<P> Sexual differentiation in humans is the process of development of sex differences in humans . It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination . Sexual differentiation includes development of different genitalia and the internal genital tracts, breasts, body hair, and plays a role in gender identification . </P> <P> The development of sexual differences begins with the XY sex - determination system that is present in humans, and complex mechanisms are responsible for the development of the phenotypic differences between male and female humans from an undifferentiated zygote . Females have two X chromosomes, and males have a Y chromosome and an X chromosome . At an early stage in embryonic development, both sexes possess equivalent internal structures . These are the mesonephric ducts and paramesonephric ducts . The presence of the SRY gene on the Y chromosome causes the development of the testes in males, and the subsequent release of hormones which cause the paramesonephric ducts to regress . In females, the mesonephric ducts regress . </P> <P> Divergent sexual development, also known as intersexuality, can be a result of genetic and hormonal factors . </P>

What determines whether a person is male or female
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