Tortoiseshell cat

Tortoiseshell is a cat coat coloring named for its similarity to tortoiseshell pattern. Like tortoiseshell-and-white or calico cats, tortoiseshell cats are almost exclusively female.^[1][2][3][4] Male tortoiseshells are rare and are usually sterile.^[a][6][4]

Tortoiseshell cats, or torties, combine two colors other than white, either closely mixed or in larger patches.^[2] The colors are often described as red and black, but the "red" patches can instead be orange, yellow, or cream,^[2] and the "black" can instead be chocolate, gray, tabby, or blue.^[2] Tortoiseshell cats with the tabby pattern as one of their colors are sometimes referred to as torbies or torbie cats.^[7]

"Tortoiseshell" is typically reserved for multicolored cats with relatively small or no white markings. Those that are predominantly white with tortoiseshell patches are described as tricolor,^[2] tortoiseshell-and-white, or calico (in Canada and the United States).^[8]

Tortoiseshell markings appear in many different breeds, as well as in non-purebred domestic cats.^[8] This pattern is especially preferred in the Japanese Bobtail breed,^[9] and exists in the Cornish Rex group.^[10]

Tortoiseshell cats have particolored coats with patches of various shades of red, grey, and black, and sometimes white. 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.^[11]

Occasionally tabby patterns of black and brown (eumelanistic) and red (phaeomelanistic) colors are also seen. These patched tabbies are often called a tortie-tabby, a torbie or, with large white areas, a caliby.^[11] Not uncommonly there will be a "split face" pattern with black on one side of the face and orange on the other, with a dividing line running down the bridge of the nose. Tortoiseshell coloring can also be expressed in the point pattern, referred to as a tortie point.^[11]

Leonard Doncaster was the first to prove that tortoiseshell is the female heterozygote of orange and black, the corresponding male being orange. In the course of his studies he discovered that the rare tortoiseshell male is often sterile.^[12]

Tortoiseshell and calico coats result from an interaction between genetic and developmental factors. The primary gene for coat color (B), for the colors brown, chocolate, cinnamon, etc., can be masked by the co-dominant gene for the orange color (O), which is on the X chromosome and has two alleles: orange (X^O) and not-orange (X^o) that produce orange phaeomelanin and black eumelanin pigments, respectively. Typically, the alleles are notated as an uppercase O for orange, or a lowercase o for not-orange. Tortoiseshell and calico cats are labeled X^OX^o, indicating O-gene heterozygosity. The (B) and (O) genes can be further modified by a recessive dilute gene (dd) which softens the colors.^[13] Orange becomes cream, black becomes gray, etc. Various terms are used for specific colors, for example, gray is also called blue, orange is also called ginger. Therefore, a tortoiseshell cat may be a chocolate tortoiseshell or a blue/cream tortoiseshell or the like, based on the alleles for the (B) and (D) genes.^[14]

Female cats are homogametic (XX) and undergo the phenomenon of X-inactivation,^[15][16] in which one of the X chromosomes is turned off at random in each cell in very early embryonic development.^[17] The inactivated X becomes a Barr body. Cells in which the chromosome carrying the orange (O) allele is inactivated express the alternative non-orange (o) allele, determined by the (B) gene. Cells in which the non-orange (o) allele is inactivated express the orange (O) 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; this produces the characteristic brindled appearance consisting of an intimate mixture of orange and black cells, with occasional small diffuse spots of orange and black.

In tri-colored calico cats, a separate gene interacts developmentally with the coat color gene. This spotting gene produces white, unpigmented patches by delaying the migration of the melanocytes to the skin surface. There are a number of alleles of this gene that produce greater or lesser delays. The amount of white is artificially divided into mitted, bicolor, harlequin, and van, going from almost no white to almost completely white. In the extreme case, no melanocytes make it to the skin and the cat is entirely white (but not an albino). In intermediate cases, melanocyte migration is slowed, so that the pigment cells arrive late in development and have less time to intermingle. Observation of tri-color cats will show that, with a little white color, the orange and black patches become more defined, and with still more white, the patches become completely distinct. Each patch represents a clone of cells derived from one original cell in the early embryo.^[18]

Male cats, like males of other therian mammals, are heterogametic (XY).^[19] The single X chromosome does not undergo X-inactivation, ergo coat color is determined by which O-gene allele is present. Accordingly, the cat's coat will be either entirely orange or melanistic (respectively X^OY or X^oY). Very rarely (approximately 1 in 3,000^[20]) a male tortoiseshell or calico is born; these typically have an extra X chromosome (XXY), a condition known in humans as Klinefelter syndrome, and their cells undergo an X-inactivation process like in females. As in humans, these cats often are sterile because of the imbalance in sex chromosomes.^[21] Some male calico or tortoiseshell cats may be chimeras, which result from fusion in early development of two (fraternal twin) embryos with different color genotypes; these torties can pass only one color to their offspring, not both, according to which of the two original embryos its testes are descended from. Others are mosaics, in which the XXY condition arises after conception and the cat is a mixture of cells with different numbers of X chromosomes.

• 
  Black tortoiseshell ("tortie") short-haired cat
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  Blue tortoiseshell Birman cat
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  Black tortoiseshell-and-white tricolor ("calico") cat
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  Black tortoiseshell tabby-and-white tricolor ("caliby") cat
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  Black colourpoint tortoiseshell ("tortie point") cat
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  Black and red mottling on the paw-pads of a tortoiseshell cat
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  An extreme case of slow melanocyte migration from the skin and fur of a tri-color calico cat
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  A 16-year-old female tortoiseshell Ragdoll cat. While tortoiseshell Ragdolls are actually colourpoint tortoiseshell-and-white (calico) in color, the nomenclature is different in this breed's descriptions of colors and patterns.^[22]

In the folklore of several cultures, cats with tortoiseshell coloration are believed to bring good luck.^[23] In Ireland, tortoiseshell cats are considered to bring good luck to their owners.^[23] In the United States, tortoiseshells are sometimes referred to as money cats.^[24] In Japan, tortoiseshell cats are considered to bring good luck against shipwrecks.^[23] There are some additional interpretations of the luck of tortoiseshell cats, such as the one in England that describes an announcement of misfortune when a strange tortoiseshell cat enters a house.^[23] In England, if a woman dreams of a tortoiseshell cat, it can be interpreted as a warning that she should take care of her so-called friends.^[23]

Some studies have found that people believe tortoiseshell cats are more likely to be aggressive and have owners report stronger prey interest^[25][26] - the slang term "tortitude" was coined in reference to this perceived behavior.^[27] There is, however, little existing scientific evidence on the matter.^[27][28] One study found that there was not a relationship between coat color and tameness.^[29] Based on varying study results, assumptions cannot be made between cat coat color and personality.^{[30][page needed][verification needed]}

• Calico cat
• Deaf white cat
• Tabby cat

• Atkins, Carla. Cats: An Owner's Guide (2003). San Diego, California: Thunder Bay Press. ISBN 1-59223-097-0

Wikimedia Commons has media related to Tortoiseshell cats.

Wikimedia Commons has media related to Calico cats.

• "Tortoiseshell Cat Tortoiseshell Color Pattern, Tortie Cat" at animal-world.com
• Tortoiseshell and tri-color cats
• Calico cat
• tortiecats.free.fr, website showing the diversity of tortie and calico coats