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The reproductive organs of boys and girls are determined by chromosomes (genes). Human cells have twenty-three pairs of chromosomes. One of each pair comes from your mother and the other from your father. The chromosomes determine how the body develops and functions. The length of each pair of chromosomes is identical with the exception of the sex chromosomes of the male. The chromosomes are numbered 1- 23. The mother will provide an X chromosome to all her eggs but the father provides either an X-chromosome from his mother or a Y-chromosome from his father. The sex chromosomes (X and Y) are the chromosome pair number 23.
Ordinary cells duplicate themselves completely, including the pairs of chromosomes, before dividing into two, by a process called mitosis. During production of egg cells and sperm cells, only one of each pair of chromosomes is replicated, this process being called meiosis. As a result, the gametes, the eggs and sperm, have twenty-three single chromosomes; they are not in pairs. The fertilised egg, which is now called the embryo, will have twenty-three pairs of chromosomes - the twenty-three from the egg and the twenty-three from the sperm. If the fertilised egg has two X-chromosomes, the resulting child will be a girl and if there is one X-chromosome and one Y-chromosome the child will be a boy. Until the eighth week after conception, there is no obvious difference between the external genital area of the two sexes. Over the next few weeks the differences become apparent.
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