Possible events in the evolution of the XY homology region. (a) Structure of the X and Y chromosomes of the human precursor, where the region is unique to the X chromosome at Xq21. As in Figs. 1 and 2, the asterisk (*) represents the region surrounding DXYS1 and the arrow represents the general order of markers sY20–sY52. (b) This region was then transposed to the Y chromosome at Yp11. (c) A paracentric inversion occurred on the Y chromosome to generate the structure in d, where the DXYS1 region was separated from the remainder of the XY homology region. (e) The X-specific region at DXS214 originated as an insertion into X or a deletion from Y, potentially via an L1-mediated mechanism (see text).
