Final answer:
In Siamese cats, a temperature-sensitive enzyme is the factor that affects hair color, becoming active in cooler parts of the body, causing the darker coloration in these areas. Therefore, the correct option is A
Step-by-step explanation:
In Siamese cats, temperature affects a temperature-sensitive enzyme, which in turn influences hair color. The correct answer to the question is a. enzyme. This temperature-sensitive enzyme becomes active in the cooler extremities of the cat, leading to the distinctive darker coloration seen in the face, ears, paws, and tail of Siamese cats, while the warmer parts of the body have lighter fur.
Comparable to the situation in rabbits with the Himalayan phenotype, the enzyme in Siamese cats does not fold correctly at higher temperatures, preventing it from catalyzing the pigment production. Additionally, while the gene for coat color is located on the X chromosome in cats, it is not this gene but its product (the enzyme) which is directly impacted by temperature.