SINGLE LOCUS. “By this type of process, a single locus takes control of sex determination, that is, derived states with a single sex-determining gene can evolve. Single locus sex determination, such as that of mammals, with a single Y-linked gene, SRY, controlling sex, might also be able to evolve directly, but no detailed model has yet been worked out to show how this could occur.”
http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html