Labrador Retriever coat colour genetics
The genetic basis of coat colour in the
.Background
Labrador Retrievers are a popular
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.[3] 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.[4][5][6] 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.[7]
Genes for black, chocolate, and yellow colouration
Pheomelanin in yellow Labradors
According to Candille, et al. (2007), dog coat color can largely be explained by three genes:
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
Eumelanin colour
The three recognised colours of Labrador Retrievers result from differences in two
In dogs, three
These represent
Eumelanin distribution
A second gene affects whether these eumelanin pigments will be expressed in the fur or solely in the skin. Called the 'extension' (E) trait, this is directed by the melanocortin 1 receptor (MC1R). This receptor signals the pigment-producing cell in response to melanocortins and results in deposition of eumelanin into the hair. Mutations in this protein have been shown to be involved in pale or red colour phenotypes in a range of species, including humans, horses, pigs, cattle, mice, fur seals, mammoths and the Kermode bear, as well as colouration in whiptail lizards.[13]
In most dogs, activity of MC1R is modulated by two signaling molecules, a repressor that is a product of the
A recessive mutation in this E gene truncates the protein, producing a non-functional receptor incapable of directing eumelanin deposition in the fur.
A variant of the functional MC1R allele that produces a facial 'mask' in other breeds of dogs (Em) is also present in Labradors, but since the colour of the mask is determined by the B locus, in Labradors the mask this gene produces is indistinguishable from the overall coat colour.[18]
Eumelanin gene interactions
The interplay between these two genes determines the colour of a Labrador Retriever, and is widely used as an example of epistasis. If a dog possesses the dominant phenotype for the extension allele (genotype EE or Ee), then it will display the fur colouration determined by its brown locus genotype, 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 results in the three coat colours seen:
- 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 will have a genotype with at least one dominant E allele, but must have only 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 or 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 disqualified in the showring, although are eligible for registration under current standards.[1] Aging-related declines in eumelanin production can cause the exposed skin in a Labrador with black skin pigmentation begin to appear lighter, but Dudley dogs have this colouration throughout their lives.
These genes
In a study of Labrador retrievers in the United Kingdom, it was found that chocolate labradors had a shorter average lifespan than either black or yellow labradors. They were also found to suffer from more skin and ear disorders. It is unknown whether this is a direct consequence of their melanin genotype, or is due to other recessive genes, amplified through the inbreeding used to propagate the chocolate phenotype.[19]
The dilute gene in the Labrador Retriever
The American Kennel Club (AKC) and other kennel clubs around the world recognize three coat colours in the Labrador: black, yellow and chocolate.[5] In recent years, other colours have become more prominent in the breed through cross breeding with other breeds. Breeders refer to these colours as 'silver', 'charcoal' and 'champagne'. These dogs typically have a metallic-looking sheen to the hair. These are conformation disqualifications within the breed and are linked with a skin disease known as Color Dilution Alopecia. The gene affecting this colour variation in all dog breeds is the recessive 'dilution' (D) locus. It is possible for each of the standard colour genotypes to be diluted if the dog carries two copies of the recessive dilute allele, dd. Dogs that carry at least one D will not have a diluted coat. If two dogs carrying the Dd genotype are bred, diluted offspring could be produced.[20] Studies have linked the diluted trait to a mutation in the melanophilin (MLPH) gene.[21][22][23][24]
The dilution factor was not originally a visible part of the genetics of Labrador Retrievers, and therefore, controversy surrounds the topic.[25] Novel pigmentation can arise through long-masked recessive traits being brought to the fore by inbreeding to select for other traits, through undisclosed outbreeding with other breeds to introduce novel traits, or through spontaneous mutation. There are many breeders in the United States who specialize in breeding these diluted Labradors. The standard for Labrador Retriever does not include dilution colours, and stipulate that any dilute is a breed disqualification, although the American Kennel Club will register purebred Labs that are dilute in colour under the colours of black, yellow or chocolate.[26] The Labrador Retriever Club, Inc. states that there is no silver gene in pure-bred Labrador Retrievers.[27] However, the American Kennel Club has maintained that their registry is based on parentage, not colour.
Mosaics and other "mis-marks"
At least one example of a Labrador Retriever mosaic for pigmentation has been described.[28] This male dog exhibited random but distinct black and yellow patches throughout his coat. He was the result of a black female heterozygous for yellow (B_Ee) bred to a yellow male (B_ee), and was mated with Labradors of each of the recognised colours. The resulting puppies were all consistent with the inheritance pattern of a yellow Labrador with black pigment. The most probable cause was either a somatic mutation early in development or a fusion between two zygotes that left some cells with genetics capable of producing dark fur, and others including the reproductive cells incapable of doing so.
Other "mis-marks" such as brindle, 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
Notes
- ^ Candille, et al. 2007 note that "Mc1r activation causes exclusive production of eumelanin, whereas Mc1r inhibition causes exclusive production of pheomelanin (5, 10). Thus, gain-of-function Mc1r mutations cause dominant inheritance of a black coat, whereas gain-of-function Agouti mutations cause dominant inheritance of a yellow coat."
References
- ^ a b c Carol Coode, Labrador Retrievers Today, Howell Book House: New York, 1993.
- ^ a b Jane B. Reese et al., Campbell Biology, 9th Ed., Benjamin Cummings, Boston, 2011, p. 273.
- ^ a b c d Christopher B. Kaelin and Gregory S. Barsh, "Genetics and Pigmentation in Dogs and Cats", Annual Review of Animal Bioscience, 1: 125-156 (2013)
- ^ O. Winge, Inheritance in Dogs with Special Reference to the Hunting Breeds, (Ithaca, NY: Comstock Publishing, 1950)
- ^ a b C. C. Little, Inheritance of Coat Color in Dogs, (Ithaca, NY: Comstock Publishing, 1957)
- ^ a b c Sheila M. Schmuts, Tom G. Berryere and Angela D. Goldfinch, "TYRP1 and MC1R genotypes and their effects on coat color in dogs", Mammalian Genome, 13: 380-387 (2002)
- .
- ^ PMID 17947548. "Production of yellow versus black pigment in dogs is controlled by three genes: Mc1r, Agouti, and CBD103. Dogs carrying wild-type alleles for all three genes have a yellow coat resulting from Agouti antagonism of Mc1r signaling in melanocytes (yellow Great Dane, top). Dogs carrying a loss-of-function mutation at Mc1r have a yellow coat, regardless of their genotype at Agouti or CBD103 (yellow Labrador Retriever, middle). Dogs carrying wild-type alleles for Mc1r and Agouti, together with the dominant black allele of CBD103 (KB) have a black coat resulting from the interaction between a β-defensin and Mc1r (black Curly Coated Retriever, bottom)."
- ^ Sheila M. Schmutz and Tom G. Berryere, The Genetics of Cream Coat Color in Dogs, Journal of Heredity, 98: 544-548 (2007)
- ^ Xiao Xu, et al., The Genetic Basis of White Tigers, Current Biology, 23: 1031-1035 (2013)
- ^ S. M. Schmutz and T. G. Berryere, Genes affecting coat colour and pattern in domestic dogs: a review, Animal Genetics, 38: 539-549 (2007)
- ^ J.A. Kerns, M. Oliver, G. Lust and G. S. Barsh, "Exclusion of Melanocortin-1 Receptor (Mc1r) and Agouti as Candidates for Dominant Black in Dogs", Journal of Heredity, 94: 75-79 (2003)
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- ^ Ruvinsky, A., Sampson, J. The Genetics of the Dog, 2001, Wallingford, Oxfordshire, UK, ebook.
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- ^ E. K. Conant, R. Juras, and E. G. Cothran, (2011) "Incidence of the mask phenotype M264V mutation in Labrador Retrievers", Research in Veterinary Science 91: e98-e99
- ^ Paul Heltzel (October 23, 2018). "Why chocolate labs don't live as long as other retrievers". National Geographic. Archived from the original on April 3, 2019. Retrieved May 11, 2019.
- ^ Coode, C. 1993. Colour Inheritance. Labrador Retrievers Today. Howell Book House, New York. 28-32.
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- ^ Kurtz, K. 2013. Genetic aspects and controversies of coat color inheritance in the Labrador retriever. Animal Science 314, Michigan State University.
- ^ Vanderwyk, Jack 2012. Analysis of the ‘silver’ Labrador population. https://web.archive.org/web/20170724123017/http://labradornet.com/silverlabsanalysis.html
- ^ "Silver Labradors - The Labrador Retriever Club, Inc". thelabradorclub.com. Archived from the original on 2018-01-07.
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