Figure 1. The XO and XY*^X mouse models.

A. XY. The Y short arm (Yp) gene complement of an XY male (represented to scale in the magnified view) comprises seven single copy genes, two duplicated genes and one multi copy gene. The pseudoautosomal region (PAR) located distally on the Y long arm mediates pairing and crossing over with the X PAR during meiosis to generate the XY sex bivalent. B–D. The diminishing Yp gene complements for the three XO male mouse models that lack the Y long arm. B. XSxr^aO. The Yp-derived Sxr^a sex-reversal factor, attached distal to the X PAR provides an almost complete Yp gene complement. C. XSxr^bO. The Sxr^a-derived deletion variant Sxr^b has a 1.3 Mb deletion (Δ^Sxr-b) removing 6 single copy genes and creating a Zfy2/1 fusion gene spanning the deletion breakpoint (†). D. XOSry. This model has only one Y chromosome gene, namely the testis determinant Sry provided as an autosomally located transgene. E. Y*^X. This mini sex-chromosome is an X chromosome with a deletion from just proximal to Amelx to within the DXHXF34 repeat adjacent to the X centromere. † represents the deletion breakpoint. This X chromosome derivative has a complete PAR that can pair with the PAR of XSxr^a, XSxr^b or X to form a ‘minimal sex bivalent’. Scale bar for magnified views is 150 kb.