ANN ARBOR, Mich. -- Think back to middle school biology class, when you learned that boys have an X and a Y chromosome inside each cell, and girls have two X's.
That small difference explains why boys are more at risk for a range of disorders -- from autism to hemophilia -- linked to genetic defects on their lone X chromosome. Meanwhile, most girls who carry the same defect on one X chromosome don't get sick, because their other X chromosome runs the show.