Cat coat genetics

Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. Understanding how can be challenging because many genes are involved. 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 Siberian could wear point coloration, the stereotypical coat of a Siamese.

Solid colors

Eumelanin

The browning gene ''B/b/b^l'' codes for TYRP1 (), 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 ''b^l'' (cinnamon), with ''b^l'' being recessive to both ''B'' and ''b''. Chocolate is a rich dark brown color, and is referred to as chestnut in some breeds. Cinnamon is a light reddish brown, but is sometimes not reddish at all.

Sex-linked orange/red

The sex-linked ''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). This gene is located on the X chromosome. The orange allele is O, and is codominant with non-orange, 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. Male tortoiseshell cats are known to exist, but, as expected from the genetics involved, they are rare and often exhibit chromosomal abnormalities. 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.

This color is known as red by breeders. Other 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.

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.

Dilution and Maltesing

The ''Dense pigment'' gene, ''D/d'', codes for melanophilin (''MLPH''; ), a protein involved in the transportation and deposition of pigment into a growing hair. 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), cinnamon fur becomes fawn, and red fur becomes cream. 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.

Other genes

* ''Barrington Brown'' is a recessive browning gene that dilutes black to mahogany, brown to light brown and chocolate to pale coffee. It is different from the browning gene and has only been observed in laboratory cats.
* The ''Dilution modifier'' gene, ''Dm'', "caramelizes" the dilute colors as a dominant trait. The existence of this phenomenon as a discrete gene is a controversial subject among feline enthusiasts.
* A mutation at the extension locus ''E/e'' (the melanocortin 1 receptor, ''MC1R'') changes black pigment to amber or light amber, similar in appearance to red and cream. Kittens are born dark but lighten up as they age. Paws and nose still exhibit the original undiluted color, this in contrast to other diluted colors, where paws and nose have the diluted color. This phenomenon was first identified in Norwegian Forest cats.
* Another recessive mutation at extension was discovered which causes the russet color in Burmese cats. It is symbolized as e^r. Like amber cats, russet cats lighten as they age.
* A modifying factor has also been hypothesized in shaded silver and chinchilla Persians whose fur turns pale golden in adulthood, due to low levels of phaeomelanin production. These cats resemble shaded or tipped goldens, but are genetically shaded or tipped silvers. This is probably related to the phenomenon known as "tarnishing" in silvers.

Tabbies

Tabby cats are striped due to the agouti gene. Their stripes have an even distribution of pigment, while the background is made up of banded hairs. Tabby cats usually show the following traits:

* M on forehead. (Visible in ticked tabby cats, but hard to discern in shaded silver/golden, and tipped cats)
* Thin pencil lines on face. (Visible in ticked tabby cats, but hard to discern in shaded silver/golden, and tipped cats)
* Black " eyeliner" appearance and white or pale fur around eyeliner.
* Pigmented lips and paws.
* A pink nose outlined in darker pigment.
* Torso, leg, and tail banding. (Torso banding disappears in the ticked tabby.)

Agouti

The ''Agouti'' gene, with its dominant ''A'' allele and recessive ''a'' allele, controls the coding for agouti signaling protein (ASIP; ). The wild-type ''A'' produces the agouti shift phenomenon, which causes hairs to be banded with black and an orangish/reddish brown, this revealing the underlying tabby pattern (which is determined by the ''T'' alleles at the separate tabby gene). The ''non-agouti'' or "hypermelanistic" allele, ''a'', does not initiate this shift in the pigmentation pathway and so homozygotes ''aa'' have pigment production throughout the entire growth cycle of the hair—along its full length. As a result, the non-agouti genotype (aa) is solid and has no obvious tabby pattern (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). ''Agouti'' is found on chromosome A3.

A major exception to the solid masking of the tabby pattern exists: the ''O'' allele of the ''O/o'' locus is epistatic over the ''aa'' genotype. That is, in red or cream colored cats, tabby striping is displayed despite the genotype at the agouti locus.

However, some red cats and most cream cats show a fainter tabby pattern when they have no agouti allele to allow full expression of their tabby alleles. That is, in genetically red cats (''O'' males and ''OO'' and ''Oo'' females) the ''aa'' does still have an effect, especially in dilute coats (when having ''dd'' genotype at the D gene locus), where the tabby pattern is sometimes not expressed except on the extremities.

Mackerel or blotched

The ''Tabby'' gene on chromosome A1 accounts for most tabby patterns seen in domestic cats, including those patterns seen in most breeds. The dominant allele ''Ta^M'' produces mackerel tabbies, and the recessive ''Ta^b'' produce classic (sometimes referred to as blotched) tabbies. The gene responsible for this differential patterning has been identified as ''transmembrane aminopeptidase Q'' ('' Taqpep'', ). 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.

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). The classic tabby is most common in Iran, Great Britain and in lands that were once part of the British Empire and Persian Empire.

Spotted tabby

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.

Ticked tabby

The ''Ticked'' (''Ti'') locus on chromosome B1 controls the generation of ticked coats, a non-patterned agouti coat having virtually no stripes or bars but still considered a tabby coat. Ticked tabbies are rare in the random-bred population, but fixed in certain breeds such as the Abyssinian and Singapura. ''Ti^A'' 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''). Two different alleles are responsible for the ''Ti^A'' ticked tabby phenotype: a cysteine to tyrosine substitution at residue 63 ( C63Y) and an alanine to valine substitution at residue 18 ( A18V). Both variants are present in the Abyssinian breed, and the A18V variant is found in the Burmese breed. 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 (''Ti^ATi⁺'') but be nearly or completely nonexistent in homozygotes (''Ti^ATi^A''). 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.

It was once thought that ''Ti^A'' was an allele of the ''Tabby'' gene, called ''T^a'', dominant to all other alleles at the locus.

Other genes

* Other genes (''pattern modifier'' genes) are theorized to be responsible for creating various type of spotting patterns, many of which are variations on a basic mackerel or classic pattern. There are also hypothetical factors which affect the timing and frequency of the agouti shift, affecting agouti band width and the number and quality of alternating bands of eumelanin and phaeomelanin on individual hairs.
* There is a gene not yet identified, but believed to be related to the agouti gene in the Chausie breed that produces silver-tipped black fur similar to Abyssinian ticked fur, known as "grizzled." This phenomenon is purported to have been inherited from the hybridization of the domestic cat to the jungle cat (''Felis chaus'').
* The ''inhibited pigment'' gene, ''I/i''. The dominant allele (I) produces ''tipped'' hairs that are fully colored only at the tip and have a white base. This allele appears to interact with other genes to produce various degrees of tipping, ranging from deeply tipped silver tabby to lightly tipped shaded silver and chinchilla silver. The inhibitor gene interacts with the non-agouti genotype (I-aa) to produce the color known as smoke. The homozygous recessive genotype when combined with the agouti gene (iiA-), produces tabby coloration, which can vary along a spectrum ranging from a deeply patterned brown tabby, to a lighter "golden tabby", to the very lightly colored shaded or chinchilla golden colors. Orange cats with the inhibitor gene (I-O-) are commonly called "cameo".

Tortoiseshells and calicos

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. 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 North America 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 red/cream and black/blue, 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 red/cream and black/blue, and is called a calico. With intermediate amounts of white, a cat may exhibit a calico pattern, a tortie pattern, or something in between, depending on other epigenetic factors. Diluted calico cats with lighter coloration are sometimes called calimanco or clouded tiger.

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.

Variations

* The basic tortoiseshell pattern has several different colors depending on the color of the eumelanin (the B locus), and dilution (the D locus).
* Tortoiseshell tabbies, also known as torbies, display tabby patterning on both colors. Calico tabbies are also called calibys or tabicos.

White spotting and epistatic white

White spotting and epistatic white (also known as dominant white) were long thought to be two separate genes, but in fact they are both on the KIT gene. 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. The Birman

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