"Is she your twin? Fraternal twins obviously can have all kinds of differences in their DNA, since they're developing from two completely separate fertilized eggs; they're as genetically similar as any pair of siblings, they just happen to share their mother's womb at the same time. Identical twins, though, they should be the same; they start out as the same egg, after all, but once they've split to form separate zygotes, each instance of cell division can introduce a slight change in the DNA of one zygote that won't have occurred in the other. And if that change occurs in the regulatory DNA for hair color, then the identical twins will have different colored hair," he said, running with the blonde-hair thing on purpose, "which I'm fairly certain you already knew."