Evolution of mutation pair f→f-/a→A. The mutant A is introduced at low frequency in the genotype a/A;m/f-. Through interbreeding, a range of other genotypes are produced in subsequent generations (represented by arrows). Phenotypic sex (i.e. indicated above the arrows) of these novel genotypes depends on the values of k and h (see Figure 2(e)). Three potential outcomes are possible: transition to a new sex determining locus (Region I), transition coupled with a change in heterogamety (Region II), or recruitment of allele A without change in the sex determining locus (Region III).