Let's talk about you and me. We are humans, and as humans, we're mostly born as one of two biological sexes. Most of us are born with either a penis or a vagina, which usually corresponds to having either an X and a Y chromosome or two X's. Some of us, though, have just one sex chromosome or kick it with three (XXX, XYY, or XXY). A few people are even born XX with just a teensy bit of a Y chromosome that makes them, for the most part, present as outwardly male.