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 bad gene, because it will keep on reappearing by mutation. In this case, we can work out the equilibrium frequency of the mutation: the equilibrium is between the rate at which the mutant gene arises by recurrent mutation, and its elimination by natural selection. This is known as mutation-selection balance.
Mutation-selection balance can maintain a genetic polymorphism.
How can we calculate the equilibrium gene frequencies that result from mutation-selection balance?