Cat coat genetics

The genetics of cat coat coloration, pattern, length, and texture is a complex subject, and many different genes are involved.

Genes involved in albinism, dominant white, and white spotting

 * The dominant C gene and its recessive alleles determine whether a cat is a complete albino (either pink-eyed or blue-eyed), a temperature sensitive albino (Burmese, Siamese, a blend known as Tonkinese), or a non-albino. If a cat has the dominant C allele, then the cat is non-albino and full pigment production occurs. The C gene codes for the enzyme tyrosinase, the first step in pigment production.


 * The white masking gene, W/w. The dominant allele of this gene masks all other colors by preventing pigment producing cell migration to the skin during embryologic development. In other words, the cat has a greatly reduced number of melanocytes. Although often called a "dominant" gene it would more correctly be called epistatic, as it masks the effects of all other color genes. A cat that is WW or Ww will be white, no matter what other color genes it may have.  A cat that is homozygous recessive (ww) will express normal pigmentation. Some cats with the W allele of this gene are deaf and/or have depigmentation of the iris of one or both eyes, resulting in blue eye color. White cats are also more likely to get skin cancer.


 * The white spotting or piebald spotting gene, S/s, has variable expression, so that an SS cat often has more extensive white patching than an Ss cat. It is this gene that creates the familiar white blaze across the face, a white bib, tuxedo pattern, or dappled paws. This gene can turn a cat's eyes blue if the reduced number of melanocytes.  affects pigmentation of the eyes.  A hypothetical Sb allele ("gloving gene") may cause the mittens in Birman and Snowshoe breeds.  Some researchers believe that there are separate white spotting genes for distinct forms of white pattern, such as the white locket that some cats have on their neck or bellies.

Genes involved in orange, black, brown, and diluted colors

 * The sex-linked orange gene, O, determines if there will be orange fur. This gene is located on the X chromosome.  In cats with orange fur, phaeomelanin (orange pigment) completely replaces eumelanin (black or brown pigment).


 * Males have only one X chromosome, so only have one allele of this gene. O results in orange fur, and o results in non-orange fur.


 * Since females have two X chromosomes, they have two alleles of this gene. OO results in orange fur, oo results in non-orange fur, and Oo results in a tortoiseshell cat, in which some parts of the fur are orange and others areas non-orange. A cat with Oo and white spotting genes is commonly called a calico.  The reason for the patchwork effect in female cats heterozygous for the O gene (Oo) is "X chromosome inactivation" - one or the other X chromosome in every cell in the embryo is randomly inactivated (see Barr body), and the gene in the other X chromosome is expressed.


 * Rufous polygenes, as yet unidentified, can affect the richness of the orange gene's expression.

For a cat to be tortoiseshell, calico, or one of the diluted variants such as blue-cream, the cat must simultaneously express two alleles, O and o, which are located on the X chromosome. Males normally cannot do this, as they have only one X chromosome, and therefore only one allele, and so calico cats are normally only female. Male tortoiseshell or calico cats occur only if they have chromosomal abnormalities such as the genotype XXY (in which case they are sterile), are chromosomal mosaics (only portions of their cells have the genotype XXY, so these cats can be fertile), or are chimeras (a single individual formed from two fused embryos, at least one of which was male).


 * The browning gene B/b/bl codes for tyrosinase related protein-1, an enzyme involved in the metabolic pathway for eumelanin pigment production, and in its dominant form, B, will produce black color. Recessive variants are b, producing brown (or chocolate), and bl producing light brown or cinnamon.


 * 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 Dense pigment gene, D/d, codes for melanophilin, a protein involved in the transportation and deposition of pigment into a growing hair. When a cat has two of the recessive d alleles, black fur becomes "blue" (appearing gray), chocolate fur becomes lilac, cinnamon fur becomes fawn, and orange fur becomes cream.


 * Dilution modifier gene, Dm, which "caramelizes" the dilute colors as a dominant trait. The existence of this phenomenon as a discrete gene is a controversial subject among feline enthusiasts.


 * There is also a theoretical "black modifier" gene, Bm, which in its recessive form, bmbm, causes tabby cats to turn into the colors amber or light amber. This gene could be more appropriately called "agouti modifier" and is probably related to the extension locus (the melanocortin receptor) or its ligands, melanocyte stimulating hormone and agouti signaling protein. This phenomenon was first identified in Norwegian Forest Cats. Other forms of extension mutations have been seen in many breeds (and domestic cats), resulting in unique forms of tabby expression.


 * A black modifier has also been hypothesized in shaded silver and chinchilla Persians whose fur turns pale golden in adulthood. A proposed mechanism is that this due to elongation of the pigment granules, although it has not been established if this pigmentation is phaeomelanin or eumelanin. These cats resemble shaded or tipped goldens, but are genetically shaded or tipped silvers with an additional modifier gene. This could be related to the phenomenon known as "tarnishing" in silvers.

One can deduce that a grey male cat with a white bib and paws:
 * has the o variant of the orange gene on its only X chromosome (because the grey color corresponds to black, not orange)
 * has at least one S variant of the white Spotting gene (because it has the white bib and paws)
 * has two w genes (because it expresses a fur color)
 * has the dominant B gene (because its fur color is a shade of black rather than brown)
 * has two d (dilute) genes (because its fur is grey, rather than black)

Genes involved in fur pattern and shading

 * The agouti gene, A/a which codes for agouti signaling protein. The dominant, wild-type A causes the agouti shift phenomenon which causes hairs to be black pigmented at the tips and orange pigmented at the roots (revealing the underlying tabby pattern), while the recessive non-agouti or "hypermelanistic" allele, a, prevents this shift in the pigmentation pathway.  In its homozygous form, aa, this results in black pigment production throughout the growth cycle of the hair.  Thus, the non-agouti genotype (aa) masks or hides the tabby pattern (Mc and mc), although sometimes a suggestion of the underlying pattern can be seen (called "ghost striping").  The O gene is also epistatic over the aa genotype.  That is, the A to a mutation does not have a discernible effect on red or cream colored cats, resulting in these cats displaying tabby striping independent of their genotype at this locus.  This explains why you can usually see the tabby pattern in the orange patches of tortoiseshell cats, but not in the black or brown patches.


 * The primary tabby pattern gene, Mc/mc, which sets the basic pattern of stripes that underlies the coat: the basic wild-type tabby gene, Mc, produces what is called a mackerel striped tabby (stripes look like thin fishbones and may break up into bars or spots); while a recessive mutant, mc, produces a blotched or classic tabby pattern (broad bands, whorls, and spirals of dark color on pale background usually with bulls-eye or oyster pattern on flank), common to Great Britain and in lands that were once part of the British Empire.


 * Secondary tabby pattern genes such as Ta / ta, at which locus a dominant mutation produces an Abyssinian ticked or non-patterned agouti tabby, having virtually no stripes or bars. (This is one type of unpatterned tabby; the other type of unpatterned tabby is the tipped / shaded / smoke cat.  See inhibited pigment gene, below.)  The dominant form of the Abyssinian gene masks out all other tabby patterns.


 * 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 genes which affect banding frequency, width, and size.

Tabby cats (AA or Aa), normally have: Most or all striping disappears in the chinchilla or shaded cat, but you can still identify the cat as a tabby from these other features.
 * 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.)


 * There is an interesting 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. The "grizzled" phenomenon is purported to have been inherited from the hybridization of these cats to Jungle Cats.


 * 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 shaded silver and chinchilla silver. The inhibitor gene interacts with the non-agouti gene (I-aa) to produce the color known as smoke. The homozygous recessive allele (ii), when combined with the agouti gene, produces tabby coloration, which can vary between a deeply patterned brown tabby, to a warmer "golden tabby", or even the very lightly colored shaded or chinchilla golden colors. Orange cats with the inhibitor gene (I-O-) are commonly called "cameo".

The genetics involved in producing the ideal 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 melanin inhibitor gene sometimes does a poor job blocking pigment, resulting in an excessively gray undercoat, or in tarnishing (yellowish or rusty fur). Likewise, poorly-expressed non-agouti or over-expression of melanin inhibitor will cause a pale, washed out black smoke. Here are the genetic influences on tipped or shaded cats:
 * Agouti gene.
 * Genes (such as Ta) causing unstriped body type.
 * Genes affecting number and width of bands of color on each hair.
 * Hypothetical wide band gene(s). Without a wide undercoat, the cat appears as a normal tabby.
 * Silver or melanin inhibitor gene.
 * Genes causing sparkling appearance (glitter in the Bengal, satin in the Tennessee Rex, grizzle in the Chausie).
 * Genes to clear up residual striping (hypothetical Chaos, Confusion, Unconfused, Erase, and Roan factors).
 * Various polygenes (sets of related genes), epigenetic factors, or modifier genes, as yet unidentified, believed to result in different degrees of shading, some deemed more desirable than others by fanciers.

Genes involved in fur length and texture
Cat fur length is governed by the Long hair gene in which the dominant form, L codes for short hair, and the recessive l codes for long hair. A rare recessive shorthair gene has been observed in some lines of Persian cat (silvers) where two longhaired parents have produced shorthaired offspring.

There are many genes resulting in unusual fur. These genes were discovered in random-bred cats and selected for. Some of the genes are in danger of going extinct because the breeders have not marketed their cats effectively, the cats are not sold beyond the region where the mutation originated, or there is simply not enough demand for the mutation.

There are various genes producing curly coated or "rex" cats. New types of rex pop up spontaneously in random-bred cats now and then. Here are some of the rex genes that breeders select for:


 * r = Cornish Rex
 * gr (provisional) = German Rex (same locus as Cornish, but apparently different allele)
 * re = Devon Rex
 * ro = Oregon Rex (extinct?)
 * Se = Selkirk Rex completely dominant, however heterozygous cats may have a fuller coat that is preferred in the show ring.
 * Lp (provisional) = LaPerm (rexing allele is incompletely dominant: Lp/lp individuals have wavy rather than curly coat)

There are also genes for hairlessness, which produce the French hairless cat (genotype h/h), the British hairless cat (genotype hd/hd), the Russian Donskoy and Peterbald (Hp/Hp or Hp/hp), and the Canadian Sphynx cat (genotype hr/hr). Some rex cats are prone to temporary hairlessness, known as baldness, during moulting.

Here are a few other genes resulting in unusual fur:


 * The Wh gene (dominant, possibly incomplete) results in Wirehair cats. They have bent or crooked hair producing springy, crinkled, coarse fur.


 * A hypothetical Yuc gene, or York Chocolate undercoat gene, results in cats with no undercoat. However, the proportional relationship between guard, awn, and down hair production varies greatly between all breeds.


 * A recessive autosomal gene for Onion hair which causes roughness and swellings on the hairs. The swelling is due to enlargement of the inner core of medulla cells.


 * A recessive autosomal gene spf for sparse fur. As well as sparse coat, the hairs are thin, straggly and contorted and there is brown exudate around the eyes and nose and on the chest and stomach. A similar condition is linked to Ornithine Transcarbamylase Deficiency in mice.