In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β h
emoglobin 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?
In equatorial Africa, there were 20% sickle cell allele present in population. During the time of slave trade, large number of people transfers from equatorial Africa to US. Then the frequency of sickle cell allele changes in the overall US population due to migration of African people by the process of gene flow.
Gene flow:
Transfer of genetic variation from one population to another population is known as gene flow or allele flow or gene migration.
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.
Your question is rather unclear but from what I can tell either there were not an even amount of Petri dishes or some of them were left open while others were not.
