Dog coat genetics


Dogs have a wide range of coat colors, patterns, textures and lengths. Dog coat qualities are governed by how genes are passed from dogs to their puppies and how those genes are expressed in each dog. Dogs have about 19,000 genes in their genome but only a handful affect the physical variations in their coats. Dogs have two copies of most genes, one from the dog's mother and one from its father. Genes of interest have more than one version, or allele. Usually only one or a small number of alleles exist for each gene. In any one gene locus a dog will either be homozygous where the gene is made of two identical alleles or heterozygous where the gene is made of two different alleles.
To understand genetically why a dog's coat physically looks the way it does requires an understanding of only a handful of canine coat genes and their alleles. For example, to understand how a black and white greyhound with wavy hair got its coat you'd need to look at three genes: the dominant black gene with its K and k alleles, the spotting gene with its many variable alleles, and the curl gene with its R and r alleles.

Genes associated with coat color

Each hair follicle is surrounded by many melanocytes, which make and transfer the pigment melanin into a developing hair. Dog fur is colored by two types of melanin: eumelanin and phaeomelanin. A melanocyte can be signaled to produce either color of melanin.
Dog coat colors are from patterns of:
  • Eumelanin — black, chocolate brown, grey or taupe pigment;
  • Phaeomelanin — tan pigment, including all shades of red, gold and cream pigment; and/or
  • Lack of melanin — white.
By 2020, more than eight genes in the canine genome have been verified to determine coat color. Each of these has at least two known alleles. Together these genes account for the variation in coat color seen in dogs. Each gene has a unique, fixed location, known as a locus, within the dog genome.
Some of the loci associated with canine coat color are:

Pigment shade

Several loci can be grouped as affecting the shade of color: the Brown, Dilution, and Intensity loci.

B (brown) locus

The gene at the B locus is known as tyrosinase related protein 1. This gene affects the color of the eumelanin pigment produced, making it either black or brown. TYRP1 is an enzyme involved in the synthesis of eumelanin. Each of the known mutations appears to eliminate or significantly reduce TYRP1 enzymatic activity. This modifies the shape of the final eumelanin molecule, changing the pigment from a black to a brown color. Color is affected in coat and skin.
There are four known alleles that occur at the B locus:
  • B = Black eumelanin. An animal that has at least one copy of the B allele will have a black nose, paw pads and eye rims and dark brown eyes.
  • b = Brown eumelanin - such as chocolate or liver. An animal with any matched or unmatched pair of the b alleles will have brown, rather than black, hair, a liver nose, paw pads and eye rims, and hazel eyes. Phaeomelanin color is unaffected. Only one of the alleles is present in the English Setter, Doberman Pinscher and Italian Greyhound, but in most breeds with any brown allele two or all three are present. It is unknown whether the different brown alleles cause specific shades or hues of brown.
B is dominant to b.
File:Silver Labrador Retriever Cooper.jpg|thumb|Labrador : KB for solid eumelanin coat; B/_ for black eumelanin lightened by d/d dilution. Labrador dogs with the dilute gene often suffer from color dilution alopecia.

D (dilute) locus

The melanophilin gene at the D locus causes a dilution mainly of eumelanin, while phaeomelanin is less affected. This dilution gene determines the intensity of pigmentation. MLPH codes for a protein involved in the distribution of melanin - it is part of the melanosome transport complex. Defective MLPH prevents normal pigment distribution, resulting in a paler colored coat.
There are two common alleles: D, and d that occur in many breeds. But recently the research group of Tosso Leeb has identified additional alleles in other breeds.
  • D = Not diluted. Black or brown eumelanin, reddish or orangish tan phaeomelanin.
  • d = Diluted. Diluted fur color: black eumelanin diluted to bluish grey ; brown eumelanin diluted to taupe or "Isabella". Phaeomelanin is diluted from red to yellowish tan; this phaeomelanin dilution is not as dramatic as the eumelanin color shift. Slight to moderate dilution of the paw pads and eye rims towards bluish grey if B/- or taupe if b/b, and slight to moderate reduction of eye color from brown towards amber in a B/- animal, or from hazel towards light amber in a b/b animal.
D is completely dominant to d.
This dilution gene can occur in almost any breed, where blue gene is the most common. Also, there are some breeds that come in dilute but with no specific color, such as the Weimaraner or the Slovakian Pointer. Some breeds that are commonly known to have dilution genes are "Italian greyhounds, whippets, Tibetan mastiffs, greyhounds, Staffordshire bull terriers, and Neapolitan mastiffs".

Color gene interactions

I (intensity) locus

The alleles responsible for pheomelanin dilution was found to be the result of a mutation in MFSD12 in 2019. and occurs in breeds that do not exhibit dark gold or red phenotypes.
Two alleles are theorised to occur at the I locus:
  • I = Non-diluted pigment
  • i = Diluted pigment
It's been observed that I and i interact with semi-dominance, so that there are three distinct phenotypes. I/i heterozygotes are paler than I/I animals but normally darker than i/i animals.
  • i results in diluted phaeomelanin such as cream, yellow, and white. Unlike d/d, it allows the skin and eyes to remain dark.
It does not effect eumelanin pigment, i.e. leaving a cream Afghan with a very black mask.
This is not to be confused with the cream or white in Nordic Breeds such as the Siberian Husky, or cream roan in the Australian Cattle Dog, whose cream and white coats are controlled by genes in the Extension E Locus.

Red Pigment

Pigment Intensity for dogs who are darker than Tan has been attributed to a mutation upstream of KITLG, in the same genes responsible for coat color in mice and hair color in humans.
The mutation is the result of a Copy Number Variant, or duplication of certain instructions within a gene, that controls the distribution of pigment in a dog's hair follicle. As such, there are no genetic markers for red pigment.
  • Dogs with a higher CNV were observed to have darker, richer colors such as deep gold, red, and chestnut.
  • Dogs with a lower CNV were observed to have lighter gold and orange colors.
This mutation not only effects Pheomelanin, but Eumelanin as well. This mutation does not effect all breeds the same.

Eumelanin vs. Phaeomelanin Pigment Switching

Several loci affect coat color by changing whether and where on a dog eumelanin or phaeomelanin are produced. Within a single hair, there can be alternating bands of eumelanin and pheomelanin. By changing that pigment switching, the result can be a hair with only eumelanin or only phaeomelanin. Some alleles affect the entire coat; others affect pigment switching for only certain parts of the body, resulting in patterns like a darker facial mask.
There are three well studied loci: the Agouti, Extension and Black loci. Intercellular signaling pathways tell a melanocyte which type of melanin to produce. These three loci affect different parts of this cell signaling pathway.
MC1R is a receptor on the surface of melanocytes. When active, it causes the melanocyte to synthesize eumelanin. When MC1R is inactive, the melanocyte produces phaeomelanin. ASIP inactivates MC1R, thereby causing phaeomelanin synthesis. DEFB103 in turn prevents ASIP from inhibiting MC1R, thereby increasing eumelanin synthesis.

A (agouti) locus

The alleles at the A locus are related to the production of agouti signalling protein and determine whether an animal expresses an agouti appearance, and, by controlling the distribution of pigment in individual hairs, what type of agouti. Agouti signaling protein tells cells to make phaeomelanin, not eumelanin. Overactive ASIP therefore leads to yellower/redder hairs; under-active ASIP leads to darker hairs. There are several known alleles that occur at the A locus:
  • Ay = Fawn or sable. Higher ASIP activity promotes phaeomelanin production. Tan with black whiskers and varying amounts of black-tipped and/or all-black hairs dispersed throughout. Fawn typically referring to dogs with clearer tan and sable to those with more black shading.
  • aw = Wild-type agouti. Each hair with 3-6 bands alternating black and tan. Dorsal hairs are darker, and ventral hairs are paler. The most common coloration in wolves. Also called wolf sable.
  • at = Tan point. Black with tan patches on the face and underside - including saddle tan. Phaeomelanin production is limited to tan points; dark portions of the dog are solid eumelanin hairs.
  • a = Recessive black. Loss-of-function ASIP mutation leads to only eumelanin production. Solid black coat.
  • ayt = Recombinant fawn has been identified in numerous Tibetan Spaniels and individuals in other breeds, including the Dingo. Its hierarchical position is not yet understood.
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/recessiveness relationships in different families and not the existence of a single hierarchy in one family.
  • Ay is incompletely dominant to at, so that heterozygous individuals have more black sabling, especially as puppies and Ayat can resemble the awaw phenotype. Other genes also affect how much black is in the coat.
  • aw is the only allele present in many Nordic spitzes, and is not present in most other breeds.
  • at includes tan point and saddle tan, both of which look tan point at birth. Modifier genes in saddle tan puppies cause a gradual reduction of the black area until the saddle tan pattern is achieved.
  • a is only present in a handful of breeds. Most black dogs are black due to the K locus allele KB for dominant black.
Border Collies is one of the few breeds that lack agouti patterning, and only have sable and tan points. However, many border collies still test to have agouti genes.