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Boy or girl?

A newly fertilised egg usually has 46 chromosomes, arranged in 23 pairs. One pair of these chromosomes decides whether the embryo will grow into a boy or a girl: the sex chromosomes, X and Y - named after their shapes. A fertilised egg with two X-chromosomes will grow into a girl, and one with one X and one Y will grow into a boy.

Girls have two X-chromosomes (left) while boys have one X- and one Y-chromosome (right).
Girls have two X-chromosomes (left) while boys have one X- and one Y-chromosome (right).
Applied Imaging Corporation / Corbis

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