Klinefelter syndrome is a genetic disorder where a male, instead of having just an X and a Y chromosome, has two Xs, and a Y. And those two Xs tend to feminise the man a bit, so they can have some breast growth. They often are quite tall, and they’re usually infertile. It’s a rare disorder, but as genetic disorders go, it’s by no means a terribly serious one.

Dr Norman Swan, Physician and Journalist