TYRP1, or tyrosinase-related protein 1, is a protein that plays a role in the creation of the pigment eumelanin. This pigment is what causes the darkening of a dog’s coat color, creating black or brown coats. In the dominant form of the TYRP1 gene (aka the B Locus), enough eumelanin is produced so that the dog's coat appears black instead of chocolate.

A mutation in the TYRP1 gene can occur, causing a change in the production of the eumelanin. This dilutes the black color pigment into a brown color. This mutated gene is known as the "b" allele. When a dog is homozygous for the mutation, meaning he has two copies of the recessive allele (b/b), all black pigment appears brown.

Variants in the TYRP1 gene, or B locus, are the most common cause of brown, chocolate or liver coat color. There are currently a total of five known TYRP1 gene mutations that impact the genes functionality, explaining the majority of dogs with brown coat and nose color. One exception is the brown or chocolate French bulldog, which remained a mystery as to the cause.

Genetic basis

Recently, research conducted in University of Bern in Switzerland revealed a nonsense variant in HPS3, c.2420G>A or p.(Trp807*). In this study, the brown or chocolate dog was homozygous for the mutant allele.  

The Intensity-locus (I-Locus) in dogs is linked to the gene encoding the Major Facilitator Superfamily domain-containing protein 12 (MFS12), which plays a role in phaeomelanin production (light pigment). In dogs with the wild-type genotype, hair expressing only phaeomelanin displays as a red, gold, or tan coat. Conversely, a recessive mutation in MFS12 leads to a reduction in phaeomelanin production, resulting in a lighter color spectrum from cream to white.

Inheritance

The wild-type I-Locus allele is denoted as I, while the recessive allele responsible for pigment dilution is marked as i. The allele displays semi-dominance, implying that dogs with the I/I genotype are
unaffected, while those with I/i will exhibit less pigment dilution compared to dogs with i/i.

The impact of the I-Locus dilution is most noticeable in dogs with the recessive red mutation on the E-Locus (e/e), as it influences the entire coat. Furthermore, in dogs with patterns like black-and-tan or
saddle tan, the tan portions of their coat will also undergo dilution to cream or white.

Coat colours in dogs are determined by two types of pigment: pheomelanin and eumelanin. Pheomelanin has a brownish colour and shows up in different shades ranging from blond in the Golden Retriever til red-brown in the Irish Setter. Eumelanin is black or brown and can be diluted to grey (blue) or liver.

Genetic basis

Despite the fact that ee-red is a pretty rare color in Border Collies, the gene MC1R itself encodes for a protein that plays a central role in coat colors of dogs. The gene encodes for a receptor (MelanoCortin 1Receptor) that controls whether the pigment cell produces pheomelanin or eumelanin.

The most common mutation is a change in a single nucleotide which causes the protein to be shortened by eleven amino acids. This renders the receptor non functional. Previously this allele was labeled as e but with the discovery of other loss-of-function mutations this is now labeled as e1.

Traditionally the brown coat colour was called "red" in some English speaking countries. This is confusing because the E-locus produces a red/blond coat color. Therefore the color produced by homozygous ee dogs is often called "ee-red" to prevent any confusion.

The MLPH gene codes for a protein called melanophilin, which is responsible for transporting and fixing melanin-containing cells. A mutation in this gene leads to improper distribution of these cells,
causing a diluted coat color. The mutation causing color dilution is recessive, and two copies of the mutated gene (the D allele or the D locus) are needed to produce the diluted coat color.

The MLPH mutation affects both eumelanin and phaeomelanin pigments. These pigments control the color of the dog. Black, brown, and yellow dogs can all be affected by the D locus. However, the effects of the dilution are more pronounced in black dogs. A diluted black dog becomes known as a blue dog.

There have been four different alleles identified in a dog’s genes that signal the agouti coloration (this
coloration can be exemplified as the coloration of the wild brown rabbit), also known as the A locus. These alleles are Ay, aw, at, and a. These alleles are dominant in a hierarchy. However, this is all dependent on whether or not the dog carries the dominant black gene at the K locus or the recessive gene on the E locus. If a dog carries one or both of these genes, the A locus is muted and the agouti coloration will not appear on the dog. This is because both the K locus and the E locus are dominant over the A locus.

The "Ay" Allele

The Ay allele is the most dominant of all four alleles on the A locus. The Ay gene produces a range of coat colors like light fawn colors, darker red colors, or even sable. This variation of color is due to variances in the expression of this gene.

The "aw" Allele

The aw allele produces a color known as "wild sable." This coat coloration is sometimes called the "wild type," or in some breeds, "wild boar." With this coloration, the hairs switch pigmentation from a
black color to a reddish or fawn color. This color is sometimes seen in German Shepherds and other shepherd breeds. It is recessive only to the Ay allele. This also means that it is dominant to the at and a alleles, and will be expressed before the at and a alleles.

The "at" Allele

Both the black-and-tan and tricolor phenotypes (expressed traits) are caused by the at allele. A tricolor dog is black-and-tan, with white. White is generally just an absence of color, rather than a pigment the dog expresses. For a dog to be black-and-tan or tricolor, he must be n/n for the dominant
black gene (the K locus). This means that the K locus is not expressed, and the dog will not be black. Furthermore, the dog must have either two copies of the at allele, or have one copy of the at allele and one copy of the a allele.

The "a" Allele

If a dog is ky/ky on the K locus, the dog must be n/n for Ay, aw, and at in order for the dog to express the a/a coloration. A dog that is solid black with the recessive K locus must also have the recessive a/a
allele in order to express the black coloration.

The term "furnishings" refers to the longer moustache and eyebrows seen in dogs with wire hair, as well as some other breeds. Some breeds are fixed for this trait, such as the Airedale Terrier, which is known for their longer mustaches. However, in other breeds, this trait can be variable. In breeds such as the Portuguese Water Dog, furnishings can be variable. However, furnishings are required for a dog to adhere to the breed standard. In the Portuguese Water Dog, dogs without furnishings are referred to as having an "improper coat."

Furnishings are a dominant trait, meaning that a dog only needs to have one copy of the furnishings gene to show the physical trait. The breed standard for Labradoodle, Goldendoodles and Portuguese Water Dog requires “furnishings.” In these dog breeds, a lack of the dominant RSPO2 variant causes an improper coat, characterized by short hair on the head, face and legs.

Hair curl or wavy coat is a dominant characteristic caused by 2 separate known mutations in the KRT71 gene. This gene codes for keratin, a protein that determines the type of hair a dog will have.

The C2 variant causes a slightly different type of curl or wavy coat than the first described curl variant. In addition to hair curl the C2/C2 allele is also believed to be a genetic risk factor for follicular dysplasia in some breeds. Because the hair curl gene is dominant, a dog only needs to have single copy of either curl variant to express that phenotype.

Hair length in dogs is a fascinating mix of simple Mendelian inheritance and complex breed-specific variations. While most traits are governed by the L Locus, recent findings in 2025 and early 2026 have expanded our understanding of how multiple variants interact.

Some breeds of dogs, such as Labradors, always have a short-haired coat. Some breeds, such as Poodles, always have long hair. Other breeds can have either type of coat, like the Dachshund. This type of long coat is caused by a recessive genetic mutation in the FGF5 gene. The FGF5 gene controls the hair cycle and tells the hair when to stop growing. Because it is a recessive mutation, a dog must have two copies of the recessive long-hair allele (l/l) to cause the dog to have long hair.

Harlequin is a coat pattern recognized only in Great Danes. The harlequin pattern is a result of a complex interaction between variances in the merle and the harlequin loci (M and H loci). The harlequin variant acts as a modifier of merle. All harlequin dogs must have one or two copies of the mutation responsible for the merle coloration pattern. Harlequin patterns cannot be expressed in dogs that are not merle or only have red pigment.

The dominant merle gene produces a coat consisting of dark spots on a diluted background. If a merle dog also inherits a single copy of the harlequin gene, the dark spots increase in size and the background pigment is eliminated. Harlequin is presumed to be homozygous embryonic lethal.

The dark mask trait is represented as a darker coloration of the coat covering the eyes and mainly the muzzle, as a consequence of the expression of black pigment (eumelanin) in these areas of the face. The variant is located in the MC1R gene (also known as the extension locus or E locus) which is involved in the switch from pheomelanin (red/yellow pigment) to eumelanin production in melanocytes. Therefore, the MC1R gene is also involved in determining the dark color of the entire coat of the dog.

Genetic basis

The dark mask variant is also known as "Em" located at the MC1R gene or E locus. The presence of one or two copies of the c.790A>G variant will increase the likelihood of expressing the dark mask phenotype, as it is a dominant trait. In the case where the dog's coat color< is dark, the trait may be unrecognizable.

The condition known as "bob-tail" or "short tail" is caused by a mutation in the Brachyury gene (a gene that codes for an important protein). The inherited trait is autosomal dominant. This means that a
dog only has to carry a single copy of the gene in order for the dog to have a bob-tail. Dogs that carry one (BT/bt) or two copies (BT/BT) of the mutation will have a natural short tail. Dogs with two copies of the normal gene (who are bt/bt) will have normal tail length.

In the homozygous state (BT/BT), the Brachyury mutation is lethal in utero. For this reason, breeding two dogs with the bob-tail gene generally results in somewhat reduced litter sizes. This is important to realize when breeding short-tailed dogs with one another.

Coat coloration is controlled by several different genes in dogs. The K locus, also known as the dominant black gene, is due to a mutation in a Beta-defensin gene (CBD103). This gene binds proteins and other pigment type cells to produce the different variations of the K locus. This gene occurs in certain breeds and has an important impact on many types of colorations.

The K locus is dependent on the E locus. If the E locus genotype is e/e (recessive), the K locus is not expressed. However, if the E locus is coded as E/E or E/e, the K locus is still expressed.

There is no single basis for white spotted patterns that occur in animals like cats, dogs and horses. In horses, random white spotting, or deletions of color, have been determined to be caused by more than
half a dozen known genetic factors.  In more than 25 different dog breeds, a mutation found in a gene called the Microphthalmia Associated Transcription Factor-(MITF) is associated with a piebald spotting.

In many breeds, piebald behaves as a “dosage”-dependent trait. This means that the amount of white a dog expresses depends on how many copies of the S locus allele a dog receives. Dogs like the
French Bulldog who have a single copy of the MITF variant (S locus allele) will express a limited white spotting pattern. Dogs that have 2 copies (S/S) of the variant will exhibit more white with very little
color. In some breeds, dogs that are coded as S/S are completely white, while dogs that are n/S have what is referred to as the mantle coloration. The mantle coloration is seen in several breeds of dogs, butis perhaps best exemplified by the Great Dane (black with a white chest/throat and feet).