Role of The E Locus
In Labradors, melanin will only be made if the cell receives a signal to do so, this signal is coded for by the E Locus, which codes for the melanocyte stimulating hormone receptor (MC1R). Labradors that are homozygous dominant for the E allele (E/E) have a mutant, extremely active form of MC1R where eumelanin is constantly being produced, so the dog will appear either black or chocolate. Labradors that are homozygous recessive for the E allele (e/e) also have a mutant for of MC1R, but this mutant is a “loss of function” receptor that cannot produce eumelanin. Labradors that are homozygous recessive for the E allele will only produce pheomelanin and will appear yellow. The recessive E allele (e/e) masks the B allele which is an example of recessive epistasis. This means that if the genotype for an individual is homozygous recessive, it will not matter what the B locus allele is for coat colour, all individuals will be yellow because the black/brown pigment will not be produced. The E allele masks the dominant C allele (red/yellow pigment) and is an example of dominant epistasis. This means that if there is a single dominant E allele, either black or brown pigment will be produced and the red/yellow pigment coded for by the C allele will not be visible. (Davol 1999).
It is interesting to note that in 2000, Everts et al. isolated and cloned the gene responsible for the MC1R receptor in dogs. The yellow Labrador has a substitution in this gene which changes the codon for Arginine to a stop codon. This nonsense mutation is found in yellow Labradors as well as Golden retrievers. Seventeen different breeds of dogs were tested for this mutation and none had it, suggesting that this mutation leads to the yellow coat colour and that this mutation has a common origin in the two breeds.
Read more about this topic: Labrador Retriever Coat Colour Genetics, Biochemical Pathway
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