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Chromosomes are present in the cells of all living organisms, in varying numbers and specific to each species. In the human being, there are 23 pairs of chromosomes contained in the nucleus of each of our cells: 22 pairs common to the two sexes and one pair called sex chromosomes. In the female, the chromosomes form a pair of identical chromosomes, the X chromosomes. In the male, the sex chromosomes are different: one of the chromosomes is an X chromosome and the other, much shorter, is called the Y chromosome.