Cat Genetics v2.0: Colours
Coloration is a physical trait of cats that is visible, is not associated with disease conditions (with some exceptions), and that has been desirable since the beginning of the development of cat breeds. The genetics of cat coloration is a good practical example of basic genetics and of heredity. We will see the function of single genes, the passage of genes to the next generation, and importantly, the interaction of genes with each other to give the combinations of color traits seen between and within the cat breeds. The good news is that the basics of color genetics in the cat are relatively simple. The not so good news is that simplicity plus simplicity plus simplicity….will pretty quickly give us complexity.
There are a number of genes that contribute to the color and the pattern of color seen in the cat; these genes can be grouped into four levels of function:
- Genes for basic colors, including Colorpoint (Locus C), Agouti (Locus A), Orange (Locus O) and Locus E. These are the genes that make the pigments that give the basic colors to the cat.
- Genes for modification of the basic colors, including Brown (Locus B), Dilution (Locus D), and also Dilute-Modifier and Silver. These are the genes that modify the colors defined by the basic color genes.
- Genes for white spotting, including Dominant White (Locus W), and White Spots (Locus S). These genes don’t make color, rather they are responsible for the lack of color (i.e. white).
- Genes for color patterns, including Tabby, Spotted and Ticked. These genes group the colors between hairs and within hairs to give color patterns.
Keep in mind that genes come in two copies, i.e. our genetic system is in duplicate (see Cat Genetics v1.0: Basics). Also, that the effect of one copy of a gene can be dominant over the other copy, which is said to be recessive. For a given gene an individual cat can have two copies that are the same (either two dominant or two recessive copies), or one copy that is dominant and one copy that is recessive. Furthermore, the functions of one gene can mask the functions of another gene.
Genes for Basic Colors
Color is produced by special chemicals called pigment. Pigment is produced in special cells called melanocytes, which are found within the hair, in the skin and in the eyes. For the cat, the color and pattern of the pigment within the hair is the most important.
The two basic pigments that contribute to coloration are black and yellow pigments. It is the interplay between the expression of genes for these two basic pigments that give the cat its basic coloration. It is important to realize that a single hair can be all black, all yellow, or have bands of black and yellow or even bands of modified black and yellow.
Colorpoint and Locus C
The C Locus involves a master gene for pigment formation. When the gene is active, both black and yellow pigments can be formed. When the gene is mutated and not active, no pigment is formed. A recessive mutation at the C Locus will give a white (albino) cat, but this is not something that is desirable for cat breeders. However, cats have a very interesting series of additional mutations at the C Locus that give the Colorpoint traits. Colorpoint refers to the fact that ears, paws and tails of the cat are more darkly pigmented compared to the body. This is the key component of coloration in several breeds including Siamese and Burmese, and can give colorpoint variations such as Tonkinese. There is an order of dominance of these variations (mutations) as follows:
Agouti (Locus A)
The Agouti gene, found at Locus A is the master gene for yellow pigment production. In the cat, the Agouti gene is either active, allowing yellow pigment to be formed, or not active, which eliminates yellow pigment formation but still allows black pigment to be produced. A normal (dominant) Agouti gene allows cat hairs to be banded yellow and black, and thus allows Tabby genes to be expressed (lots of fun, see below). A recessive mutation at Agouti is responsible for black cats, called “seal” is some breeds such as the Siamese. Quite frankly the Agouti gene is more interesting in dogs as it gives dominant yellow as well as black and tan colorations not seen in cats (see Dog Genetics v2.0: Colours). Although we have the Agouti gene within our genome, it is not involved in the color traits of hair in people. Pity.
The Orange gene (Locus O) is interesting for two reasons. First of all, it is unique to cats. Secondly, it is found on the X chromosome, making the Orange coloration sex linked. There are two versions of the gene, orange (O) and black (o). Orange (O) blocks black pigment and allows only yellow pigment to be formed, while black (o) blocks yellow pigment and allows only black pigment to be formed. Female cats, possessing two X chromosomes, can have orange fur (O/O), black fur (o/o), or a stylish mix of orange and black hairs to give Tortoiseshell fur (O/o). Male cats, with only one X chromosome, can only have orange fur or black fur, and cannot be tortoiseshell. The rare tortoiseshell males that do occasionally occur usually have an extra X chromosome (XXY). Unfortunately, Orange is not as yet cloned, so we do not know what type of gene it is. Functionally Orange acts a bit like the Agouti Dominant Yellow that is seen in dogs (see Dog Genetics 102: Colors). Dogs do not have sex linked color patterns.
Locus E, also known as Extension, is an important gene for allowing black pigment production. Having said this, Locus E is not very important for most cat breeds although it is responsible for the Amber color (e/e) that can be seen in the Norwegian Forest Cat breed. Locus E is much more important in the dog and the horse where (e/e) is responsible for the beautiful golden coloration of the Golden Retriever dog and the rich red coloration of the Irish Setter dog and the Chestnut coloured horse (see Dog Genetics v2.0: Colours ; Horse genetics v2.0: Colours).
Genes for Modification of Colors
The color modifying genes do not make the color pigments, but they do influence the shade of the color, usually by reducing its intensity i.e. washing it out. Modifying genes include Brown (Locus B), Dilution (Locus D), and also Dilute-Modifier and Silver.
Brown (Locus B)
The Brown gene (Locus B) in the cat affects the intensity of black pigment. Brown is a recessive trait that will modify the normal black pigment (B) to a brown color (b/b) or to a cinnamon color (b’/b’). Normal coloration is dominant over brown, which in turn is dominant over cinnamon.
Dilution is another gene that modifies the normal colors and will wash out or dilute both black pigment and yellow pigment. Dilution is a recessive trait that will modify normal black pigmentation to give a grey color (d/d) often called Blue. Dilution can combine with B locus Brown (b/b) to give Lilac (Lavender) coat colour and with B locus Cinnamon (b’b’) to give Sable (Fawn) coat colour.
There is a gene called Dilute-Modifier in the cat that in the presence of d/d will cause a further washing out of pigment colors. Locus B (Brown) now gives ‘Carmel’ shades of colors while Locus O (Orange) gives Apricot. The Dilute-Modifier gene is not as yet cloned or characterized.
Silver is a gene that modifies yellow pigment to a silver color but does not effect black pigment. Unfortunately, the Silver gene is not as yet cloned.
Genes for White Spotting
White markings are a prominent feature of cat color genetics. Interestingly, white is not a color per se, but is rather a lack of color. In fact, white results from a developmental lack of cells (called melanocytes) that produce pigmentation.
Dominant White (Locus W)
Dominant White, from Locus W, can give an all white cat. Dominant White as it’s name indicates is dominant in that only one copy of the mutated gene (W) is needed to give the trait. The recessive and normal version of the gene is called w. Dominant White will mask the effects of the Colorpoints (Locus C), Agouti (Locus A), and Brown and Cinnamon (Locus B). Dominant white can be confused with recessive white (albino, c/c, coming from the C locus). Unfortunately, Dominant white can be associated with deafness, particularly when it is present in two copies (W/W).
White spots (Locus S)
Cats can have a full range of white markings, from none, to moderate, to predominant. The Locus S is involved in producing white markings, but the gene has not been cloned and it is not known if Locus S represents one gene or several genes. While a comprehensive understanding of the genetics of white spots in cats remains for the future, some breed specific advances have been made. The White Gloving pattern seen in the Birman breed is caused by a mutation in the Kit gene, and displays recessive genetics.
Cats are known for their striking color patterns, including stripes, spots and swirls of color. When it comes to color patterns, dogs are poor cousins to cats. Color pattern effects are produced by different levels of expression of the basic pigments, black and yellow, both between groups of hairs and also within the hair shaft itself. Collectively, color patterns are called Tabbies but this oversimplifies the genetics as there are several genes involved in giving patterned effects. Tabbies require a normal functioning Agouti locus (A/-), and they will be masked in a black cat (a/a). Tabby patterns can overlay the basic color genes as well as the modifying color genes. Three genes associated with color patterns are mentioned, including Classic Tabby, Ticked Tabby and Spotted Tabby.
Mackerel and Classic Tabby
Mackerel Tabby and Classic Tabby are caused by the same gene (Ta), with Mackerel being dominant over Classic Tabby. In Classic (or Blotched) Tabby, swirls and blotches of black and yellow pigment occur. Mackerel Tabby gives the familiar alternating stripes of black and yellow pigment seen in cats. Think little tigers.
In the Ticked Tabby pattern, an alternation of yellow and black pigment occurs within the hair shaft itself giving the cat a stylish ‘salt and pepper’ sheen to its coat as seen in Abyssinian cats. Ticked is dominant over non ticked. Although the Ticked Tabby locus is distinct from the Classic Tabby locus, it can be masked by the Classic Tabby patterns.
The Spot Tabby locus is responsible for spots of black pigment on a yellow pigment background. Think of little leopards. Spots are seen in traditional cat breeds such as the Ocicat and in hybrid breeds such as the Bengal. The genetics of Spot are not well understood.
Other Traits to Mention
The genetics of short hair and long hair is relatively straight forward. Short hair is the dominant copy (L) while long hair is the recessive copy (l/l) of the responsible gene.
Curly Hair Coat
Curly Coat is an interesting trait coded for by the K gene. Dominant mutations (Ks) give the hair coat of the Selkirk Rex, while recessive mutations (kre, khr) give the hair coat of the Devon Rex and the Sphinx, respectively.
Colors and Heredity
We have just talked about the genetics of cat colors. We should also mention the heredity of cat colors, that is to say, the passage of the genes for coat colors from one generation to the next. To help the discussion, think of the cat genome as a deck of playing cards (see Cat Genetics v1.0: Basics) where each gene (or card) is represented twice. From this deck of cards, with its own particular combination of cards is constructed a house of cards, or in this instance, a cat. Between generations, the house of cards is collapsed and the deck of cards gets shuffled and then cut in two. A new house (cat) is built from a new combination of cards, half coming from mom and the other half from dad. When you think of all the known genes with all their variations that are involved in establishing the color of a cat, then add in the genes not as yet characterised, this can give a whole lot of new possible combinations between generations. That’s genetics. And the challenge of breeding.
This review of the genetics of cat colors is not meant to be exhaustive. It is the beginning of the story rather than the end of one. Further information and details are provided elsewhere: