An enzyme catalyzes the production of the pigment responsible for dark fur color in Siamese cats and Himalayan rabbits. This enzyme does not function at higher temperatures; it is thermolabile. Rabbits raised at 5C are all black. If raised at 20C, they are white with black paws, ears, and noses; they are all white when raised at 35C. Explain the mixed black and white coloring at 20C.