Sex differences in embryos can be detected as early as the 2nd day after conception, with male embryos in both humans (Ray, Conaghan, Winston, & Handyside, 1995) and mice(Burgoyne et al., 1995) being observed to have more cells and a higher metabolic rate (Wilson & Davies, 2007). Around the 6th week post-conception, the SRY gene on the Y chromosome interacts with products of genes on the X chromosome and autosomes to stimulate the primordial fetal gonad to develop into testes (Koopman, 1999). Testicular hormones, including testosterone and Mullerian inhibiting hormone, are produced beginning between 12-16 weeks gestation, which then triggers the separate processes that will masculinize and defeminize the developing organism. In the absence of these factors, the fetus develops into a female.
