Typically in mammals, the gender of an organism is determined by the sex chromosomes. In the case of humans, this happens to be the X and the Y chromosomes. So as you may recall, if you are XX, you are female. If you are XY, you are male.
The gender of an organism is determined by the extra X or Y chromosome they receive from the father. They get the first X from their mother and the second from their father if it is a combination on two X's (XX) it is a female and it the combination is an X and a Y (XY) it means the organism is male.
