 A useful simplification of equation (3) can be achieved by making some assumptions about the genotype fitnesses, and adopting a new notation. Let us suppose that the \(A_2\) allele is completely recessive (as is often the case for deleterious mutants). This means that the \(A_1 A_1\) and \(A_1 A_2\) genotypes have identical fitness. Therefore, genotypic fitnesses can be written \(w_{11} = 1,\) \(w_{12} = 1,\) \(w_{22} = 1 - s,\) where \(s\) denotes the difference in fitness of the \(A_2 A_2\) homozygote from that of the other two genotypes. \((s\) is known as the selection coefficient against \(A_2 A_2)\). Since we are assuming that the \(A_2\) allele is deleterious, it follows that \(s \gt 0\). Substituting these genotype fitnesses in equation (3) yields: