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Cats and genetics

Genetics is a tough subject, but there's a few examples of everyday cats that everyone can recognize that will help remember some of the more esoteric vocabulary terms.

First lets think of a Siamese cat. Siamese cats are have gray-white bodies, and black feet, faces, ears, and the tips of their tails are also black. Why?

Second lets think of Calico cats. Calico cats are generally white-appearing cats with black and orange solid spots. Why?

If you don't recognize those cats immediately, its probably time for a Google image search, but lets move into the explanations:

Siamese cats have black at the "ends" of their extremities and bodies. What would create color in a cat but only at the periphery? And what is different about the ends of our own bodies? Ever notice, especially in cold weather, that your fingers and toes are the first to feel numb? Blood flow into the extremities is more limited than throughout the main body cavity, and obviously the extremities are cooler.

This is also the case in Siamese cats, but the black color is actually the result of a heat-activated mutant gene. At the body's core, where temperatures are higher, the mutant is activated and the color is gray. At the extremities where its colder, the mutant is not activated, and the cat's color is black. In fact, during the winter months Siamese cats are "blacker". Siamese cats would actually be completely black cats if it weren't for this nifty heat-activated mutation.

Calico cats, on the other hand, have no real pattern to their coloring, and definitely can't be explained in relation to body heat differences. Since the coloring is "systemic" and unchanging regardless of how distal we get from the bodies core, there must be a problem with somatic cells throughout the body, and in fact there is. Female cats are XX, just like female humans. Since those X's are "identical", the cat only really needs one of them to maintain cellular function, and so one X will be turned off in every single cell (this "turned off X" is also known as the "Barr body").

It just so happens that the color genes between white, orange, and black are all located on the X chromosome. When one chromosome is turned off, the only X chromosome left activated will express the color in that area of the body. So different colored spots are the result of different X "turn-offs" in different locations of the body, because which chromosome becomes the Barr body is not preferentially selected (It's like a coin-flip for orange or black).