What sex?

Half of your genes come from your father and half come from your mother. They are bundled up as a set of 46 chromosomes. One pair of these, the sex chromosomes, determines what sex you are. An embryo with one X- and one Y-chromosome will be a boy, and one with two X-chromosomes will be a girl. The presence of a Y-chromosome turns on a 'male switch' in the developing embryo.