Recurrent disadvantageous mutation

Natural selection will act to eliminate any allele that decreases the fitness of its bearers, but what about a recurrent disadvantageous mutation that keeps arising at a certain rate?

Selection can never finally eliminate the gene, because it will keep on reappearing by mutation. In this case, we can work out the equilibrial frequency of the mutation: the equilibrium is between the mutant gene's creation, by recurrent mutation, and its elimination by natural selection. This is known as mutation-selection balance.