Six Biological Markers

You probably learned that XX means female and XY means male. That's the headline, not the whole story. Biologists track six biological markers when they describe sex: chromosomes, gonads, hormones, internal anatomy, external anatomy, and brain structure. In most people these six line up neatly. But they are set at different stages of development, by different signals — so now and then they don't all agree.

By the broadest medical count, roughly 1 in 100 people are born with bodies that don't fit the standard male or female template. These intersex variations cover a wide range — extra sex chromosomes like XXY, cells that can't read testosterone, and dozens of other natural conditions. None of them is a mistake. Each is one of the ways human development actually plays out.