A farmer intends to artificially inseminate cows with semen from a bull which has been chosen based on characteristics of colour and muscle mass - HSC - SSCE Biology - Question 10 - 2020 - Paper 1

The introduction of a recessive allele into a population through artificial insemination is best described as an example of gene flow. Gene flow refers to the transfer of genetic variation or alleles from one population to another. In this case, by introducing semen from a bull with a rare recessive allele, the farmer is adding new genetic material to the existing cow population.

Other options such as genetic drift and natural selection do not apply as they are processes that occur over time in relation to population genetics rather than a direct introduction of alleles. Selective breeding refers to the process where humans breed specific organisms for desired traits, which is not what is being illustrated in the context of the question.