page_title: Labrador Retriever coat colour genetics
section_title: Pheomelanin in yellow Labradors
hierarchical_section_title: Labrador Retriever coat colour genetics / Pheomelanin in yellow Labradors
caption_attribution_description: Yellow and "fox red" Labrador Retriever.
context_page_description: The genetic basis of coat colour in the Labrador Retriever has been found to depend on several distinct genes. The interplay among these genes is broadly used as an example of epistasis.
context_section_description: The E locus also determines whether the phenotype due to the third genetic locus affecting coat colour will be evident. This locus is recognised as affecting coat colour through the expression of pheomelanin, the pigment responsible for red and yellow pigmentation. The effects on pheomelanin pigmentation are only seen if there is no eumelanin expressed in the fur, else the dark eumelanin will mask any pheomelanin present.  Thus these differences are visible only in yellow Labradors, which as a result range in colour from light cream to copper-red.
  It had long been thought that the genetic locus for this trait was the same seen regulating pheomelanin in other mammals, subsequently identified as tyrosinase.  This enzyme makes both eumelanin and pheomelanin, and when subject to a knockout mutation results in albinism.  A less extreme mutation of the same tyrosinase gene, the so-called Chinchilla trait, produces a dilution that selectively affects pheomelanin alone, similar to the phenotype observed in yellow Labradors.  Thus, as with Chinchilla-related pheomelanin dilution in other species, this trait in yellow Labradors has been represented by the letter C. However, genetic analysis of the inheritance of coat colour in yellow Labradors has shown that the locus responsible is entirely distinct from the Chinchilla trait of the tyrosinase gene, and likewise is distinct from SLC45A2, the so-called cream gene responsible for the dilution of pheomelanin in buckskin, palomino and cremello horses and also for the absence of pheomelanin in the white tiger, while a mutation in SLC7A11 found to cause pheomelanin dilution in mice was not found in a survey of cream-coloured dogs.  The gene responsible for colour variation within the yellow Labrador remains unidentified, but the variation it causes is formally recognised as part of the normal pigmentation spectrum seen within the breed.  A pattern of incomplete dominance is observed: individuals with two copies of the more active allele (CC) would be the darkest, those with one copy of each (Cc) would be intermediate, and those having two copies of the lower-expressing version (cc) would be the lightest.
caption: Colour variation within yellow Labradors due to differences in pheomelanin expression.
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