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The coat of the domestic dog (Canis lupus familiaris) refers to the hair that covers its body. A dog's coat may be a double coat, made up of a soft undercoat and a tougher topcoat, or a single coat, which lacks an undercoat. Double coats have a top coat, made of stiff hairs to help repel water and shield from dirt, and an undercoat to serve as insulation. The terms fur and hair are often used interchangeably when describing a dog's coat, however in general, a double coat, e.g., like that of the Newfoundland and most mountain dogs, is referred to as a fur coat, while a single coat, like that of the Poodle, is referred to as a hair coat.
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Colors, patterns, lengths and textures
There are a greater variety of coat colors, patterns, lengths and textures found in the domestic dog than in its wolf relations, even though dogs and wolves belong to the same species (Canis lupus). Different breeds use different names for longhaired and shorthaired types, there is no standard nomenclature for length, breed standards give acceptable lengths by measurement. Coat colors in dogs were not likely initially selected for by humans but were probably the inadvertent outcome of some other selection process (i.e. selection for tameness). Research has found that tameness brings associated physical changes, including coat coloring and patterning.
Domestic dogs often display the remnants of countershading, a common natural camouflage pattern. The basic principle of countershading is when the animal is lit from above, shadows will be cast on the ventral side of the body. These shadows could provide a predator or prey with visual cues relating to the movement of the animal. By being lighter colored on the ventral side of the body, an animal can counteract this, and thereby fool the predator or prey. An alternative explanation is that the dorsal and ventral sides of an animal experience different selection pressures (from the need to blend into different backgrounds when viewed from above and below) resulting in differing coloration.
Genetic basis of color and pattern
Modern breeds of dog exhibit a diverse range of coat colorings, patterns, lengths and textures. In recent years, the understanding of the genetic basis for coat coloring and patterning and coat length and texturing has increased significantly.
There are currently eight known genes within the canine genome that are associated with coat color. Each of these genes occurs in at least two variants, or alleles, which accounts for the variation in coat color between animals. Each of these genes exists at a fixed location, or locus, of the animal's genome. The loci associated with canine coat color are:
A (agouti) locus
The alleles at the A locus are related to the production of agouti signalling protein (ASIP) and determine whether an animal expresses an agouti appearance, and if so what type, by controlling the distribution of pigment in individual hairs. There are five suspected alleles that occur at the A locus:
Most texts suggest that the dominance hierarchy for the A locus alleles appears to be as follows: Ay > aw > at > a, however research suggests the existence of pairwise dominance/recessive relationships in different families and not the existence of a single hierarchy in one family. This means, for example, that Ay may be incompletely dominant over at. Recent studies reveal saddle tan to be a modification of the tanpoint phenotype caused by interaction of other genes with ASIP.
B (brown) locus
The alleles at the B locus are related to the production of tyrosinase related protein 1 (TYRP1) and determine the degree to which an animal expresses tyrosinase, an enzyme related to the production of melanin, in its coat and skin (including the nose and paw pads). There are two known alleles that can occur at the B locus:
B is dominant to b. An animal that has at least one copy of the B allele will have a black nose, paw pads and eye rims while an animal that is homozygous for any of the b alleles will have a liver nose, paw pads and eye rims.
D (dilute) locus
The alleles at the D locus (the melanophilin gene or MLPH) are related to the dilution of eumelanin and/or phaeomelanin and determine the intensity of pigmentation. There are two known alleles:
D is dominant to d. Homozygosity of d is sometimes accompanied by hair loss and recurrent skin inflammation, a condition referred to as either color dilution alopecia (CDA) or black hair follicular dysplasia (BHFD) depending upon the breed of dog.
E (extension) locus
The alleles at the E locus (the melanocortin receptor 1 gene or MC1R) determines whether an animal expresses a melanistic mask or a grizzle overlay, as well as determining whether an animal expresses eumelanin in its coat. Expression of eumelanin will result in a black or brown coat, while a lack of expression of eumelanin will result in a red or yellow coat. There are four known alleles that occur at the E locus:
The dominance hierarchy for the E locus alleles appears to be as follows: Em > EG > E > eh > e. The Grizzle allele has been studied only in Salukis and Afghan Hounds, the latter in which it is referred to as "Domino". Its placement in the dominance hierarchy has not been solidified. The eh sable extension allele has been studied only in English Cocker Spaniels and produces sable in the presence of all K locus alleles. The expression is dependent upon the animal being homozygous for at and not possessing Em or E. The expression of EG is dependent upon the animal being homozygous for at and not possessing Em or KB. An animal that is homozygous for e will express a red or yellow coat regardless of the alleles at other loci (unless the animal is homozygous for ca at the C locus in which case it will be albino).
H (harlequin) locus
DNA studies have not yet isolated the gene at the H locus, but the traits associated with it have been mapped to chromosome 9. The H locus is a modifier locus (of the M locus) and the alleles at the H locus will determine if an animal expresses a harlequin pattern (white base with black patches). There are two alleles that can occur at the H locus:
H is dominant to h. Breeding data suggests that H is embryonic recessive lethal and that therefore all harlequins are H/h. The Harlequin allele is specific to Great Danes. As H is a modifier locus of the M locus, in order for the Harlequin pattern to be expressed, one copy of the H allele (at the H locus) and one copy of the M allele (at the M locus) must be present (i.e. H/h and M/m).
K (dominant black) locus
The alleles at the K locus (the β-Defensin 103 gene or DEFB103) determine the coloring pattern of an animal's coat. There are three known alleles that occur at the K locus:
The dominance hierarchy for the K locus alleles appears to be as follows: KB > kbr > ky. The coloring of an animal that possesses at least one KB will be determined by the alleles it possesses at the B and E loci. An animal with one kbr allele and no KB allele will express a brindle pattern to its coat unless it is homozygous for e (at the E locus) or possibly homozygous for a (at the A locus). An animal that is homozygous for ky will express the agouti pattern in accordance with the alleles it has at the A locus.
M (merle) locus
The alleles at the M locus (the SILV gene) determine whether an animal expresses a merle pattern to its coat (patches of sporadic colored and white hairs and other patches of solid color). There are two alleles that can occur at the M locus:
M is dominant to m. Both heterozygosity and homozygosity of the merle gene (i.e. M/m and M/M) are linked to a range of auditory and ophthalmologic abnormalities.
S (spotting) locus
The alleles at the S locus (the microphthalmia-associated transcription factor gene or MITF) determine the degree and distribution of spotting of an animal's coat. There is disagreement as to the number of alleles that occur at the S locus, with researchers postulating either two or four alleles. The four alleles postulated are:
S is dominant to s. DNA studies have not yet confirmed the existence of all four alleles, with some research suggesting the existence of at least two alleles (S and sp) and other research suggesting the possible existence of a third allele (si). It has been suggested that what appears to be the result of an sw allele is in fact the result of plus and minus modifiers acting on one of the other alleles. It is thought that the spotting that occurs in Dalmatians is the result of the interaction of three loci (the S locus, the T locus and F locus) giving them a unique spotting pattern not found in any other breed.
In 2014, a study found that a simple repeat polymorphism in the MITF-M Promoter is a key regulator of white spotting and that white color had been selected for by humans.
Postulated color and pattern loci
There are at least five additional theoretical loci thought to be associated with coat color in dogs. DNA studies are yet to confirm the existence of these genes or alleles but their existence is theorised based on breeding data:
C (colored) locus
The alleles at the theoretical C locus are thought to determine the degree to which an animal expresses phaeomelanin, a red-brown protein related to the production of melanin, in its coat and skin. Five alleles are theorised to occur at the C locus:
The C locus in dogs is not well understood and the theorised alleles are based on those present in other species. True albinism has not been conclusively shown to exist in dogs. It is thought that an animal that is heterozygous for the C allele with one of the other alleles will express a result somewhere between the two alleles.
F (flecking) locus
The alleles at the theoretical F locus are thought to determine whether an animal displays small, isolated regions of white in otherwise pigmented regions (not apparent on white animals). Two alleles are theorised to occur at the F locus:
It is thought that F is dominant to f.
G (progressive greying) locus
The alleles at the theoretical G locus are thought to determine if premature greying of the animal's coat will occur. Two alleles are theorised to occur at the G locus:
It is thought that G is dominant to g.
I (intensity) locus
The alleles at the theoretical I locus are thought to affect phaeomelanin expression. Two alleles are theorised to occur at the I locus:
It is thought that I and i are co-dominant, so that animals with i/i will be paler than animals with I/i.
T (ticking) locus
The alleles at the theoretical T locus are thought to determine whether an animal displays small, isolated regions of pigment in otherwise white regions (not apparent on non-white animals). Two alleles are theorised to occur at the T locus:
It is thought that T is dominant to t.
Genetic basis of length and texture
Research indicates that the majority of variation in coat growth pattern, length and curl can be attributed to mutations in three genes, the R-spondin-2 gene or RSPO2, the fibroblast growth factor-5 gene or FGF5, and the keratin-71 gene or KRT71.
The L (length) locus
The alleles at the L locus (the fibroblast growth factor-5 gene or FGF5) determine the length of the animal's coat. There are two known alleles that occur at the L locus:
L is dominant to l. A long coat is demonstrated when a dog has pair of recessive 'l' alleles at this locus.
The W (wired) locus
The alleles at the W locus (the R-spondin-2 gene or RSPO2) determine the coarseness and the presence of "facial furnishings" (e.g. beard, moustache, eyebrows). There are two known alleles that occur at the W locus:
W is dominant to w. Animals that are homozygous for l (i.e. l/l) and possess at least one copy of W will have long, soft coats with furnishings, rather than wirey coats.
The R (curl) Locus
The alleles at the R locus (the keratin-71 gene or KRT71) determine whether an animal's coat is straight or curly. There are two known alleles that occur at the R locus:
R is dominant to r.
Interaction of Length & Texture Genes
These three genes responsible for the length and texture of an animal's coat interact to produce eight different phenotypes:
Hair growth
The coat of most dogs grows to a specific length and then stops growing, while the coats of some dogs grow continuously in a manner similar to human hair growth. A long-coated dog with a single dominant furnishings allele (wire coat), will demonstrate this feature. Examples of breeds of dog whose coats grow continuously are:
Corded Coats
Corded coats, like those of the Puli and Komondor are thought to be the result of continuously growing curly coats. Other breeds with continuously growing curly coats, such as the Poodle, can also be groomed to cord.
Hairless
Some breeds of dog do not grow hair on parts of their bodies and may be referred to as "hairless". Examples of "hairless" dogs are the Xoloitzcuintli (Mexican Hairless Dog), the Peruvian Inca Orchid (Peruvian Hairless Dog) and the Chinese Crested. Research suggests that hairlessness is caused by a dominant allele, which is homozygous lethal.
Genetic testing and phenotype prediction
In recent years genetic testing for the alleles of some genes has become available Software is also available to assist breeders in determining the likely outcome of matings.
Colors
The same color may be referred to differently in different breeds. Likewise, a same term may mean different colourations in different breeds.
Brown, chocolate and liver are the most common terms used to refer to the bb-dilution of black pigment to a dark brown. Depending on breed and exact shade, terms such as mahogany, midtone brown, grey-brown, blackish brown are used. Sedge and deadgrass are used to describe the desired Chesapeake Bay Retriever color that resembles "that of its working surroundings" as closely as possible.
Red refers to reddish shades of orange, brown, and tan. Terms used include orange, red-gold, cinnamon, tan, and ruby. Genetically a dog called red is usually a clear sable (with little to no eumelanin tipping on hairs) or a ruddy recessive yellow.
In some breeds, "red" refers to what would usually be called brown, chocolate, or liver. A "red merle" is always a liver-based merle. In Australian Cattle Dogs, blue stands for a densely ticked liver-based colouration with an overall red-grey appearance.
Gold refers specifically to a rich reddish-yellow and its variants, whereas yellow can refer to any shade of yellow and tan. Terms used include yellow-gold, lion-colored, fawn, apricot, wheaten, tawny, straw, yellow-red, mustard, sandy, honey, apricot, blond, lemon. Dogs called golden or yellow tend to be recessive yellow, but can also be sable.
Cream refers to a pale yellowish or tannish colour which can be almost white.
Fawn typically refers to a yellow, tan, light brown, or cream dog that has a dark melanistic mask.
With Weimaraners, fawn refers to their typical brownish grey colouration that with other breeds is usually called lilac.
Black is a pure black that can get grizzled as the dog ages, or have a tendency to gain a brownish cast when exposed to the elements.
Blue is a cool-toned, metallic grey. It typically means a dd-dilution of black pigment, a grey colouration that is grey from birth, but has a wide range of breed-specific meanings.
In Kerry Blue Terriers, Poodles, and Bearded Collies, "blue" refers to colouration that is black at birth and progressively greys out as the dog matures. In Australian Shepherds, Rough Collies, and Shetland Sheepdogs, blue means a blue (black-based) merle. In Australian Silky Terriers, blue means a saddle-type black and tan pattern, where the black parts of the coat progressively fade to a steel grey as the dog matures, and in Australian Cattle Dogs, blue stands for a densely ticked black-based colouration with an overall blue-grey appearance.
Grey simply means a grey colouration of any shade. It can be used as an alternative synonym of blue, but tends to mean some other type of grey than the dd-dilution of black ("true" blue). Synonyms include silver, pepper, grizzle, slate, blue-black grey, black and silver, steel. Greys of a dusty or brownish cast are often lilac, a dd-dilution of liver, and this colouration does not have much of a commonly recognised name. Across various breeds, it is called lavender, silver-fawn, isabella, fawn, café au lait or silver beige.
In Poodles, a blue is a very slowly fading, very dark steel grey, whereas a silver is a quicker to clear, much lighter grey that can range from a pale platinum to a steel grey. Both are black at birth with minimal markings to indicate future change. Similarly, café au lait is a slower and darker and silver beige a quicker and lighter progressively greying brown, i.e. liver.
White: Such a light cream that it is seen and described as pure white, making them distinct from albino dogs. A white dog, as opposed to an albino one, has dark pigment around the eye rims and nose, often coupled with dark-colored eyes. There is often some coat identifiable as cream between the dog's shoulder blades.
Patterns
The same pattern may be referred to differently in different breeds.
Show coats
The nature and quality of a purebred dog's coat is important to the dog fancy in the judging of the dog at conformation shows. The exact requirements are detailed in each breed's breed standard and do not generalise in any way, and the terminology may be very different even when referring to similar features. See individual breed articles for specific information.
Every hair in the dog coat grows from a hair follicle, which has a three phase cycle, as do most mammals. These cycles are: anagen, growth of normal hair; catagen growth slows, and hair shaft thins; telogen, hair growth stops, the follicle rests, and the old hair falls off—is shed. At the end of the telogen phase, the follicle begins the cycle again.
Hypoallergenic coats
Some dog breeds have been promoted as hypoallergenic (which means less allergic, not free of allergens) because they shed very little. However, no canine is known to be completely nonallergenic. Often the problem is with the dog's saliva or dander, not the fur. Although poodles, bichons, yorkies, and terriers are commonly represented as being hypoallergenic, the reaction that an individual person has to an individual dog may vary greatly. In treating dog related allergies, it has been found that "Factors related to individual dogs seem to influence the allergenicity more than breed..."