"it just so happens that the direction for male pattern baldness is recessive. for it to happen, you must have the same pattern on both maps. the big problem is that this gene also happens to be on the part of the map where y is incomplete. males only get one shot at it, while females get two. this makes it much more common in males, as all things being equal, we'd expect to see it happen half the time in males and 1/4 of the time in females. this isn't exactly true, but this is a very simplified discussion, so let's roll with it. as a quick aside, you're probably wondering why you don't see bald females very often. it does happen, of course, and many females end up with two maps that have the directions for male pattern baldness. the problem is that the symptom of the disease is triggered by the presence of testosterone. this is a chemical that men's bodies produce in abundance, but that female bodies barely produce at all. this means that you mother could actually have the directions for this to happen, but you'd never know because there's not enough testosterone in her body to make it happen"