in humans, sex is normally either male or female. for example: john's sex is male. jane's sex is female. most of the time, the sex of a person depends on what sex chromosomes that person got from his or her parents. a woman's ovum (egg cell) contains one x chromosome. a man's sperm contains either an x or a y chromosome. when a sperm and ova combine to form a fertilised egg, the baby may get either of these chromosomes from its father. if the baby gets two x chromosomes, it will develop into a female. if the baby gets one x and one y, it will develop into a male. before babies are born, their sex parts are already formed, but not ready yet. during puberty, the sex parts finish developing as well as other parts of the body like breasts and body hair.