Sex chromosome complements of the mice with MSYq deficiencies and relevant control mice. (a) XYRIII control, illustrating the previously documented male specific gene content of the mouse Y chromosome. The short arm (shown expanded) carries seven single copy genes, one duplicated gene (Zfy), and multiple copies of Rbmy. MSYq carries multiple copies of the Ssty gene family. (b) XYTdym1 control. This male has a normal Y gene complement except that a 11 kb deletion has removed the testis determinant Sry; the Sry deletion is complemented by an Sry transgene located on an autosome. (c) The variant YRIIIqdel has a deletion removing about two-thirds of MSYq. (d) The variant YTdym1qdel has a large deletion removing about nine-tenths of MSYq, together with the small 11 kb deletion removing Sry (complemented by an Sry transgene). (e) XSxraY* X mice are male because of the presence of the YRIII short arm derived, sex reversal factor Sxraattached distal to the X pseudo-autosomal region (PAR). Sxracomprises most of the Y short arm except for a substantial reduction in copies of Rbmy. The Y* X chromosome is in effect an X chromosome with a deletion from just proximal to Amel (close to the X PAR boundary) to within the DXHXF34 sequence cluster adjacent to the X centromere. It provides a second PAR, which is essential in order to avoid meiotic arrest. CEN, centromere; kb, kilobase; TEL, telomere.