Tortoiseshell describes a coat coloring found mostly in female cats. Cats of this color are mottled, with patches of orange or cream and chocolate, black or blue. They are sometimes called torties for short.
The term "tortoiseshell" is typically reserved for cats with brindled coats that have relatively few or no white markings.
Those that are largely white with red and brown patches (rather than a brindled aspect) are described as tortoiseshell-and-white (in the United Kingdom) or calico (in Canada and the United States). Tortoiseshells and calicos are not specific breeds of cat. The tortoiseshell markings appear in many different breeds as well as in non-purebred domestic cats. This pattern is especially preferred in the Japanese Bobtail breed.
Tortoiseshell cats have coats with patches of red, brown or black, chocolate, cream, or cinnamon. The size of the patches can vary from a fine speckled pattern to large areas of color. Typically, the more white a cat has, the more solid the patches of color. Dilution genes may modify the coloring, lightening the fur to a mix of cream and blue, lilac or fawn. The markings on tortoiseshell cats are usually asymmetrical. Occasionally tabby patterns of eumelanistic and pheomelanistic colors are also seen (these are often then called "tortie-tabby", "torbie" or "caliby"). Tortoiseshell also can be expressed in the point pattern.
Tortoiseshell and calico coats result from an interaction between genetic and developmental factors. The primary gene for coat color (C) is located on the X Chromosome and has two co-dominant alleles, Orange (XO) and Black (XB), that produce orange phaeomelanin and black eumelanin pigments, respectively. The cells of female cats, which like other mammalian females have two X Chromosome (XX), undergo the phenomenon of X-inactivation, in which one or the other of the X-chromosomes is turned off at random in each cell in very early development. The inactivated X becomes a Barr body. Cells in which the chromosome carrying the Orange (XO) allele is inactivated express the alternative Black (XB) allele. Cells in which the Black (XB) allele is inactivated express the Orange (XO) allele. Pigment genes are expressed in melanocytes that migrate to the skin surface later in development. In bi-colored tortoiseshell cats, the melanocytes arrive relatively early, and the two cell types become intermingled, producing the characteristic brindled appearance consisting of an intimate mixture of orange and black cells, with occasional small diffuse spots of orange and black.
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