I would like to point out a few things, that I hope don't make me look to odd. First off, red hair is recessive. This means, even if both parents have red hair a child may often not. However, black hair and brown hair I believe are both dominate. JKR says on her site, that blood is a dominate gene. This means more likely then red hair, if both parents have it, a child will. This also means if you have even just a tiny bit somewhere down the line, a child might show up with it later on. So I like to think there's sometimes squibs out there who are like those fluke kids who have red hair when both their parents have brown. These squibs or flukes pass magic on down the genetic line. This means we end up with supposidely full blood "muggle" children having magic and it making no sense.