All embryos go through the initial stages of growth and development in the same way. By the time an embryo is five weeks old, embryonic gonads have developed that can become either male or female gonads. One gene located on the Y chromosome determines if the embryo will develop male or female gonads. The SRY gene is only found on the Y chromosome. If the embryo has a Y chromosome, SRY gene expression will occur after 7 weeks. The protein coded by the SRY gene will cause the embryonic gonads to develop into testes. A fetus with testes will become male. If there is no expression of the SRY gene by week 13, the embryonic gonads develop into ovaries. A fetus with ovaries will become female. The embryo is by default female; the expression of the SRY gene causes a change in gonad development.

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