Human females have a pair of X chromosomes (XX) and produce gametes (ova) that all contain an X chromosome . Human males produce two types of sperms, 50% carrying an X chromosome and 50% carrying a Y chromosome . During fertilisation, the ovum (carrying the mother's X chromosome) can fuse with either a Y-carrying sperm to form a male (XY) or an X-carrying sperm to form a female (XX) . Therefore, the X chromosome from the mother is transmitted to both sons and daughters.