<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> <P> A baby's sex is determined at the time of conception . When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female (XX) or male (XY). To be female, one needs to be (XX), whereas to be a male, (XY) is needed . It is the Y chromosome that is essential for the development of the male reproductive organs, and with no Y chromosome, an embryo will develop into a female . This is because of the presence of the sex determining region of the Y chromosome, also known as the SRY gene . </P> <P> A fetus doesn't develop its external sexual organs until the second month of pregnancy--seven weeks after conception . The fetus appears to be sexually indifferent, looking neither like a male or a female . Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs . This process is called sexual differentiation . The precursor of the internal female sex organs is called the Müllerian system . </P>

When did you become genetically male or female