As the cells of an early female embryo divide they randomely inactivate one of the two X chromosomes. By chance, some cells end up with an active X from their mother, and others the X chromosome they received from their father. As the embryo develops, the descendants of the early dividing cells will inherit the same paternal or maternal active X chromosomes and the specific gene forms on those chromosomes. On average, half of the cells express their maternal X and half express their paternal X. The result of X inactivation is a mosaicism of expressed X-linked traits.