Figure 4.—

Protected polymorphism of two sex-determination factors on different linkage groups. Recessive deleterious alleles on the ancestral y chromosome can prevent the W allele from spreading to fixation, leading to a permanent state of multifactorial sex determination (equilibrium frequencies indicated by the dashed lines) if the deleterious alleles have fixed on the y chromosome and recombination with the x has been fully repressed. Even a tiny amount of recombination ( here), however, introduces genetic variation on the y chromosome and purges its deleterious alleles. When this happens, W ultimately fixes and the ancestral XY system is lost. In this simulation, a locus segregating for deleterious alleles (parameters: , , , and deleterious mutation rate ; allele frequencies in male and female gametes are shown by dotted lines with open and solid triangles, respectively) is linked to Y and an autosomal sex-antagonistic locus (parameters: , , , , , and ; allele frequencies are not shown) is linked to W.