Tortoiseshell is a coat coloration in domestic cats named for its similarity to tortoiseshell pattern. The coats of tortoiseshell cats, or torties for short, are two colors other than white in an asymmetrical distribution, either closely mixed ('brindled') or in larger patches. The two colors always consist of one eumelanistic (black, blue, chocolate, lilac, cinnamon or fawn) and one phaeomelanistic (red or cream) color. The most common tortoiseshell coloration is black tortoiseshell (black and red). Tortoiseshell can occur in combination with other cat coat patterns, such as tabby and colorpoints. Tortoiseshell cats with the tabby pattern in their eumelanistic color are tortoiseshell tabby cats, sometimes referred to as torbies.
Like the tricolored tortoiseshell-and-white or calico (in North American English) cats, tortoiseshell cats are almost exclusively female.Atkins (2003), p. 61 Male tortoiseshells are rare and are usually sterile.Atkins (2003), p.105 Tortoiseshell markings appear in many different cat breeds, as well as in non-purebred domestic cats. This pattern is especially preferred in the Japanese Bobtail breed.
Pigmentation
Tortoiseshell coats result from an interaction between genetic, developmental, and environmental factors. Coat colors in domestic cats are produced by the interaction of orange-based phaeomelanin (O) and black-based eumelanin (B) pigments. Tortoiseshell cats are bicolored and expresses a combination of both next to each other in their coat. The primary gene for cat coat color coloration (B) produces the brown-toned colors — black, blue, chocolate, lilac, cinnamon, and fawn. This gene can be masked by the co-dominant gene for the orange color (O), which produces the orange-toned colors — red and cream. The orange gene is located on the X chromosome and has two alleles: orange (XO) and non-orange (Xo), that produce the orange phaeomelanin and black eumelanin pigments, respectively. Typically, the alleles are notated as an uppercase O for orange, or a lowercase o for non-orange.
+Pigment color overviewPigmentAlleleGeneBasic (D)Dilution (d)phaeomelaninorangeOredcreameumelaninnon-orangeoBblackbluebchocolate lilacblcinnamonfawn
The (B) and (O) genes can be further modified by a recessive dilute gene (dd) which softens the basic colors. Red becomes cream, black becomes blue, chocolate becomes lilac, and cinnamon becomes fawn. All tortoiseshells form a combination of either two basic colors — red combined with black, chocolate or cinnamon — or two dilute colors — cream combined with blue, lilac or fawn. Therefore, a tortoiseshell cat may be a chocolate tortoiseshell (chocolate and red) or a blue tortoiseshell (blue and cream) or the like, based on the alleles for the (B) and (D) genes. However, due to genetic dominance, the most common tortoiseshell coloration is black tortoiseshell (black and red). Various terms are colloquially used for specific colors, for example, black is also called "brown", blue is also called "grey", red is also called "orange", "ginger", and "yellow". Even though there exist three dilute tortoiseshell colors, blue is sometimes colloquially referred to as "dilute", which is the most common dilute coloration.
Sex linkage Female cats are homogametic (XX) and undergo the phenomenon of X-inactivation, in which one of the X chromosomes is turned off at random in each cell in very early embryonic development. 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 bicolored 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. Tortoiseshell cats have a combination of orange-based O and black-based o on two XX-chromosomes; labelled as XOXo, indicating O-gene heterozygosity. Orange and black females have homozygous O-genes; labelled as XOXO (orange) and XoXo (black).
Male cats, like males of other therian mammals, are heterogametic (XY). The single X chromosome does not undergo X-inactivation, ergo coat color is determined by which O-gene allele is present. Accordingly, the male cat's coat will be either entirely orange (O; XOY ) or melanistic black (o; XoY).
Leonard Doncaster was the first to prove sex linkage of the tortoiseshell coat coloration; i.e. that tortoiseshell is the female heterozygote of orange and black (XOXo) with the corresponding male being orange (XOY). In the course of his studies he discovered that the rare tortoiseshell male is often sterile.
Very rarely (approximately 1 in 3,000) a male tortoiseshell 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. Some male tortoiseshell cats may be chimaeras, 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.
Variations
Color and distribution Tortoiseshell cats have particolored coats; either bicolored with parts of various shades of orange and non-orange pigments, or tricolor in which tortoiseshell is combined with white spotting. The size of the patches can vary from a fine brindled pattern to large patched areas of color. Typically, the higher degree of white spotting a cat has, the more distinct the different colored patches are. In general, the markings on tortoiseshell cats are asymmetrically distributed. Dilution genes modify the coloring, lightening the coat coloration from red with either black, chocolate or cinnamon to a mix of cream with either blue, lilac or fawn. These three dilute tortoiseshell colors can be affected by a dilute modifier resulting in apricot mixed with their three caramel variants. Furthermore, the all tortoiseshell colors can occur in combination with the silver inhibitor gene, resulting in their respective silver tabby or smoke versions.
Tricolor (tortoiseshell-and-white)
In tricolor cats, also known as tortoiseshell-and-white or in North American English as calico, a separate gene interacts developmentally with the tortoiseshell coat color genes. This white 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 divided by cat registries into the categories mitted, true bi- or tricolor, 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, known as "epistatic" or "dominant white" (genetically different from albinism). In intermediate cases, melanocyte migration is slowed, so that the pigment cells arrive late in development and have less time to intermingle. Observation of tricolor 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.
Pattern Apart from the different color combinations the tortoiseshell pattern also occurs in combination with other genetic cat coat patterns. The phaeomelanistic red and cream colors will always show a tabby pattern, even if they are genetically "solid" or "self" (meaning non-agouti, i.e. non-tabby). The eumelanistic color (black, blue, chocolate, lilac, cinnamon, and fawn) makes it possible to visibly determine whether a tortoiseshell cat is tabby or solid. The tortoiseshell tabbies are often called tortie-tabby, or torbie/y for short. In North America the combination of calico and torby, caliby, is used for tortoiseshell tabbies with large white areas. Tortoiseshell coloring can also be expressed in combination with one of the colorpoint restriction patterns, colloquially referred to as a tortie point.
Folklore
In the folklore of several cultures, cats with tortoiseshell coloration are believed to bring good luck. In Ireland, tortoiseshell cats are considered to bring good luck to their owners. In the United States, tortoiseshells are sometimes referred to as money cats. In Japan, tortoiseshell cats are considered to bring good luck against shipwrecks. 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. In England, if a woman dreams of a tortoiseshell cat, it can be interpreted as a warning that she should take care around her so-called friends.
Behavior Some studies have found that people believe tortoiseshell cats are more likely to be aggressive and with stronger prey interest—the slang term "tortitude" was coined in reference to this perceived behavior. There is, however, little existing scientific evidence on the matter. One study found that there was not a relationship between coat color and tameness. Based on various study results, assumptions cannot be made between cat coat color and personality or temperament.
Gallery
See also Calico cat Deaf white cat Tabby cat
References
Atkins, Carla. Cats: An Owner's Guide (2003). San Diego, California: Thunder Bay Press.
External links
Tortoiseshell and tricolor cats