Beyond XX and XY

The 1 in 100 figure covers all intersex variation. Zoom in on just the sex chromosomes, and a narrower pattern shows up: about 1 in 400 people carry a count other than plain XX or XY — an extra X, a missing one, an extra Y. Many never find out. A man with XXY chromosomes can live a completely typical life and only learn of it during fertility testing.

The takeaway: sex runs on several interacting systems at once. Chromosomes send the first signal, but hormones, receptor sensitivity, and the timing of gene activity all shape the final result. No single part decides alone.