So, remember how I blithely mentioned about the agouti gene, which restricts black pigmentation in horses to the points, ie nose, ears, feet and tail? Today I'm going to tell you how it works.
We covered ways in which proteins can fail to fold properly due to gene mutations on Day Fourteen. However, there are proteins which can fold properly at, say, 30 degrees celsius, but the increased molecular motion will make them fall apart at the 37-40 degrees that is normal mammalian body temperature.
So, if you have one of those mutations in a melanin-producing protein, you can see why the extremities of the body, where surface temperature is likely to be colder, are darker, because more of the melanin-producing protein is present in its working form.
The same effect can be seen in many mammals- Siamese cats are very well known, as are Himalayan rabbits and guinea pigs, all of which in their most common form are white with dark noses, ears, feet and tails.
And it explains why Siamese kittens are born pure white. Awww.
Temperature sensitive mutant plants and even moulds are a very useful biological model- it's one of the few ways to study the effects of a gene being turned on or off in the same individual organism as opposed to across a population.