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The bioloigcal sex is determined by the sex chromosomes x and y, for a girl they have ** and for a boy XY. These chromosomes are responsible for the development of the embryo into a biologically sexed male or female by triggering the development of glands. These glands then produce hormones that influence our external and internal genitalia. For a male it is the hormone tesosterone and for a female oestrogen. Up until three months, foestus' all appear externally female, as this point if the foetus has XY chromosomes the testes will normally produce tesosterone which causes external male genitalia to develope.

The role of hormones in gender development can be seen by studying individuals who have been exposed prenatally to hormone levels. Normally external genitalia are in accord with genetic sex. However, in some cases a genetic male embryo is exposed to too little male hormone and the result is tht the new born appears externally female. For a female foetus she may have been exposed to too much tesosterone which can lead to abnormal genitalia.