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 (one from its mother and one its father) or heterozygous where the gene is made of two different alleles (one inherited from each parent).
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 (white) spotting gene with its many variable alleles, and the curl gene with its R and r alleles.
Each hair follicle is surrounded by many melanocytes (pigment cells), which make and transfer the pigment melanin into a developing hair. Dog fur is colored by two types of melanin: eumelanin (brownish-black) and phaeomelanin (reddish-yellow). A melanocyte can be signaled to produce either color of melanin.
Dog coat colors are from patterns of:
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:
Several loci can be grouped as affecting the shade of color: the Brown (B), Dilution (D), and Intensity (I) loci.
The gene at the B locus is known as tyrosinase related protein 1 (TYRP1). 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 (including the nose and paw pads).
There are four known alleles that occur at the B locus:
B is dominant to b.
The melanophilin gene (MLPH) 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 (normal, wild-type MLPH), and d (defective MLPH) that occur in many breeds. But recently the research group of Tosso Leeb has identified additional alleles in other breeds.
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".
The alleles responsible for pheomelanin dilution (changing of a dog's coat from tan to cream or white) 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:
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.
It does not effect eumelanin (black/brown/blue/lilac) 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.
Pigment Intensity for dogs who are darker than Tan (shades of gold to red) 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.
This mutation not only effects Pheomelanin, but Eumelanin as well. This mutation does not effect all breeds the same.
Several loci can be grouped as controlling when and where on a dog eumelanin (blacks-browns) or phaeomelanin (reds-yellows) are produced: the Agouti (A), Extension (E) and Black (K) loci. Intercellular signaling pathways tell a melanocyte which type of melanin to produce. Time-dependent pigment switching can lead to the production of a single hair with bands of eumelanin and phaeomelanin. Spatial-dependent signaling results in parts of the body with different levels of each pigment.
MC1R (the E locus) is a receptor on the surface of melanocytes. When active, it causes the melanocyte to synthesize eumelanin; when inactive, the melanocyte produces phaeomelanin instead. ASIP (the A locus) binds to and inactivates MC1R, thereby causing phaeomelanin synthesis. DEFB103 (the K locus) in turn prevents ASIP from inhibiting MC1R, thereby increasing eumelanin synthesis.
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, by controlling the distribution of pigment in individual hairs, what type of agouti. There are four known alleles that occur at the A locus:
Most texts suggest that the dominance hierarchy for the A locus alleles appears to be as follows: A > a > a > 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.
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.
The alleles at the E locus (the melanocortin receptor one gene or MC1R) determine whether an animal expresses a melanistic mask, as well as determining whether an animal can produce eumelanin in its coat. There are three known, plus two more theorized, alleles that occur at the E locus:
The dominance hierarchy for the E locus alleles appears to be as follows: E > E > E > e > e.
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: K > k > k.
Alleles at the Agouti (A), Extension (E) and Black (K) loci determine the presence or absence of brindle and its location:
The Merle (M), Harlequin (H), and Spotting (S) loci contribute to patching, spotting, and white markings. Alleles present at the Merle (M) and Harlequin (H) loci cause patchy reduction of melanin to half (merle), zero (harlequin) or both (double merle). Alleles present at the Spotting (S), Ticking (T) and Flecking (F) loci determine white markings.
DNA studies have isolated a missense mutation in the 20S proteasome β2 subunit at the H locus. 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 vs merle pattern. There are two alleles that occur at the H locus:
H/h heterozygotes are harlequin and h/h homozygotes are non-harlequin. Breeding data suggests that homozygous H/H is embryonic lethal and that therefore all harlequins are H/h.
The alleles at the M locus (the silver locus protein homolog gene or SILV, aka premelanosome protein gene or PMEL) determine whether an animal expresses a merle pattern to its coat. There are two alleles that occur at the M locus:
M and m show a relationship of both co-dominance and no dominance.
There are other new discovery on M locus and it would be useful to add the supplementary category on "M(merle) Locus" part. Since the original section only talk about just one allele M, but there are some variation on the one allele and derive a number of new alleles, which will lead to the other production of pigment.
One of the variation of M allele is Mc and Mc+. Although just one copy of Mc is not long enough to make visible change on coats, the combination of Mc or more than two copies of Mc would lead to odd shade of black/liver.
Another type of variation of M allele is Ma and Ma+. This kinds of allele would lead to visibly merle-patterned dog if there are two copies of Ma. It is important to be supplement because if the dog with atypical merle bred to dog with any longer merle allele, the double merle health problems might occur.
The alleles at the S locus (the microphthalmia-associated transcription factor gene or MITF) determine the degree and distribution of white spotting on an animal's coat. There is disagreement as to the number of alleles that occur at the S locus, with researchers sometimes postulating a conservative two or, commonly, four alleles. The alleles postulated are:
In 2014, a study found that a combination of simple repeat polymorphism in the MITF-M Promoter and a SINE insertion is a key regulator of white spotting and that white color had been selected for by humans to differentiate dogs from their wild counterparts.
Based on this research the degree of White Spotting is dependent on the Promoter Length (Lp) to produce less or more color. A shorter Lp creates less white (Solid Colored and Residual White dogs) while a longer Lp creates more white (Irish Spotting and Piebald).
What separates Piebald from Irish White and Solid is the presence of a SINE insertion (Short Interspersed Element) in the S locus genes that changes the normal DNA production. The result is Piebald and Extreme Piebald. The only difference between the two recognized forms of Piebald is the length of the Lp.
Because of this variability, a dog's Phenotype will not always match their Genotype. The Beagle for example is fixed for spsp Piebald, yet there are Beagles with very little white on them, or Beagles that are mostly white. What makes them Piebald is the SINE Insertion, but the Lp length is what changes how their patterns are expressed.
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.
People have postulated several alleles at the C locus and suggested some/all 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 have been theorised to occur at the C locus:
However, based on a 2014 publication about albinism in the Doberman Pinscher and later in other small breeds, the discovery was made that multiple alleles in the C locus are highly unlikely, and that all dogs are homozygous for Normal Color production, excluding dogs who carry albinism.
There are 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:
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:
(See ticking below, which may be another name for the flecking described here)
It is thought that F is dominant to f.
The alleles at the theoretical G locus are thought to determine if progressive 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.
The alleles at the theoretical T locus are thought to determine whether an animal displays small, isolated regions of pigment in otherwise s-spotted white regions. Two alleles are theorised to occur at the T locus:
It is thought that T is dominant to t. Ticking may be caused by several genes rather than just one. Patterns of medium-sized individual spots, smaller individual spots, and tiny spots that completely cover all white areas leaving a roan-like or merle-like appearance (reserving the term large spots for the variation exclusive to the Dalmatian) can each occur separately or in any combination.
The alleles at the theoretical U locus are thought to limit phaeomelanin production on the cheeks and underside. Two alleles are theorized to occur at the U locus:
It is thought that U is recessive to u but due to lack of genetic studies these assumptions have only been made through visual assessment. The urajiro pattern is expressed in the tan (phaeomelanin) areas of any dog and does not effect black (eumelanin) pigment.
Miscolours occur quite rarely in dog breeds, because genetic carriers of the recessive alleles causing fur colours that don't correspond to the breed standard are very rare in the gene pool of a breed and there is an extremely low probability that one carrier will be mated with another. In case two carriers have offspring, according to the law of segregation an average of 25% of the puppies are homozygous and express the off-colour in the phenotype, 50% become carriers and 25% are homozygous for the standard colour. Usually off-coloured individuals are excluded from breeding, but that doesn't stop the inheritance of the recessive allele from carriers mated with standard-coloured dogs to new carriers.
In the breed Boxer large white markings in heterozygous carriers with genotype S s or S s belong to the standard colours, therefore extreme white Boxers are born regularly, some of them with health problems. The cream-white colour of the Shiba Inu is not caused by any spotting gene but by strong dilution of pheomelanin. Melanocytes are present in the whole skin and in the embryonic tissue for the auditory organs and eyes, therefore this colour is not associated with any health issues.
The occurrence of a dominant coat colour gene not belonging to the standard colours is a suspicion for crossbreeding with another breed. For example, the dilute gen D in the suddenly appeared variety "silver coloured" Labrador Retriever might probably come from a Weimaraner. The same applies for Dobermann Pinschers suffering from Blue dog syndrome.
Somatic mutation, a mutation that can occur in body cells after formation of the embryo, can be passed on to next generations. A pigment somatic mutation can cause patches of different colors (mosaicism) to appear in the dog's coat.
Every hair in the dog coat grows from a hair follicle, which has a three phase cycle, as in most other mammals. These phases are:
Most dogs have a double coat, each hair follicle containing 1-2 primary hairs and several secondary hairs. The primary hairs are longer, thicker and stiffer, and called guard hairs or outer coat. Each follicle also holds a variety of silky- to wiry-textured secondary hairs (undercoat) all of which are wavy, and smaller and softer than the primary hair. The ratio of primary to secondary hairs varies at least six-fold, and varies between dogs according to coat type, and on the same dog in accordance with seasonal and other hormonal influences. Puppies are born with a single coat, with more hair follicles per unit area, but each hair follicle contains only a single hair of fine, silky texture. Development of the adult coat begins around 3 months of age, and is completed around 12 months.
Research indicates that the majority of variation in coat growth pattern, length and curl can be attributed to mutations in four genes, the R-spondin-2 gene or RSPO2, the fibroblast growth factor-5 gene or FGF5, the keratin-71 gene or KRT71 and the melanocortin 5 receptor gene (MC5R). The wild-type coat in dogs is short, double and straight.
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 dominance of L > l is incomplete, and L/l dogs have a small but noticeable increase in length and finer texture than closely related L/L individuals. However, between breeds there is significant overlap between the shortest L/L and the longest L/l phenotypes. In certain breeds (German Shepherd, Alaskan Malamute, Cardigan Welsh Corgi), the coat is often of medium length and many dogs of these breeds are also heterozygous at the L locus (L/l).
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, but the dominance of W > w is incomplete. W/W dogs have coarse hair, prominent furnishings and greatly-reduced shedding. W/w dogs have the harsh wire texture, but decreased furnishings, and overall coat length and shedding similar to non-wire animals.
Animals that are homozygous for long coat (i.e., l/l) and possess at least one copy of W will have long, soft coats with furnishings, rather than wiry 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:
The relationship of R to r is one of no dominance. Heterozygotes (R/r) have wavy hair that is easily distinguishable from either homozygote. Wavy hair is considered desirable in several breeds, but because it is heterozygous, these breeds do not breed true for coat type.
Corded coats, like those of the Puli and Komondor are thought to be the result of continuously growing curly coats (long + wire + curly) with double coats, though the genetic code of corded dogs has not yet been studied. Corded coats will form naturally, but can be messy and uneven if not "groomed to cord" while the puppy's coat is lengthening.
These three genes responsible for the length and texture of an animal's coat interact to produce eight different (homozygous) phenotypes:
Breeds in which coat type Is not explained by FgF5, RSPO2 and KRT71 genes:
Genotypes of dogs of these 3 breeds are usually L/L or L/l, which does not match with their long-haired phenotype. The Yorkshire and Silky Terriers share common ancestry and likely share an unidentified gene responsible for their long hair. The Afghan Hound has a unique patterned coat that is long with short patches on the chest, face, back and tail. The Irish Water Spaniel may share the same pattern gene, although unlike the Afghan Hound, the IWS is otherwise genetically a long-haired (fixed for l/l) breed.
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 of the forkhead box transcription factor (FOXI3) gene, which is homozygous lethal. There are coated homozygous dogs in all hairless breeds, because this type of inheritance prevents the coat type from breeding true. The hairlessness gene permits hair growth on the head, legs and tail. Hair is sparse on the body, but present and typically enhanced by shaving, at least in the Chinese Crested, whose coat type is shaggy (long + wire). Teeth can be affected as well, and hairless dogs have sometimes incomplete dentition.
The American Hairless Terrier is unrelated to the other hairless breeds and displays a different hairlessness gene. Unlike the other hairless breeds, the AHT is born fully coated, and loses its hair within a few months. The AHT gene, serum/glucocorticoid regulated kinase family member 3 gene (SGK3), is recessive and does not result in missing teeth. Because the breed is new and rare, outcrossing to the parent breed (the Rat Terrier) is permitted to increase genetic diversity. These crosses are fully coated and heterozygous for AHT-hairlessness.
Some breeds (e.g., Rhodesian Ridgeback, Thai Ridgeback) have an area of hair along the spine between the withers and hips that leans in the opposite direction (cranially) to the surrounding coat. The ridge is caused by a duplication of several genes (FGF3, FGF4, FGF 19, ORAOV1 and sometimes SNP), and ridge is dominant to non-ridged.
There are many genes and alleles that cause long hair in dogs, but most of these genes are recessive. This means that longhaired hybrid breeds usually have to have two longhair or longhair carrier parents, and the gene can also be passed on for many generations without being expressed.
There are lots of variations of allele that would affect the dog's hair. The allele that causes bristles is actually dominant. Dogs with both the longhair and line coat genes will be "coarse", which means longer line coats of fur. Examples of such coats include the Korthals Griffon, and possibly the Irish Wolfhound.
The most common colour of dog nose is black. However, a number of genes can affect nose colour.
The genes also affect the eye colours of dogs. There are two main types of eye colours patterns.
All hepatic dogs (bb) have amber eyes. Amber eyes vary from light brown to yellow, chartreuse, or gray. Dogs with melanin can occasionally see amber eyes.[article refers to Dr Sheila M. Schmutz]
Blue eyes in dogs are often related to pigment loss in coatings.
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.
The genes responsible for the determination of coat colour also affect other melanin-dependent development, including skin colour, eye colour, eyesight, eye formation and hearing. In most cases, eye colour is directly related to coat colour, but blue eyes in the Siberian Husky and related breeds, and copper eyes in some herding dogs are not known to be related to coat colour.
The development of coat colour, skin colour, iris colour, pigmentation in back of eye and melanin-containing cellular elements of the auditory system occur independently, as does development of each element on the left vs right side of the animal. This means that in semi-random genes (M merle, s spotting and T ticking), the expression of each element is independent. For example, skin spots on a piebald-spotted dog will not match up with the spots in the dog's coat; and a merle dog with one blue eye can just as likely have better eyesight in its blue eye than in its brown eye.
All known genes are on separate chromosomes, and therefore no gene linkage has yet been described among coat genes. However, they do share chromosomes with other major conformational genes, and in at least one case, breeding records have shown an indication of genes passed on together.
There are size genes on all 39 chromosomes, 17 classified as "major" genes. 7 of those are identified as being of key importance and each results in ~2x difference in body weight. IGF1 (Insulin-like growth factor 1), SMAD2 (Mothers against decapentaplegic homolog 2), STC2 (Stanniocalcin-2) and GHR(1) (Growth hormone receptor one) are dose-dependent with compact dwarfs vs leaner large dogs and heterozygotes of intermediate size and shape. IGF1R (Insulin-like growth factor 1 receptor) and HMGA2 (High-mobility group AT-hook 2) are incomplete dominant with delicate dwarfs vs compact large dogs and heterozygotes closer to the homozygous dwarfed phenotypes. GHR(2) (Growth hormone receptor two) is completely dominant, homozygous and heterozygous dwarfs equally small, larger dogs with a broader flatter skull and larger muzzle. It is believed that the PMEL/SILV merle gene is linked to the HMGA2 size gene, meaning that alleles are most often inherited together, accounting for size differences in merle vs non-merle litter mates, such as in the Chihuahua and the Great Dane (merles usually larger) and Shetland Sheepdog (merles frequently smaller).