There are 46 chromosomes in a human, 23 pairs. Everyone has a set of chromosomes from their father and a matching set from their mother. They include a pair of sex chromosomes. The mother's eggs always contain an X chromosome, while the father's sperm contains either a Y chromosome or an X chromosome. That determines the sex of the child. To produce sex cells (gametes), the stem cells go through a different division process called meiosis. This reduces the 23 pairs (diploid) to 23 singles (haploid). These, when combined by fertilisation, produce the new set of 23 pairs.