Figure 1.

A proposed path for the evolution of the human sex chromosomes. Lahn and Page [6] postulate four inversions on the human Y chromosome, which suppressed recombination between the 'proto' sex chromosomes. Each inversion (designated 1-4) is thought to have reduced the size of the pseudoautosomal region (white) and enlarged the non-recombining portions of the X (yellow) and Y (blue) chromosomes. Time points at which the human X and Y may have diverged from the sex chromosomes of other mammals are indicated.