Tortoiseshell describes a coat coloring found in cats. Cats of this colour are mottled, with patches of red and black, chocolate, or cinnamon. They are sometimes called torties for short.
The term "tortoiseshell" (also called calimanco or clouded tiger in North America) is typically reserved for cats with brindled coats that have relatively little or no white markings. Those that are largely white with red and black patches (rather than a brindled aspect) are described as tortoiseshell-and-white (in the UK) or calico (in the United States).
Both calico and tortoiseshell cats are almost always female.
Tortoiseshell coats are caused by a combination of specific genetic traits. In female cats, where this trait occurs almost exclusively, it is a result of X-inactivation, in which different patches of fur receive coding for different hair color due to the activation of an X chromosome from either the mother or the father. Calico coloring is a mix of phaeomelanin based colors (red) and eumelanin based colors (black, chocolate and cinnamon).
Coat coloration in cats is complex, and controlled by several genes. One gene involved has two alleles: the Orange allele, O, which is the dominant form, (XO), and produces orange fur; and the "Black" allele, "o", which is the recessive form, (Xo), and produces black fur.
For a cat to be a tortoiseshell or calico, it must simultaneously express both of the alleles, O and o, which are two versions of the same gene, located at the same locus on the X chromosome. Males normally cannot do this: they can have only one allele, as they have only one X chromosome, consequently virtually all tortoiseshell or calico cats are females. Occasionally a male is born (the rate is approximately 1 in 3,000). These may have Klinefelter's syndrome, carrying an extra X chromosome, and will almost always be sterile or they may be a chimera resulting from the fusion of two differently coloured embryos.
The spotting gene causes white patches to cover the colored fur. Although there is no genetic difference, the amount of white is artificially divided into mitted, bicolour, harlequin, and van, going from almost no white to almost completely white.
In normal female tortoiseshell cats and in Klinefelter males, the position of the patches depends on which X-chromosome is active in each cell and which is inactivated to become a Barr body.
In some cats, mainly calico cats with a prominently white coat, the texture of hair may change between colours.
Cats Are Not Peas: A Calico History of Genetics - by Laura Gould, Copernicus Books, ISBN 0-387-94796-5