Sex chromosomes in the human female are XX; whereas those of the male are XY. During the very early stages of gestation period, the development of the embryo is predisposed in certain directions by the possession of the sex chromosomes one contributed by the mother and the other by the father. The mother can contribute only X chromosome which predisposes towards the development of female structural, functional and temperamental characteristics, but the father can contribute either an X or a Y chromosome predisposing in the first instance to female or in the second to male structural, functional and temperamental characteristics. In short, the sex of the child to be born is governed by the chromosomal contribution of the father. A child conceived will develop into a female on receiving an $\mathrm{X}$ chromosome from the mother and an X chromosome from the father (XX), or into a male on receiving an $\mathrm{X}$ chromosome from the mother but a $\mathrm{Y}$ chromosome from the father (XY). Therefore, sex in humans is not polygenic.