Sex cells are different than the body cells.
Sex cells contain 23 chromosomes. There are no pairs.
This is because the nucleus of the sex cell of a man and the nucleus of the sex cell of a woman fuse together.
This way the child has 50% of its genetic information form the father and 50% of the mother.
Pair 23 will determine the sex of the child. This depends of chromosome 23 of the spermcell is a X or a Y chromosome.