Labrador Retriever coat colour genetics

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.

Labrador Retrievers are a popular breed of dog in many countries. There are three recognised colours, black, chocolate, and yellow, that result from the interplay among genes that direct production and expression of two pigments, eumelanin and pheomelanin, in the fur and skin of the dog. The recognized colours are due to two genes, while a third gene affects the range of colouration observed within the yellow Labrador. These individual genes do not act independently of each other, and their interaction in affecting the trait of coat colour is used to demonstrate the genetic principle of epistasis, where multiple genes affect 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 colouration of dogs in general and Labradors in particular have relied heavily on analogy to the traits characterized in mice and other mammals. Initial genetic studies of coat colour in dogs published in the 1950s concluded that there were two main genes involved, one distinguishing blacks from browns, and the other distinguishing blacks from reds and yellows. A 1977 study using crosses within a population of purebred Labradors showed the involvement of two specific genes in production of the three main coat colours of Labradors and described the underlying genetics of these colour varieties.

The three recognised 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, and is referred to as the B (brown) locus. The variation displayed by this locus is observed in many mammals, reflecting a so-called 'dilution', a lightening, of black eumelanin to a brown colour. Initial genetic research excluded a role for the melanocortin 1 receptor and the Agouti locus as being the cause of the black dilution trait in dogs. Instead, TYRP1 (tyrosine related protein 1) was found to be responsible. This enzyme is localised to melanosomes, the cellular organelles that produce and store pigments, and serves to catalyze oxidation of eumelanin precursors. In dogs, three mutations in 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 the range of dogs, and hence are thought to have preceded the divergence of distinct breeds, and all three are found within Labrador Retrievers. Each of the mutations appears to eliminate or significantly reduce enzymatic activity, and the colouration phenotypes (the visible traits) produced by the three mutations are indistinguishable. These represent recessive mutations in the TYRP1 gene, and since mammals have two copies of each gene, one from each parent, an animal with at least one copy of the fully functioning TYRP1 protein (represented as 'B') will display the dominant trait, black pigmentation, while to display 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.

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Wikipedia