The allele responsible for sickle-cell anaemia first appeared in the tropical and subtropical regions of Asia, the Middle East and Africa. It entered the Un...

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The introduction of the sickle-cell allele into the United States due to the forced migration of individuals from Africa is an example of gene flow. Gene flow refers to the transfer of genetic material from one population to another through migration and interbreeding. In this case, the allele was transferred from African populations to the American population. This is an example of micro-evolution, which refers to small-scale evolutionary changes that occur within a population.