 It is easy to see why equation (4) is said to describe selection-mutation balance—natural selection is continually removing \(A_2\) alleles from the population, while mutation is continually re-creating them. Equation (4) tells us the equilibrium frequency of \(A_2\) that will be maintained, as a function of the rate of mutation from \(A_1\) to \(A_2\) and the magnitude of the selective disadvantage suffered by the \(A_2 A_2\) homozygote. Though equation (4) was derived under the assumption that the \(A_2\) allele is completely recessive, it is straightforward to derive similar equations for the cases where the \(A_2\) allele is dominant, or partially dominant. In those cases, \(A_2\)’s equilibrium frequency will be lower than if it is completely recessive; for selection is more efficient at removing it from the population. A deleterious allele that is recessive can ‘hide’ in heterozygotes, and thus escape the purging power of selection, but a dominant allele cannot.