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Tortoiseshell is a coloring found in cats caused by a combination of specific genetic traits. It is a mix of phaeomelinin based colors (red) and euemelinin based color (black, chocolate and cinnamon). The pattern results in a cat with patches of red and patches of black, chocolate or cinnamon. The size of the patches can vary from a fine speckled pattern to large areas of colour. Dilution genes may modify the coloring, lightening the fur to a mix of cream and blue, lilac or fawn. Occasionally silver or ginger tabby patterns are also seen (sometimes called "torbies"). Tortoiseshell also can be expressed in the points pattern. The description "tortoiseshell" (also called 'calimanco' or 'clouded tiger' in North America) is correctly reserved for cats with coats that have no white markings, while those that are largely white with orange and black patches are described as calico (in the US) or tortoiseshell-and-white (in the UK). Tortoiseshells and calicos are not specific breeds of cat. The tortoiseshell markings appear in many different breeds.
Coat coloration in cats is complex. The genes involved include the Orange gene, O, which in its dominant form, O, produces orange fur, and in its recessive form, o, produces black fur.
For a cat to be calico, it must simultaneously express both of the two genes, O and o, which are located at the same location on the X chromosome. Males normally cannot do this: they can have only one allele, as they have only one X chromosome. Over 90% of tortoiseshell cats are females. Occasionally a male is born. These have Klinefelter's syndrome, carrying an extra X chromosome, and are almost always sterile.
The position of the patches depends on which X- chromosome is active in each cell and which is inactivated to become a Barr body.