall U. S. population?

gene flow

Explanation:

Gene flow, in simple terms, is a transfer of gene from one population to another. Also called as migration, where there is movement of individuals, and the genetic material they carry from one population to another. It also involves successful breeding of these individuals in their new locations. When people with sickle cell anemia were brought into the US, transfer of the gene responsible for sickle cell disease changed the frequency of the sickle cell allele in overall US population.