Vet Chat #2 Tortoiseshell cats

What is a tortoiseshell cat, and why do we love them so much?  Contrary to popular rumour (probably spread by the cats themselves!) tortis are not a distinct breed, but rather are named for their colouring, a mixture of orange and black. Calico cats are a version of tortoiseshell with white patches as well as orange and black. Variations include chocolate, blue or grey instead of black, and yellow or cream instead of orange. Whatever the final mix, it is unofficially recognised that tortis come with their own particular brand of loveable, crotchety, “don’t touch me” personalities. Why do we never see male tortoiseshells? The simple answer is that coat colour is coded on a sex-linked gene – read no further if you have no interest in animal genetics. But if you do, here is the story. “Orange” and “black” are two alternative alleles of a single gene that occurs on the X chromosome, not the Y chromosome. White colouring is coded elsewhere. Normal males can of course only be orange OR black (+/- white) as they have only one X chromosome …. that is, unless they have a genetic anomaly such as Klinefelter’s syndrome, where they have an XXY genotype instead of being XY. Cats that are XXY are sterile, and I have never seen one as far as I am aware. Female cats can express both colours, orange and black, but not as simply as you would think. At a very early point of development , and as a normal process that happens in all mammals including cats, one X gene in every embryonic cell is randomly “switched off” so that males and females all get to express the same amount of X-linked information. In cats, depending on how early the “switching off” happens, and whether or not they have both colour alleles, larger or smaller patches of fur become orange OR black. With the added white gene expressed (or not), this gives us our beautiful, grumpy, love-me-but-don’t-touch-me-till-I-say-so tortoiseshell moggies.

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