Labrador Retriever coat colour genetics

The three recognised colours of Labrador Retriever (top to bottom): Chocolate, Black and Yellow.

The genetic basis of coat colour in the Labrador Retriever (a highly popular breed of dog) has been studied in detail, and found to depend on several distinct genes. The interplay between these genes is broadly used as an example of epistasis.

Background

Labrador Retrievers are a popular breed of dogs in many countries. There are three recognised colours, Black, Chocolate, and Yellow, that result from the interplay among genes that direct the production and expression of two pigments, eumelanin and phaeomelanin, in the fur and skin of the dog. The recognized colours are due to the interplay of two specific genes, while a third gene affects the range of colouration observed within the yellow Labrador. These individual genes do not act independent of each other, and their interaction in affecting the trait of coat color is used to demonstrate the genetic principle of epistasis, the interplay between multiple genes in affecting a single trait.

The genetics of mammalian colouration has been studied in detail, and similar mechanisms have been identified across many species. For this reason, much of the early work on the coloration of dogs in general and Labradors in particular have relied heavily on analogy to the traits characterized in mice and other mammals. Work done in 1977 using crosses within a population of purebred Labradors showed the involvement of two genes in the production of the three main coat colours of Labradors and described the underlying genetics of these colour varieties.

Genes

Labradors showing the two eumelanin colour phenotypes: Black (BB,Bb) and Chocolate (bb).

The three colours of Labrador Retrievers result from differences in two genetic loci that affect pigment expression. The first of these affects the colour of the dark pigment, eumelanin, that is produced by the dog, and is referred to as the B (brown) locus. The variation displayed by this locus is observed in many mammals, causing 'dilution' of the normal black eumelanin to a brown colour. The gene responsible, TYRP1 (tyrosine related protein 1), encodes an enzyme localized to melanosomes, the cellular organelles that produce pigments, and serves to catalyze oxidation of eumelanin precursors. In dogs, three mutations on this gene have been identified, one resulting in a truncation of the protein, the other two leading to an amino acid deletion or a single amino acid substitution in the sequence of the protein. All of these mutations are found across dog breeds, and are hence though to have preceeded the divergence of distinct breeds. Each of the mutations appears to eliminate protein or significantly reduce enzymatic activity, and the colouration phenotype (the visible trait) produced by the three mutations is indistinguishable. These represent recessive mutations in the TYRP1 gene, and since mammals have two copies of each gene, an animal with at least one copy of the fully function TYRP1 protein (represented as 'B') will display the dominant trait, black pigmentation, while to display the brown pigmentation, both copies of this gene must be mutant alleles (collectively represented as 'b'). Thus a dog with the genotypes BB or Bb will express black eumelanin, while brown eumelanin will be seen in dogs with the bb genotype.

Labradors with the recessive (ee) and dominant (EE, Ee) phenotypes for the expression of eumelanin pigment in the fur.

A second gene affects whether these eumelanin pigments will be expressed in the fur, or solely in the skin. This gene, called the extension allele and represented by the letter E, corresponds to the melanocortin 1 receptor (MC1R). This receptor signals the pigment-producing cell in response to melanocortins, and directs the deposition of eumelanin into the hairs of the dog. A recessive mutation in this gene has been identified that results in truncation of the protein, resulting in a non-functional receptor. This mutation is unique to Yellow Labrador Retrievers and Golden Retrievers, and is thought to have existed in the retriever population before these individual breeds became distinct. The exact mutation has also been found to underlie the colouration of white coyotes found around Labrador, and is thought to have passed into that population through interbreeding with a Golden Retriever. As with the B locus, the presence of a single copy of the functional receptor gene ('E') in a Labrador Retriever will result in the dominant phenotype, the presence of eumelanin in the fur. If both copies of a dog's gene are the recessive mutated variant ('e'), the dog will have no eumelanin in their fur. Such a dog will appear yellow, with the eumelanin evident only in the skin of the lips, nose, and eye rims.

The interplay between these two genes produces the three recognised colours of the Labrador Retriever. If the dog possesses the dominant phenotype for the 'extension' allele (genotype EE or Ee), then it will display the fur colouration determined by their 'brown' allele, while a dog with the recessive extension trait (ee) will have a yellow coat with either black (BB, Bb) or brown (bb) exposed skin. This produces the three colours seen:

Yellow Labradors with black (top) and brown skin colouration.

• Black Labradors can have any genotype with at least one dominant allele at both the B and E loci: BBEE, BBEe, BbEE, or BbEe.
• Chocolate Labradors can have a genotype with a dominant E allele, but must have recessive b alleles: bbEE and bbEe.
• Yellow Labradors with black skin pigment will have a dominant B allele but must have recessive e alleles: Bbee or BBee.
• Yellow Labradors with pale, chocolate pigment or an absence of skin pigment, can have only recessive alleles at both loci: bbee. These dogs are often referred to as Dudleys, and are not eligible for registration under current standards. Aging-related declines in eumelanin production can cause a dog with black skin pigmentation begin to appear lighter, but Dudley dogs have this colouration throughout their lives.

A single genetic cross involving two Black Labradors, if each has a single recessive allele at both the B and the E locus, has the potential of producing all of the possible colour combinations. The ability of the E locus to potentially override the coat colour directed by the B locus is a classical example of epistasis, where multiple genetic loci affect the same phenotype, the observed trait. The E locus also determines whether the phenotype due to the third genetic locus affecting coat colour will be evident.

Colour variation within Yellow Labradors due to differences in phaeomelanin expression.

The third genetic locus is recognised as affecting coat colour within Yellow Labradors, through the expression of phaeomelanin, the pigment responsible for red and yellow pigmentation. The effects on phaeomelanin pigmentation are only seen if there is no eumelanin expressed in the fur, else the dark eumelanin will mask any phaeomelanin present, and thus these differences are visible only in the Yellow Labrador. These dogs can range in colouration from a red to a light cream colour. It had long been thought that the genetic locus for this trait was the same seen regulating phaeomelanin in other mammals, subsequently identified as tyrosinase. This enzyme makes both eumelanin and phaeomelanin, and when subject to a knockout mutation results in albinism. However, a less extreme mutation of the same tyrosinase gene, the so-called Chinchilla trait, produces a 'dilution' effect on phaeomelanin alone, similar to what was observed in Yellow Labradors. Thus this phaeomelanin trait in Labradors has been identified by the letter C, with a pattern of incomplete dominance, where an individual with two copies of the more active allele (CC) would be the darkest, one copy of each (Cc) would be intermediate, and two copies of the lower-expressing version (cc) would be the lightest. However, genetic analysis of the inheritance of Yellow Labrador coat colour shows the locus responsible to be entirely distinct from the C trait of the tyrosinase gene, and likewise distinct from SLC45A2, the gene responsible for absence of phaeomelanin in the white tiger, while a mutation in SLC7A11 found to cause phaeomelanin dilution in mice was not found in a survey of cream-coloured dogs. The gene responsible for the different colour varieties of the Yellow Labrador remains unidentified.

The silver Labrador is not a formally recognised colour variety.

A silver colour variant exists, but because its genetics are unclear and may even have resulted from interbreed crosses, it is not been formally recognised.

Mosaics and other "mis-marks"

At least one example of a Labrador Retriever mosaic for pigmentation has been described. This male dog exhibited random, but distinct black and yellow patches throughout his coat. He was the result of a black female (carrying yellow, Bb) bred to a yellow male. This dog was mated with 2 yellow females, one black female, and 2 chocolate females, and the puppies resulting from these breedings were all consistent with the inheritance pattern of a yellow Labrador with black pigment (Bbee). The most probable cause was a somatic mutation early in development that left cells capable of producing dark fur, and others including the reproductive cells incapable.

Other “mis-marks” such as brindling, tan points, white spots, and rings around the tails are not uncommon in Labradors. Each of these conditions have various underlying genetic as well as environmental causes.

See also

• Labrador Retriever
• Genetics
• Coat (dog)