For a population to be under Hardy‒Weinberg equilibrium, the following five assumptions must be met. (1) There can be no selection, meaning that all genotypes must be equally likely to survive and reproduce within the population; if selection is operating, certain alleles or genotypes will be overrepresented in the next generation. (2) There can be no migration of individuals into or out of the population; if migration occurs, alleles from outside the population will change the population's allele frequencies. (3) There can be no mutations in the DNA sequence of any individuals in the population; mutation, like migration, will change allele frequencies. (4) There must be a sufficiently large population to avoid chance events altering the allele or genotype frequencies; in small populations, sampling error—genetic drift—will affect allele frequencies but, unlike selection, there will be no consistent direction of change from generation to generation (under selection, allele A may increase each generation, whereas, under drift, A might increase in generation one, and a in generation two, a again in three, and so on). (5) Individuals within the population must mate randomly with one another regardless of their genotype; nonrandom mating will affect allele and/or genotype frequencies. For a population that is under Hardy‒Weinberg equilibrium, known allele frequencies can be used to determine the genotype frequencies in that population using the Hardy‒Weinberg equation. According to this equation, where p represents the allele frequency of one allele and q represents the frequency of the second allele, the frequency of homozygous dominant individuals is given by calculating p2, the frequency of homozygous recessive individuals is given by calculating q2, and the frequency of heterozygous individuals is given by calculating 2pq. For example, in a population with a dominant allele frequency of 60% (p = 0.6) and a recessive allele frequency of 40% (q = 0.4), the genotype frequencies for homozygous dominant, homozygous recessive, and heterozygous individuals would be 0.36 or 36%, 0.16 or 16%, and 0.48 or 48%, respectively. Kenneth R. Miller, Levine2,590 explanations

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