In my college biology course, the professor came in one day carrying a Siamese cat by the scruff of its neck, and he had a put somber expression on his face so that the class would be afraid he was going to dissect it. It turned out, of course, that he was only going to talk about its biochemistry. The reason a Siamese has black extremities is that those are the coolest parts, and somewhere in the chain of reactions leading to its pigments there is a temperature-sensitive enzyme. If you keep a Siamese cat outdoors in the winter, or keep it in a refrigerator, it will turn black all over.