Genes and you
Your gene instructions are bundled up into 46 chromosomes, a set of 23 pairs. You have a full set in almost every cell of your body. However, the sperm cells of a man, and the egg cells of a woman have only 23 chromosomes each - one from each pair. So when an egg and sperm join during fertilisation, the new cell usually has a full set of 46 chromosomes. Occasionally, this process doesn't work properly and the new cell has an extra chromosome or a chromosome with a bit missing.