Figure 1.

History of the protected B6-POSA-Y^POS strains. The original POSA strain was a semi-inbred strain created when Naval Medical Research Institute (NMRI) Swiss females were intercrossed to M. musculus domesticus poschiavinus wild-derived males. A POSA male was backcrossed to B6 over 6 generations, and one of the B6-Y^POS males that was not sex-reversed was backcrossed onto the 129 background for 13 generations. Finally, a 129-Y^POS was backcrossed over >13 generations to B6, and at each generation only the most masculinized males were selected for breeding, creating the B6.POSA-Y^POS congenic line (previous studies referred to this line as B6.129), which was then further crossed to B6 females to create subcongenic chromosomes 1–13. In each backcrossing experiment, a region of the POSA genome remained, undetected, conferring protection from sex reversal (asterisk) on all breeding males. The shaded boxes follow the transmission of the Y^POS chromosome throughout the generations.