Both the man's sperm and the woman's egg play a part in determining the gender of a baby. Every normal human cell contains 46 chromosomes, except for the male sperm and female eggs. They contain 23 chromosomes each. When a sperm fertilises an egg, the 23 chromosomes from the father pair with the 23 from the mother, making 46 in all. Chromosomes are tiny threadlike structures which each carry about 2,000 genes. Genes determine a baby's inherited characteristics, such as hair and eye colour, blood group, height and build. A fertilised egg contains one sex chromosome from its mother and one from its father. The sex chromosome from the mother's egg is always the same and is known as the X chromosome. But the sex chromosome from the father's sperm may be an X or a Y chromosome. If the egg is fertilised by a sperm containing an X chromosome, the baby will be a girl (XX). If the sperm contains a Y chromosome, then the baby will be a boy (XY).