<P> Divergent sexual development, known as intersex, can be a result of genetic and hormonal factors . </P> <P> Most mammals, including humans, have an XY sex - determination system: the Y chromosome carries factors responsible for triggering male development . The "default sex," in the absence of a Y chromosome, is female - like . This is because of the presence of the sex - determining region of the Y chromosome, also known as the SRY gene . Thus, male mammals typically have an X and a Y chromosome (XY), while female mammals typically have two X chromosomes (XX). In humans, biological sex is determined by five factors present at birth: the presence or absence of a Y chromosome, the type of gonads, the sex hormones, the internal genitalia (such as the uterus in females), and the external genitalia . </P> <P> Chromosomal sex is determined at the time of fertilization; a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell . Gonadal sex refers to the gonads, that is the testis or ovaries, depending on which genes are expressed . Phenotypic sex refers to the structures of the external and internal genitalia . </P> <P> A human fetus does not develop its external sexual organs until seven weeks after fertilization . 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 do you become genetically male or female