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We all inherit one of each pair of our chromosomes from our mother and one from the father, which protects us against genetic "mistakes". In the case of the X and Y chromosomes things are a bit different. We all have either two X chromosomes (females) or an X and a Y (males).

Having two Xs protects you from some genetic problems, eg red-green colour blindness, (an X-linked trait). If you have an X and Y you do not have this protection, which explains why such conditions are generally seen in males.