Why Tortoiseshells are Tortoiseshells

Tortoiseshell cats display a wide range of markings

Most people who are familiar with cats know about tortoiseshells: those patchy, red and black kitties. Not necessarily as well known is the fact that virtually all tortoiseshells are female. Why is that?

Torti with lots of black

In domestic cats, the basic black and red pigment genes are actually located on the X chromosomes, and are thus sex-linked. Since cats are like us and their sex is determined by the X-Y system, males have only one X chromosome, while females have two of them. A lot of cats will have all chromosomes of the same type, which will lead to the cat having all red pigment or all black pigment. However, the genes are also basically co-dominant, which leads to both colors being expressed in females that have both a red X and a black X. However, things are a bit more complicated than that.

The exact mechanism behind a tortoiseshell's patch coat is actually quite fascinating. As with all X-Y animals, females don't necessarily benefit from having more than one copy of the X chromosome. The extra dose of genetic material can actually be problematic, so the body undergoes what it needs to for what is called dosage compensation. Basically, the body only wants to work with one chromosome at a time in each cell. Due to this, cells will undergo X-chromosome inactivation, packaging the material into a tiny bundle that is not usable. This little bundle is known as a Barr body.

A cell nucleus with a Barr body. Xi is the inactive chromosome in a Barr body, while Xa marks the active one

Torti-tabby ("torbie") with lots of red

Barr body formation is directly linked to the patchy coloration of tortoiseshells. Where the cat has an active black chromosome, the fur will be black pigmented. Where the cat has an active red chromosome, the fur will be red pigmented. Also, the chromosomal inactivation happens at varying stages of development. After it happens, all cells that come from the initial chromosonally inactivated cell will be of the same type. So, if a chromosome become inactive very early in development, it will lead to a large patch of fur that's the same color. Later inactivation leads to smaller patches. This is how tortoiseshell cats can have such variation in the size of their patches.

A tortoiseshell with fairly even color distribution

Believe it or not, looking for Barr bodies is one of the easiest ways to determine if an individual has a sex chromosomal abnormality. Women with no Barr bodies are likely to be XO, while those with more than one would be XXX, XXXX, or higher. Males with Barr bodies are likely XXY, and multiple Barr bodies would be XXXY or higher. Of course, this only works for those with unusual numbers of X chromosomes and another sort of test must be done for multiple Y's.

A calico cat

Calico cats are almost exactly like tortoiseshell cats, with the only difference being the addition of the gene for white markings. 

If you see a male tortoiseshell or calico cat, it's likely to have a chromosomal abnormality such as Klinefelters syndrome (XXY) or some sort of mutation. There have been more than a few male tortoiseshells that have been found to be either chimeras, which have the DNA of two separate individuals, or mosaics resulting from some sort of mutation.

Also an interesting feature of tortoiseshells is that they cannot really be cloned. Since their coloration is based on random chromosome inactivation, the offspring will never look the same as its parent. In fact, it's more likely that the clone will be solid black or red rather than tortoiseshell since whatever cell that is collected from the donor will have already undergone the inactivation. Indeed, the first ever cloned cat, though cloned from a calico tabby, ended up a simple, uni-colored, black-pigmented tabby.

Sources are Texas A&M, Memorial University, Kimball's Biology, University of Miami, Messy Beast, Cat Fanciers' Association, Berkley, io9, and Dr. Sophia Yin. Images are from Wikimedia Commons under creative Commons licenses: one, two, three, four, five, six.