Sex Chromosomes
The sex of an individual is determined by two sex chromosomes.
Males have an X and a Y chromosome (XY).
Females have two X chromosomes (XX).
Sex determination
When a female (XX) produces gametes, the egg always contains an X chromosome. Males (XY) produce sperm cells with either an X or a Y chromosome. The sex of an individual is determined by which sperm fertilises the egg. If a sperm cell carrying a Y chromosome fertilises the egg, the child will be male (XY). If a sperm cell carrying an X chromosome fertilises the egg, the child will be female (XX).
Note the other chromosomes have not been shown here for simplicity