|Tortoiseshell cats display a wide range of markings|
|Torti with lots of black|
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|
|A tortoiseshell with fairly even color distribution|
|A calico cat|
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.