The genetic norm for the human genome is the presence of forty-six chromosomes in twenty-three pairs. One of each pair – half of your nuclear genetic composition – is donated by your mother and half by your father. Twenty-two of the pairs, while differing slightly from each other, have the same dual ‘form’ (a bit like pairs of vaguely matching black socks) while the twenty-third, the sex chromosomes, carry sex-related genetic information and are therefore quite distinct from each other (like mismatched socks of different colours).
