Home > Your genes > Where did your genes come from? > Boy or girl?
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.
Enlarge
Girls have two X-chromosomes (left) while boys have one X- and one Y-chromosome (right).
Principal Funder:
Major Sponsors:
