Probably! That or a mutation, anyway. But a few weeks ago I read about an interesting situation * in which a parent with AB blood and one with A blood can have an O child without adultery, because of another gene that sometimes suppresses the A and B antigens. That wouldn't allow for varying power levels with blood purity, and would sort of be still "one thing that makes you a wizard."

• I can't remember the name, and would appreciate if someone could remind me.