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Genetics responsible for the appearance of a cat's fur From Wikipedia, the free encyclopedia
Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. The variations among cat coats are physical properties and should not be confused with cat breeds. A cat may display the coat of a certain breed without actually being that breed. For example, a Neva Masquerade (Siberian colorpoint) could wear point coloration, the stereotypical coat of a Siamese.
The browning gene B/b/bl codes for TYRP1 (Q4VNX8), an enzyme involved in the metabolic pathway for eumelanin pigment production. Its dominant form, B, will produce black eumelanin. It has two recessive variants, b (chocolate) and bl (cinnamon), with bl being recessive to both B and b.[1] Chocolate is a rich dark brown color, and is referred to as chestnut in some breeds. Cinnamon is a light brown which may be reddish.
The sex-linked red "Orange" locus, O/o, determines whether a cat will produce eumelanin. In cats with orange fur, phaeomelanin (red pigment) completely replaces eumelanin (black or brown pigment).[2] This gene is located on the X chromosome. The orange allele is O, and non-orange is o. Males can typically only be orange or non-orange due to only having one X chromosome. Since females have two X chromosomes, they have two alleles of this gene. OO results in orange fur, oo results in fur without any orange (black, brown, etc.), and Oo results in a tortoiseshell cat, in which some parts of the fur are orange and others areas non-orange.[3] One in three thousand tortoiseshell cats are male, making the combination possible but rare- however, due to the nature of their genetics, male tortoiseshells often exhibit chromosomal abnormalities.[4] In one study, less than a third of male calicos had a simple XXY Klinefelter's karyotype, slightly more than a third were complicated XXY mosaics, and about a third had no XXY component at all.[4]
The pelt color commonly referred to as "orange" is scientifically known as red. Other common names include yellow, ginger, and marmalade. Red show cats have a deep orange color, but it can also present as a yellow or light ginger color. Unidentified "rufousing polygenes" are theorized to be the reason for this variance. Orange is epistatic to nonagouti, so all red cats are tabbies. "Solid" red show cats are usually low contrast ticked tabbies.[5]
The precise identity of the gene at the Orange locus is unknown. It has been narrowed down to a 3.5 Mb stretch on the X chromosome in 2009.[5]
The Dense pigment gene, D/d, codes for melanophilin (MLPH; A0SJ36), a protein involved in the transportation and deposition of pigment into a growing hair.[5] When a cat has two of the recessive d alleles (Maltese dilution), black fur becomes "blue" (appearing gray), chocolate fur becomes "lilac" (appearing light, almost grayish brown-lavender), cinnamon fur becomes "fawn", and red fur becomes "cream".[6] Similar to red cats, all cream cats are tabbies. The d allele is a single-base deletion that truncates the protein. If the cat has d/d genes, the coat is diluted. If the genes are D/D or D/d, the coat will be unaffected.[5]
| Basic color | Dilution | Dilute modifier, double dilution |
|---|---|---|
| Black ("brown") | Blue ("gray") | Caramel, blue-based caramel (UK) |
| Chocolate | Lilac | Taupe, lilac-based caramel (UK) |
| Cinnamon | Fawn | Fawn-based caramel (UK) |
| Red ("orange") | Cream | Apricot |
| Amber | Light amber | Unknown |
| White | N/A | N/A |
Tabby cats have a range of variegated and blotched coats, consisting of a dark pattern on a lighter background. This variety is derived from the interplay of multiple genes and resulting phenotypes. Most tabbies feature thin dark markings on the face, including the 'M' on the forehead and an eyeliner effect, pigmented lips and paws, and a pink nose outlined in darker pigment.[citation needed] However, the following different coat patterns are all possible:[citation needed]
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The agouti factor determines the "background" of the tabby coat, which consists of hairs that are banded with dark eumelanin and lighter phaeomelanin along the length of the hair shaft. The Agouti gene, with its dominant A allele and recessive a allele, controls the coding for agouti signaling protein (ASIP; Q865F0). The wild-type dominant A causes the banding and thus an overall lightening effect on the hair, while the recessive non-agouti or "hypermelanistic" allele a does not initiate this shift in the pigmentation pathway. As a result, homozygous aa have pigment production throughout the entire growth cycle of the hair and therefore along its full length.[10] These homozygotes are solidly dark throughout, which obscures the appearance of the characteristic dark tabby markings—sometimes a suggestion of the underlying pattern, called "ghost striping", can be seen, especially in bright slanted light on kittens and on the legs, tail and sometimes elsewhere on adults.
A major exception to the solid masking of the tabby pattern exists, as the O allele of the O/o locus is epistatic over the aa genotype. That is, in red or cream colored cats, tabby marking is displayed regardless of the genotype at the agouti locus. However, some red and most cream tabbies do have a fainter pattern when lacking an agouti allele, indicating that the aa genotype does still have a faint effect even if it does not induce complete masking. The mechanism of this process is unknown.
The Tabby locus on chromosome A1 accounts for most tabby patterns seen in domestic cats, including those patterns seen in most breeds. The dominant allele TaM produces mackerel tabbies, and the recessive Tab produce classic (sometimes referred to as blotched) tabbies.[11] The gene responsible for this differential patterning has been identified as transmembrane aminopeptidase Q (Taqpep, M3XFH7).[citation needed] A threonine to asparagine substitution at residue 139 (T139N) in this protein is responsible for producing the tabby phenotype in domestic cats. In cheetahs, a base pair insertion into exon 20 of the protein replaces the 16 C-terminal residues with 109 new ones (N977Kfs110), generating the king cheetah coat variant.[12]
The wild-type (in African wildcats) is the mackerel tabby (stripes look like thin fishbones and may break up into bars or spots). The most common variant is the classic tabby pattern (broad bands, whorls, and spirals of dark color on pale background usually with bulls-eye or oyster pattern on flank).[12] Spotted tabbies have their stripes broken up into spots, which may be arranged vertically or horizontally. A 2010 study suggests that spotted coats are caused by the modification of mackerel stripes, and may cause varying phenotypes such as "broken mackerel" tabbies via multiple loci. If the genes are Sp/Sp or Sp/sp the tabby coat will be spotted or broken. If it is an sp/sp gene, the tabby pattern will remain either mackerel or blotched. This gene has no effect on cats with a ticked coat.[11]
The Ticked (Ti) locus on chromosome B1 controls the generation of "ticked coats", agouti coats with virtually no stripes or bars. Ticked tabbies are rare in the random-bred population, but fixed in certain breeds such as the Abyssinian and Singapura.[13] TiA is the dominant allele that produces ticked coats; Ti+ is the recessive one. The causative gene for ticked tabby markings is Dickkopf-related protein 4 (DKK4).[14] Both a cysteine to tyrosine substitution at residue 63 (C63Y) and an alanine to valine substitution at residue 18 (A18V) result in decreased DKK4, which is associated with . Both variants are present in the Abyssinian breed, and the A18V variant is found in the Burmese breed.[13] Stripes often remain to some extent on the face, tail, legs, and sometimes the chest ("bleeding through"). Traditionally, this has been thought to happen in heterozygotes (TiATi+) but be nearly or completely nonexistent in homozygotes (TiATiA). The ticked tabby allele is epistatic to and therefore completely (or mostly) masks all the other tabby alleles, "hiding" the patterns they would otherwise express.[11]
It was once thought that TiA was an allele of the Tabby gene, called Ta, dominant to all other alleles at the locus.[15]
Tortoiseshells have patches of orange fur (pheomelanin based) and black or brown (eumelanin based) fur, caused by X-inactivation. Because this requires two X chromosomes, the vast majority of tortoiseshells are female, with approximately 1 in 3,000 being male.[17] Male tortoiseshells can occur as a result of chromosomal abnormalities such as Klinefelter syndrome, by mosaicism, or by a phenomenon known as chimerism, where two early stage embryos are merged into a single kitten.
Tortoiseshells with a relatively small amount of white spotting are known as "tortoiseshell and white", while those with a larger amount are known in the United States as calicos. Calicos are also known as tricolor cats, mi-ke (meaning "triple fur") in Japanese, and lapjeskat (meaning "patches cat") in Dutch. The factor that distinguishes tortoiseshell from calico is the pattern of eumelanin and pheomelanin, which is partly dependent on the amount of white, due to an effect of the white spotting gene on the general distribution of melanin. A cat which has both an orange and non-orange gene, Oo, and little to no white spotting, will present with a mottled blend of black/red and blue/cream, reminiscent of tortoiseshell material, and is called a tortoiseshell cat. An Oo cat with a large amount of white will have bigger, clearly defined patches of black/red and blue/cream, and is called a calico in the US.
With intermediate amounts of white, a cat may exhibit a calico pattern, a tortie pattern, or something in between, depending on other epigenetic factors. Blue tortoiseshell, or diluted calico, cats have a lighter coloration (blue/cream) and are sometimes called calimanco or clouded tiger.[18]
A true tricolor must consist of three colors: white, a red-based color like ginger or cream, and black-based color like black or blue. Tricolor should not be mistaken for the natural gradations in a tabby pattern. The shades which are present in the pale bands of a tabby are not considered to constitute a separate color.[19]
Tortoiseshell cats with small white patches are called tortillo cats, a portmanteau of Calico and Tortoiseshell.[21]
The KIT gene determines whether or not there will be any white in the coat, except when a solid white coat is caused by albinism. White spotting and epistatic white (also known as dominant white) were long thought to be two separate genes (called S and W respectively),[22] but in fact they are both on the KIT gene. The two have been combined into a single white spotting locus (W). White spotting can take many forms, from a small spot of white to the mostly-white pattern of the Turkish Van, while epistatic white produces a fully white cat (solid or self white). The KIT gene W locus has the following alleles:[22][23][24]
The colorpoint pattern is most commonly associated with Siamese cats, but due to crossbreeding may also appear in any (non-pedigree) domesticated cat. A colorpoint cat has dark colors on the face, ears, feet, and tail, with a lighter version of the same color on the rest of the body, and possibly some white. The exact name of the colorpoint pattern depends on the actual color. A few examples are seal points (dark brown to black), chocolate points (warm, lighter brown), blue points (gray), lilac or frost points (silvery gray-pink), red or flame points (orange), and tortie (tortoiseshell mottling) points. This pattern is the result of a temperature sensitive mutation in one of the enzymes in the metabolic pathway from tyrosine to pigment, such as melanin; thus, little or no pigment is produced except in the extremities or points where the skin is slightly cooler. For this reason, colorpoint cats tend to darken with age as bodily temperature drops; also, the fur over a significant injury may sometimes darken or lighten as a result of temperature change. More specifically, the albino locus contains the gene TYR (P55033).[5] Two distinct alleles causing blue-eyed and pink-eyed albinism respectively have been previously theorized.
Although the Siamese colorpoint pattern is the most famous coloration produced by TYR, there are color mutations at the locus.
The tyrosine pathway also produces neurotransmitters, thus mutations in the early parts of that pathway may affect not only pigment, but also neurological development. This results in a higher frequency of cross-eyes among colorpoint cats, as well as the high frequency of cross-eyes in white tigers.[33]
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The silver series is caused by the Melanin inhibitor gene I/i. The dominant form causes melanin production to be suppressed, but it affects phaeomelanin (red pigment) much more than eumelanin (black or brown pigment). On tabbies, this turns the background a sparkling silver color while leaving the stripe color intact, making a cold-toned silver tabby. On solid cats, it turns the base of the hair pale, making them silver smoke.[34] The term cameo is commonly used for red silver and cream silver (inhibitor gene (I-O-)) colored coats in cats.
Silver agouti cats can have a range of phenotypes, from silver tabby, to silver shaded (under half the hair is pigmented, approx. 1/3 of hair length), to tipped silver also called chinchilla or shell (only the very tip of the hair is pigmented, approx. 1/8 of hair length). This seems to be affected by hypothetical wide band factors, which make the silver band at the base of the hair wider. Breeders often notate wide band as a single gene Wb/wb, but it is most likely a polygenic trait.
If a cat has the wide band trait but no silver melanin inhibitor, the band will be golden instead of silver. These cats are known as golden tabbies, or in Siberian cats sunshine tabbies. The golden color is caused by the CORIN gene. Shaded golden and tipped golden are also possible, in the same hair length distribution as the silver-gene. However, there is no golden smoke, because the combination of wide band and nonagouti simply produces a solid cat.[35][unreliable source][36]
The genetics involved in producing the ideal tabby, tipped, shaded, or smoke cat is complex. Not only are there many interacting genes, but genes sometimes do not express themselves fully, or conflict with one another. For example, the silver melanin inhibitor gene in some instances does not block pigment, resulting in a grayer undercoat, or in tarnishing (yellowish or rusty fur). The grayer undercoat is considered less desirable to fanciers.
Likewise, poorly-expressed non-agouti or over-expression of melanin inhibitor will cause a pale, washed out black smoke. Various polygenes (sets of related genes), epigenetic factors, or modifier genes, as yet unidentified, are believed to result in different phenotypes of coloration, some deemed more desirable than others by fanciers.
The genetic influences on tipped or shaded cats are:
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Fever coat is an effect known in domestic cats, where a pregnant female cat has a fever or is stressed, causing her unborn kittens' fur to develop a silver-type color (silver-grey, cream, or reddish) rather than what the kitten's genetics would normally cause. After birth, over some weeks the silver fur is replaced naturally by fur colors according to the kitten's genetics.[37][38][39]
Cat fur can be short, long, curly, or hairless. Most cats are short-haired, like their ancestor.[40] The fur can naturally come in three types of hairs; guard, awn, and down hair. The length, density and proportions of these three hairs varies greatly between breeds, and in some cats only one or two types are found.[40][41]
Most oriental breeds only express one single layer of silky coat.[40] However, cats can also have double-layered coats out of two hair types in which the down hairs form the soft, insulating undercoat, and the guard hairs form the protective outer coat.[40]
A typical cat coat exists of all three natural hair types, but due to the equal lengths of two of these hairs, the coat is still considered double-layered.[40] Typically, the down hairs comprise the undercoat while the guard and awn hairs make up the basic top coat.[40][41] Double-coated cats with thick undercoats require daily grooming as these coats are more prone to matting.[40] Double coats are found in for example the Persian, British Shorthair, Maine Coon and Norwegian Forest cat.
Additionally, there even exist cats which express all three natural types of cat hair in different lengths and structures, which form three different layers. These cats are called triple-coated. Siberians and Neva Masquerades are known for their unique triple coats,[40] which provides double insulation to withstand their natural cold climate.
There have been many genes identified that result in unusual cat fur. These genes were discovered in random-bred cats and selected for. Some of the genes are in danger of going extinct because the cats are not sold beyond the region where the mutation originated or there is simply not enough demand for cats expressing the mutation.
In many breeds, coat gene mutations are unwelcome. An example is the rex allele which appeared in Maine Coons in the early 1990s. Rexes appeared in America, Germany and the UK, where one breeder caused consternation by calling them "Maine Waves". Two UK breeders did test mating which indicated that this was probably a new rex mutation and that it was recessive. The density of the hair was similar to normally coated Maine Coons, but consisted only of down type hairs with a normal down type helical curl, which varied as in normal down hairs. Whiskers were more curved, but not curly. Maine Coons do not have awn hairs, and after moulting, the rexes had a very thin coat.[citation needed]
Cat fur length is governed by the Length gene in which the dominant form, L, codes for short hair, and the recessive l codes for long hair. In the longhaired cat, the transition from anagen (hair growth) to catagen (cessation of hair growth) is delayed due to this mutation.[42] A rare recessive shorthair gene has been observed in some lines of Persian cat (silvers) where two longhaired parents have produced shorthaired offspring.
The Length gene has been identified as the fibroblast growth factor 5 (FGF5; M3X9S6) gene. The dominant allele codes for the short coat is seen in most cats. Long coats are coded for by at least four different recessively inherited mutations, the alleles of which have been identified.[43] The most ubiquitous is found in most or all long haired breeds while the remaining three are found only in Ragdolls, Norwegian Forest Cats, and Maine Coons.
There are various genes producing curly-coated or "rex" cats. New types of rex arise spontaneously in random-bred cats now and then. Some of the rex genes that breeders have selected for are:
There are also genes for hairlessness:
Some rex cats are prone to temporary hairlessness, known as baldness, during moulting.
Here are a few other genes resulting in unusual fur:
| Gene | Locus
Name |
Locus Symbols | Allele Variants | Description |
|---|---|---|---|---|
| ASIP | Agouti | A | A, APb, a | Agouti/tabby, charcoal (cat hybrids, i.e. Bengal and Savannah breeds), recessive black/solid |
| TYRP1 | Brown | B | B, b, bl | Black, brown/chocolate, cinnamon |
| -- | Orange | O | XO, Xo, Y | Red, black (sex-linked epistatic) |
| LVRN / Taqpep | Tabby Pattern | Ta | TaM, Tab | Mackerel, classic/blotched |
| DKK4 | Ticked Tabby | Ti | TiA,Ti+ | (Epistatic to tabby) ticked, full body ticked (see Abyssinian) |
| -- | Spotted Modifier | Sp | Sp, sp | (Modifier to tabby) spotted tabby, no modification |
| TYR | Colorpoint | C | C, cb, cs, ca, c | Full color, mink, sepia, siamese point, blue eye albino, red eye albino |
| -- | Inhibitor | I | I, i | Silver, non-silver |
| MLPH | Dilution | D | D, d | Diluted color (black=blue, chocolate=lilac, cinnamon=fawn, orange=cream), no effect |
| -- | Dilute Modifier | Dm | Dm, dm | Diluted color modified (blue/brown/cinnamon=caramel, cream=apricot), no effect |
| KIT | White | W | W, ws, w, wg, wsal | Solid white, white spotting, without white, white gloving, white fading |
| CORIN | Wide Band | wb | -, wb | Tabby agouti, shaded, tipped, smoke, silver, golden, "sunshine" (Siberian) |
| -- | Barrington Brown | Ba | Ba, ba | Diluted brown (black=mahogany, chocolate=light brown, cinnamon=pale coffee), no effect; Unverified gene |
| MC1R | Extension | E | E, e, er, ec | Normal, amber (Norwegian Forest Cat), russet (Burmese), copal (Kurilian Bobtail) |
| FgF5 | Long hair | L | L, l (M1, M2, M3, M4, M5) | Short, long (Ragdoll, Norwegian Forest Cat, Maine Coon and Ragdoll, most longhair breeds, Maine Coon) |
| KRT71 | Curly Coat | Re | Se, se/Re, re, hr | Curly coat (Selkirk Rex), normal hair, curly coat (Devon Rex), hairlessness (Sphynx) |
| LPAR6 | Rex (Cornish) | R | R, r | Normal hair, curly coat (Cornish Rex) |
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