Thomas Hunt Morgan [Public domain] via Wikimedia CommonsSex in mammals is genetically determined; females normally have two X chromosomes, and males normally have one X and one Y chromosome. But sometimes an individual with two X chromosomes is born with male genitalia, in a case of XX male sex reversal. The incidence of XX male sex reversal in humans is roughly 1 in 20,000–25,000 male births. Results from recent studies suggest that changes in expression of the gene Sox3 can cause complete XX male sex reversal in mice and may underlie some cases of unexplained XX male sex reversal in humans.
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