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Figure S1. Breeding strategy for backcrossing MF1Y⁻Sry mice onto the SJL background to yield the informative four core genotypes to dissociate sex chromosomes from gonadal sex. MF1Y⁻Sry mice are outbred mice that have a mutation in the Sry gene on the Y chromosome, and to which a transgenic Sry gene has been added back on an autosome. Males of the MF1Y⁻Sry strain or of subsequent generations are bred with female SJL mice. The breeding results in four types of mice. Two are phenotypic males, and two are phenotypic females. The phenotypic males both carry the Sry transgene, which determines testis formation, but differ in the number of X and Y chromosomes that they carry. Specifically, the XY⁻Sry male mice carry one dose of NPX genes and one dose of NPY⁻ genes (Table S1). The XXSry males carry two doses of NPX. Because the genes required for spermatogenesis are located on the NRY⁻, XXSry males are sterile. The XY⁻Sry, which are fully fertile, are therefore the male mice that are used in each subsequent backcross with SJL female mice.