I was discussing a piece of my dissertation with a group of other people from my department at a dissertation workshop last night, and some of the attendees raised some interesting cases that I'm curious how people would respond to.
Case A: Suppose an evil geneticist decides to play around with people's racial intuitions. One way to do that would be to modify the DNA of a human embryo who is the product of two white parents to give the genetic characteristics that would typically cause the visible characteristics commonly associated with black people. Would the child be black? This isn't a case of egg-switching, baby-switching, or anything like that. The child's biological parents are both white. Is the child therefore white?
Case B: God decides it would be fun to have two versions of Michael Jordan in the world and thus creates an exact duplicate of him. Is the duplicate black?
If the answer in either case is that the resulting person is black, then descent from black people isn't necessary for being black. One has only white ancestors, and the other has no ancestors. I think that would be pretty significant given that most people working in the philosophy of race think descent is a necessary condition (and many think it's even a sufficient condition).