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X and Y sperm

A man's sperm contains 22 chromosomes as well as either an X- or a Y-chromosome. A woman's egg contains 22 chromosomes plus an X-chromosome. The sperm that fertilises the egg determines the baby's sex. If the sperm carries an X-chromosome the baby will be XX and will become a girl. If the sperm carries a Y-chromosome the baby will be XY, a boy.

The man's sperm determines a baby's sex.

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