Teen athletes have traditionally been classified as "boys" or "girls" so that girls can participate in sports without having to compete against boys who are usually stronger and faster than girls. This system worked as long as everyone agreed on which athletes were girls and which athletes were boys. Today some disagree about who qualifies as girls and who qualifies as boys. One thing hasn't changed. There are two different genetic types of athletes. One type has two "X" chromosomes. The other has an "XY" combination of chromosomes. Athletes who have an "XY" combination of chromosomes are usually stronger and...