See, there's this biological phenomenon called chimerism, which is basically the opposite of identical twins? Instead of one fertilized egg producing two identical babies, two or more fertilized eggs produce one individual with a patchwork of different DNA. If one of the contributing zygotes is genetically male and the other is genetically female, chimerism can result in an intersex individual. And in people, one of the most common symptoms of chimerism is actually the person in question having more than one blood type running through their veins.