Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called x. But men have a mismatched pair in which one is a normal-sized x, while the other is a short one called y. So women are xx, while men are xy. As fig. shows half the children will be boys and half will be girls. And children will inherit an x chromosome from their mother regardless of whether they are boys of girls. Thus the sex of the children will be determined by what they inherit from their father. A child who inherits an x chromosome, from her rather will be a girl, and one who inherits a y chromosome from him will be a boy.
