The full genetic instructions for each person, known as the human genome, are carried by 23 pairs of chromosomes and consist of around 20,000-25,000 genes. An egg always contains one X chromosome, while sperm can carry an X or a Y chromosome. Whether your baby is a boy or a girl will therefore always be determined by the father. If a sperm carrying an X chromosome fertilizes the egg, the resulting embryo will be a girl. If a sperm with a Y chromosome fertilizes the egg, the resulting embryo will be a boy. In the male, both the X and Y chromosomes are active. In females, however, one of the two X chromosomes is deactivated early in the development of the embryo in order to prevent duplicate instructions. This could be the X chromosome from either the mother or the father.
