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begins in the womb.
All human individuals—whether they have an XX, an XY, or an atypical s*x chromosome combination—begin development from the same starting point. During early development the gonads of the fetus remain undifferentiated; that is, all fetal genitalia are the same and are phenotypically female. After approximately 6 to 7 weeks of gestation, however, the expression of a gene on the Y chromosome induces changes that result in the development of the testes. Thus, this gene is singularly important in inducing testis development. The production of testosterone at about 9 weeks of gestation results in the development of the reproductive tract and the masculinization (the normal development of male s*x characteristics) of the brain and genitalia. In contrast to the role of the fetal testis in differentiation of a male genital tract and external genitalia in utero, fetal ovarian secretions are not required for female s*x differentiation. As these details point out, the basic differences between the s*xes begin in the womb, and this chapter examines how s*x differences develop and change across the lifetime. The committee examined both normal and abnormal routes of development that lead individuals to become males and females and the changes during childhood, reproductive adulthood, and the later stages of life.