Similar, though smaller, differences were also found between boys and girls with normal complements of chromosomes.

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Women have two X chromosomes, and a normal copy of the gene will override the defective one.

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This dual life-cycle means that, besides disrupting normal cellular activities in the ways that other viruses do, a retrovirus may also disrupt the normal function of its host's chromosomes.

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Most normal cells in the human body contain 46 chromosomes, or bundles of genetic code.

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This happens during the production of sex cells, and its effect is to halve the number of chromosomes in a cell (the number returns to normal when two sex cells combine during fertilisation).

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Add extra copies of them to these organisms' chromosomes, or force the existing copies to produce more protein than normal, and life is prolonged.

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