TABLE 1.

Mating table

Frequency of sex	MA	Ma	mA	ma
MA	σ11	σ12	σ13	σ14
Ma		σ22	σ23 = σ14	σ24
mA			σ33	σ34
ma				σ44

A modifier of sex (M) alters the probability that a diploid individual reproduces sexually in a manner that depends on the absolute fitness of the individual. We assume that the modifier has the same effect in trans double heterozygotes, Ma/mA, as in cis double heterozygotes, MA/ma (σ₂₃ = σ₁₄), because both have fitness W_Aa. Similarly, we assume that the frequency of sex in individuals of genotype ij is the same as in those of genotype ji.