Chromosomes carry about 2000 genes. The genes determine characteristics like baby’s height, eye and hair colour.  When an egg is fertilised it has one sex chromosome from the mother and one from the father. The sex chromosome from the mothers egg is the X chromosome but the sex chromosome from the father can be either X or Y chromosome.

Normal human cells have 46 chromosomes. Male sperm and female eggs however have 23 chromosomes each. When a sperm fertilises an egg the 23 chromosomes from the father pair with the 23 form the mother to total 46. If the sperm has a Y chromosome the baby will be a boy (XY) and if the egg is fertilised by sperm containing X chromosome the baby will be a girl (XX).