Explain how the ch (Himalayan) allele and tyrosinase control the Siamese coat-color pattern. What is the underlying reason that certain parts of a Siamese cat (e.g., tail and ears) are darker than the cat's trunk?
What will be an ideal response?
The tyrosinase enzyme produced by the hypomorphic ch (Himalayan) allele is unstable and is inactivated at a temperature very near the normal body temperature of most mammals. Thus, ch is a temperature-sensitive allele. The parts of cats that are farthest away from the core of the body (the paws, ears, tail, and tip of the nose) at most times tend to be slightly cooler than the trunk. At these cooler extremities, the temperature-sensitive tyrosinase produced by the ch allele remains active, producing pigment in the hairs there. However, in the warmer central portion of the body, the slightly higher temperature is enough to cause the tyrosinase produced by the ch allele to denature, or unravel. This inactivates the enzyme and leads to an absence of pigment in the central portion of the body. Animals that are chch or chc have the Himalayan phenotype. The final allele in the series, c, is a null allele that does not produce functional tyrosinase.
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