GENETICS

THIS IS A PICTURE OF MY DAUGHTER PLAYING WITH A BUNCH OF BENGAL BABIES AT A FELLOW BREEDERS HOME... THIS PICTURE IS FEATURED IN THE BOOK "GETTING TO KNOW THE BENGAL CAT" BY GENE DUCOTE.

If you want to read about which colors are commonly seen in cats, or if you want to know what your cat's color is called, read section A.

If you are interested in the genetics of different colors and in what colors are theoretically possible, read section B. Section B is more technical. You may want to read section A first to become familiar with cat color terminology.

Domestic cats exhibit a rich variety of coat patterns and colors. The names given to these colors and patterns are increasingly based on genetic theory. Many people have expressed confusion over the names that cat fanciers apply to the coat colors and patterns, so this article has been written to help explain the names given to the possible colors and patterns, and why these names are applied.

This article attempts to introduce and describe the colors and patterns of domestic cats and the names that are given to them. It does not attempt to explain the mechanisms of inheritance nor the formulas for calculating the possible outcomes of particular breedings.

The colors in hair, skin, and eyes are caused by the presence of melanin. Melanin is deposited in the hair shafts in the form of microscopic granules which vary in shape, size, and arrangement, giving a variety of colors.

There are two chemically different kinds of melanin: eumelanin and phaeomelanin. Eumelanin granules are thought to be spherical in shape and absorb almost all light, giving black pigmentation. Phaeomelanin granules are thought to be elongated "footballs" in shape, and reflect light in the red-orange-yellow range.

Several genes can cause variation in the density of the the melanin granules, so other colors can be produced. The most variation is found in the black-based (eumelanistic) colors. The following table lists the commonly accepted names for the basic colors, by genotype:

Mutations of the gene for Black give rise to Chocolate and Cinnamon. These colors are thought to be due to a smaller number of eumelanin granules in the hair shaft. The Chocolate color is a medium to dark brown color; it is sometimes called chestnut. Cinnamon is a terra-cotta or burnt sienna color. These are alleles at the (B) locus; Chocolate is recessive to Black, and Cinnamon is recessive to Chocolate.

A mutation of the gene for Dense coloration produces Blue, Lilac, and Fawn. These colors are due to clustering of the particles of pigment in the hair shaft. This is called dilution or Maltesing. Blue is the dilute form of Black; it is commonly seen as various shades of gray. Lilac is the dilute form of Chocolate; it is described as dove or light taupe gray, and is sometimes called frost or lavender. Fawn is the dilute form of Cinnamon; it is described as "coffee and cream" or caramel color. Dilution is a mutation at the (D) locus; dilution is recessive to dense coloration.

In comparison, the red-based (phaeomelanistic) colors have much less variation. Red is usually described as orange or "marmalade", but some red cats have rather pale pigmentation and so people may describe them as yellow. Cream is the dilute form of Red, and is described as a buff color. The symbol for the gene for Red/Cream is (O); Black is recessive to Red.

The Red gene (O) is carried on the X chromosome; for this reason it is sex-linked. Males normally only have one X chromosome and so if a male carries the Red gene at all, he will be Red. Females have two X chromosomes; if both X chromosomes carry the Red gene, then the cat will be Red. However, many females carry the Red gene on only one chromosome, which allows the black-based pigmentation to show through in patches. This combination of red and black is called Tortoiseshell.

A typical Tortoiseshell is a patchwork of black and orange, usually in a random pattern. Some "Torties" have large patches of orange, others are mostly black. The Tortoiseshell can be modified by dilution, which gives a patchwork of blue and cream rather than black and orange. Such dilute Torties are usually called Blue-Cream. Finally, the black patches of a Tortie may actually be any of the black-based colors, so you may see a Chocolate Tortie or a Cinnamon Tortie, and, if dilution is also present, a Lilac-Cream Tortie or a Fawn-Cream Tortie.

The mutations described above have been seen and described in cats in Europe and and the Western Hemisphere for hundreds of years. Another set of mutations of color has been introduced with the Siamese and Burmese cats from Asia. The Burmese carries the gene for Sepia color (cb) and the Siamese carries the gene for Pointed color (cs). These are alleles at the albino (C) locus; when they are combined (cb/cs), as in the Tonkinese, "mink" colors are produced.

Not shown in the preceding table are two further mutations in the albino series which always have solid white coats, regardless of the other genes for pigmentation. These are the albino white with pale blue eyes (ca/ca), and the albino white with unpigmented (pink) eyes (c/c).

White fur is the absence of any pigmentation. A solid white coat may be caused by any of three genetic mechanisms, which are completely different:

Complete white spotting. The white spotting factor (S) is an incomplete dominant, which is affected by polygenetic modifiers and usually results in a cat that is only partially white. However, it can be so complete that it results in a completely white coat. White spotting will be discussed in a later section.

Dominant white. This mutation overrides all other genes for pigmentation, and produces a white coat and blue eyes. As its name implies, this is the effect of a dominant gene (W).

In the dominant white, the other genes for color and pattern are still present, but they are completely hidden. The only way to determine the underlying genotype is by test matings with colored cats of (reasonably) well-known genotype.

Breeding two dominant whites will mostly produce solid white kittens, but if both of the parents are heterozygous (W/w), then the underlying colors may appear on a few of the kittens. Unless the genotypes of the whites are known from pedigree or test breeding, the results are unpredictable.

Dominant white is found in mixed-breed cats, of course, and notably in Persian and Oriental Shorthair breeding programs. At one time the dominant white Oriental Shorthair was considered a separate breed by some associations, called the Foreign White. The dominant white can produce much deeper blue eye color than the albino, so it is considered desirable. It is believed that the best blue eyes in solid white Oriental Shorthairs are those that are masking Chocolate.

Deafness in white cats is associated with the white spotting factor (S), and with the dominant white (W), but not with the albino white (c/c or ca/ca).

All of the foregoing discussion has described solid colors. However, the solid or "self" colored cat is not the most common. More cats have ticked fur than solid color, and in most of them, the ticked fur alternates with the solid color in some sort of pattern, which is called tabbying.

First, ticking is the result of the agouti gene (A) which causes the individual hairs to have bands of light and heavy pigmentation. The agouti gene allows full pigmentation when the hair starts to grow, then slows down the synthesis of pigment for a while, and then turns it on for a while. As the hair approaches its normal length and stops growing, pigment synthesis stops. The result is a hair shaft that has dense pigment at the tip, then a band of yellow to orange, then a band of dense pigment, fading to yellow to orange at the root.

The agouti band can be seen in both the eumelanistic (black-based) and phaeomelanistic (red-based) colors. In both cases, the agouti band marks the period where the production of melanin has slowed down. It is fairly well accepted that the color in the agouti band of a eumelanistically-pigmented hair shaft is still eumelanin, not phaeomelanin, but it is the fact that the granules are sparse and "shredded" that gives them the yellow to orange color. The agouti band is not an alternation of eumelanin production with phaeomelanin production in the same hair shaft.

In eumelanistically-pigmented hair shafts, the agouti band is normally a drab yellow-beige color. However, the color of the agouti band can be a richer orange due to the effect of "rufousing" factors. These are polygenetic factors that have not been isolated and identified, but breeders have been able to select for them to produce "warm" background colors in the tabbies. In particular, the Brown Tabby patterns are genetically Black, but the selection of individuals with strong rufousing has produced a rich brown color in the ticked hairs.

The mutation that causes solid color is called non-agouti (a/a), and is recessive. The effect of non-agouti is to suppress the ticking, so the same density of pigment is found all along the hair shaft, except at the root, where it normally begins to fade in any case.

The tabby pattern is determined by the tabby gene (T), which causes the ticked hairs to alternate with stripes, blotches, or spots of hairs of solid color. The commonly-recognized types of tabby patterns have been given descriptive names:

Classic Tabby. Ticked hairs alternate with solid hairs in a blotched pattern, often with a circular "bullseye" on the side, or a "butterfly" on the back. This is called a Blotched Tabby in the UK.

Ticked Tabby. Ticked hairs are found uniformly over the entire coat, giving a flecked or freckled appearance. This pattern is sometimes called the Agouti Tabby or Abyssinian Tabby.

The classic tabby pattern (tb) is recessive to the mackerel tabby pattern (T). The Abyssinian pattern (Ta) is dominant to the mackerel tabby pattern (T).

The agouti and tabbying genes also apply to all the colors generated by the albino series (sepia, mink, and pointed colors), but space does not permit them to be listed here. Associations in the US only recognize Burmese and Tonkinese in non-agouti, eumelanistic colors, so no tabby patterns should be visible in those breeds. The Singapura is recognized only in the Sable Agouti Tabby color (seal sepia ticked tabby). Tabby patterns have been accepted by some associations in Siamese, and they are called "Lynx Point".

Note that there are no true solid Red or Cream colors. Breeders have produced Red and Cream cats that appear solid by selecting for rufousing polygenes that tend to "wash out" the contrast in the tabby pattern. A tell-tale 'M' can still be seen on the forehead of most "solid" reds.

The Sorrel Abyssinian is sometimes called a "red" Aby, but this is a misnomer. These are all black-based colors. True Red and Cream Abyssinians and Somalis are not accepted by the US associations.

Note that the spotted tabby pattern is not shown as a separate genotype on the above chart. It has not been conclusively proven whether the spotted tabby pattern is another distinct mutation of the tabby gene or is simply an effect of polygenetic modifiers on the mackerel tabby pattern. Some breeders point to the existance of spotted patterns in various wild cat species as support for the theory that the spotted pattern is a distinct mutation. In practice, however, the spotted breeds continue to produce a range of patterns from mackerel through spotted, and breeders must continually select for well-defined spots or mackerel tabbies will result.

Although they are not shown on the chart, Tortoiseshell cats can also have tabby patterns. In a tortoiseshell tabby, or "torbie", the same tabby pattern is applied to both the red patches and the black patches. The bands of solid and ticked fur in the red patches are continuous with the bands of solid and ticked fur in the black patches.

In the typical tabby, the ticked hairs have bands of lighter pigmentation, but they are not devoid of color. Typically, the lighter bands are a drab beige-yellow color, but rufousing can make them closer to orange.

At the other extreme, shading causes the agouti band to be lighter in color. Shading can also cause the agouti band to be wider, so that the light color extends all the way to the root. The effect is to produce a hair shaft that has a colored tip, in whatever color is determined by the color genes, and then much lighter below the tip. When the light colored portion of the hair shaft is near-white, it is called Silver, when it is yellow or a warm cream color, it is called Golden.

Several genetic theories have been proposed to explain the inheritance of shaded coloration. The earliest theory proposed a Chinchilla gene (Ch) which was thought to be an allele at the albino locus. If correct, this would imply that shaded sepia, mink, and pointed colors were impossible. Breeding experiments have disproved that theory. A more recent theory proposed another single dominant gene, called the inhibitor gene (I), but this theory was inadequate to explain the variations of shading and did not correlate with the experiences of breeders, so current theories propose at least two genes. None of the current theories have been experimentally proven, however.

All of these theories seek to explain the genetic factors that apparently suppress the synthesis of pigment after a certain point in the growth of the hair shaft. This effect interacts with the agouti and tabby patterns to produce varying degrees of shading, which are commonly called "Chinchilla", "Shaded Silver", "Silver Tabby", and "Smoke".

In the Chinchilla, all of the hairs are tipped with color, and then light-colored below the tip. Since both the ticked and the solid hairs turn light-colored before the point where the agouti band would begin, so the tabby pattern is not visible. The tipping is so light that the coat looks white at a first glance, but sparkles with color on closer inspection.

In the Shaded Silver, all of the hairs are tipped with color at about the point where the agouti band would normally begin. As in the Chinchilla, both the ticked and the solid hairs turn light-colored before the point where the agouti band would begin, so the tabby pattern is not visible. However in the Shaded Silver, the colored tips are long enough that the normal color is clearly visible, particularly along the head and spine.

In the Silver Tabby, the ticked hairs are tipped with color and then light-colored below the tip, but the solid hairs have normal coloration. The tabby pattern is actually enhanced by the greater contrast between the almost-white ticked hairs and the full color of the solid hairs.

The Smoke pattern results from the action of shading on a solid (non-agouti) coat. All of the hairs have full color well beyond the point at which the agouti band would appear, and then turn into a near-white undercoat. Such a coat looks like a solid color until you blow on it or the cat's movement reveals the contrasting white undercoat.

The same range of shadings can be seen with the Golden undercoat. These are called "Golden Chinchilla", "Shaded Golden", "Golden Tabby", and "Golden Smoke". Rather than the near-white of the Silvers, these have an undercoat that is described as warm cream or apricot.

The shaded patterns are most striking on the eumelanistic colors, because of the contrast, but they can also be applied to Red and Cream. These colors are sometimes called "cameos", but the names for the cameo colors can be equated to names commonly used for shaded eumelanistic colors:

Since shading can be applied to both black-based and red-based colors, naturally it can be applied to tortoiseshell, dilute tortoiseshell, torbie, and dilute torbie.

In theory, Golden undercoats can be applied to the red-based colors, but it is debatable whether breeders will find that combination worthwhile. The lack of contrast in a Red Shaded Golden would make the effect of shading almost impossible to see. However, Golden can be seen in the undercoats of the black patches of a Tortoiseshell Shaded Golden or a Tortoiseshell Golden Chinchilla.

White spotting is a very common mutation that causes patches of white in what is called a "piebald" pattern. The range of variation is quite remarkable: from white toes, to white feet; a white streak on the nose or a white chin, to a white bib; a white belly and legs, to white over most of the body, leaving only a few patches of color; or even a completely white coat.

White spotting can be thought of as a mask over the color that the cat naturally carries. People who have cats with just small patches of tabby markings on the head and tail and white everywhere else tend to think of them as white cats, but they are really tabbies all over. The tabby pattern is simply hidden by the white spotting.

White spotting can occur in combination with any of the colors and patterns already described. The customary way of describing the pattern is to add "and White" to the name of the color and pattern of the cat. Thus, a "Red Mackerel Tabby" would become a "Red Mackerel Tabby and White" and a "Lilac" would become a "Lilac and White".

The "Tortoiseshell and White" is given a special name (in the US); it is called "Calico". Consequently, a "Blue-Cream and White" is sometimes called a "Dilute Calico".

The white spotting factor (S) is a dominant mutation with variable expression. Cats that are homozygous (S/S) tend to have more white area than cats that are heterozygous (S/s) for white spotting, but there are other modifying genes that can affect the degree of white spotting. Non-genetic variations have been noted. Some people have observed that the white area may increase as the cat gets older.

The white spotting factor can create blue-eyed or odd-eyed cats, if it reaches one or both eyes. The white spotting factor is associated with deafness, if the white areas reach the ears. Since it usually covers the eyes if it covers the ears, the deaf cats caused by white spotting frequently have blue eyes (but not always). The deafness may affect one or both ears. It is caused by a degeneration of the cochlea (inner ear) which begins a few days after birth. The deafness is irreversible.

I have included the commonly-used genetic symbols throughout this article, but they are not essential to understanding the article. By convention, genes for discrete characteristics are symbolized by letters; usually the letters are derived from the initial letter of the gene name. Mutations of a single gene are called allomorphs, or more commonly, alleles. The dominant allele is symbolized by a capital letter and the recessive is symbolized by a lower case letter.

The ASCII character set is inadequate to show subscripts and superscripts. In conventional genetic notation, multiple alleles are distinguished by superscript letters. For example, Black would be (B), Brown would be (b), and Light Brown is (bl) (b superscript-l). Since the superscripts cannot be printed in ASCII, I have written these as two-letter symbols. I hope this doesn't cause too much confusion.

An individual cat has a pair of genes for each particular trait, one inherited from each parent. A true-breeding black cat would be symbolized by (B/B), and a chocolate (brown) cat would be (b/b). These are called homozygous because they have received the same gene from both parents. A black cat that carries the recessive gene for chocolate would be symbolized by (B/b) because is received different genes from its two parents.

A cat carrying a recessive trait, such as (B/b), is called heterozygous. It is indistinguishable from the homozygous individual, except through breeding experiments. When the presence of a dominant gene determines the visible trait, I have sometimes written the genotype in the form (B/-), where the dash indicates that the value of the second gene is unknown or does not affect the visible result.