Sex determination is decided by the 23rd pair of chromosomes commonly known as the sex chromosomes. Males have XY as their 23rd pair and females have XX. A female gamete, the ovum, can only receive only the X chromosome and the male gamete, the spermatozoon can receive either the X or the Y chromosome when meiosis occurs. Therefore the male sperm is responsible for the sex determination of the baby.

Example:
Figure 1: a Punnet square showing that there is an equal opportunity for a foetus to develop into either a male or female. It also shows how the male gamete determines the foetus sex by combining the X chromosome with either an X or Y chromosome.

Once the sperm fuses with the ovum both chromosomes will pair up and begin the first stages of cell division.

It won't be until the 6th week of gestation that we can...