Why should the X and Y chromosomes diverge, and why should the Y degenerate?
Clearly to maintain sex differences, recombination needs to be suppressed in the region of the major sex-determining locus (SRY in humans is located in a nonrecombining region just 5 kb proximal to the major pseudoautosomal region; see Figure 14.8). Additionally, environmental circumstances may have offered a selective advantage for breaking down recombination between the sex chromosomes. For example, one trigger could have been the development of sexually antagonistic genes, with alleles which may be of benefit to the heterogametic sex (XY), but harmful to the homogametic sex (XX). If such genes accumulate, then there will be a selective pressure to ensure that they are not transmitted to the homogametic sex, a restriction which can be met if they are present on a nonrecombining Y chromosome. Certainly, recombination between the present-day human X and Y chromosomes is very limited, being very largely confined to the tiny major pseudoautosomal region at the tips of the short arms (PAR
