http://www.sciencedaily.com/releases...1210125546.htm
Apparently some scientists in Europe have discovered that shutting off a gene called Foxl2, located on an autosome (or non-sex chromosome) in adult mice causes the cells in ovaries to become more like those found in testes, leading to speculation that this particular gene is required for an embryo to become female.
Current "common knowledge" (thanks, Jurassic Park) is that all embryos start out female and become male if there's a Y chromosome. It's interesting to think that it's possible that all female mammals actually only stay that way because of this Foxl2.