What is the most likely explanation for why Himalayan rabbits have dark fur only at their extremities?

The most likely explanation for why Himalayan rabbits have dark fur only at their extremities is that the protein product of the ch allele is only active at lower temperatures. This phenomenon is a classic example of temperature-sensitive allele expression. In these rabbits, the coloration is influenced by the ambient temperature.

At cooler temperatures, which are typically found at the extremities of the body such as the ears, paws, and nose, the enzyme responsible for producing the dark pigment (usually eumelanin) becomes active, leading to the development of dark fur in those areas. Conversely, the warmer body core does not activate this enzyme, resulting in a lighter color for the fur in those regions. This trait is an example of how genetics can interact with environmental factors to produce phenotypic variation.

Other options may suggest different mechanisms, such as dominance relationships at different temperatures or environmental factors influencing fur coloration. However, these do not adequately explain the specific pattern of dark fur restricted to the extremities, which is firmly rooted in the temperature-dependent expression of the allele. The correct choice highlights the underlying genetic principle that connects the specific allele's activity to the effects of temperature on fur pigmentation.