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Labrador Retriever coat colour genetics: Wikis

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The genetic basis of coat colour in the Labrador Retriever (a highly popular type of dog) has been widely studied, and found to depend on many genes. A wide range of colourings have developed over time.

This article examines the current knowledge about genetic colour determination in the breed.

Contents

Background

Labrador Retrievers are recognized in three colours, Black, Chocolate, and Yellow. In order to breed for a certain colour, it is helpful to know the genotypes of the individuals that are to be bred. Luckily, VetGen now offers the ChromaGene genetic test for Labradors that tests for the genotype of the B and E loci. The test can be found at [1]. This tool is instrumental to breeders by increasing the stud pool of dogs they can breed to (with knowledge of the genotypes of their own bitches) to produce the colours of Labradors they want while knowing the probabilities of the colours that will be produced in a single breeding as well as knowing the possibility of producing a pale pigmented yellow and can easily eliminate that possibility by breeding sires and dams whose combined genotypes have no possibilities of producing that pup.

Genotypes

In order to analyze and understand the inheritance of coat colour in Labradors, a few conventions need to be stated. First, the actual appearance of the dog (black, chocolate, or yellow) is the Phenotype. There are at least nine combinations of alleles that underlie the three colours of the Labrador. These allelic combinations are called genotypes. One allele is inherited from the gametes of the sire and the other allele is inherited from the gametes of the dam. In simple Mendelian Inheritance, one allele overrides or is dominant to the other, resulting in the dominant allele being expressed in the phenotype. Recessive alleles will be expressed in the phenotype when the individual is homozygous recessive, or carries two recessive alleles. When reporting genotypes, dominant alleles are written as capital letters (B) while their recessive counterparts are written with lower case letters (b).

Carol Coode, a well-known Labrador Retriever breeder and judge from England presents the main alleles governing the inheritance of coat colour in Labrador Retrievers as:

B = black
b = chocolate
E = ability to express pigment or coat colour
e = inability to express dark pigment or coat colour

There is another C allele that works in yellow Labradors which will be discussed later. This C allele is responsible for producing red pigment.

In 1977, Templeton, Stewart, and Fletcher published their research regarding Labrador Coat colour Inheritance in The Journal of Heredity. They used a line of purebred Labrador Retrievers developed at the University of Oregon Health Sciences Center in Portland Oregon and used a series of test crosses and experimental breeding to test their hypothesis. They hypothesized that black and chocolate coat colours are determined by simple Mendelian inheritance at the B locus and that yellow coat colour is determined by a simple Mendelian inheritance at the E locus, which is separate from the B locus. They found that black (B) coat colour is dominant to chocolate (b) coat colour and yellow coat colour allele e is recessive to E and is epistatic to the B locus. The B locus is responsible for pigmentation of eyes, lips, nose, and eye rims of yellow dogs.

The following are the possible genotypes for a given Labrador as presented by Carol Coode(1993): LabRetColors.png

  • Labradors whose Phenotype is Black, can have the genotypes: BBEE, BBEe, BbEE, or BbEe.

Black Labradors must have at least one dominant B allele that codes for black pigment and one dominant E allele which codes for the expression of pigment or coat colour.

  • Labradors whose Phenotype is Chocolate, can have the genotypes: bbEE and bbEe.

Chocolate Labradors must be homozygous recessive for the B allele (bb) and have at least 1 domnant E allele to code for expression.

  • Labradors whose Phenotype is Yellow with normal, black pigment, can have the genotypes Bbee or BBee.

In order for the desirable, dark pigment to be expressed, a yellow dog must have a dominant B allele. Yellow dogs result from being homozygous recessive for the E allele (ee) which masks expression of coat colour pigment. The presence of two recessive E alleles always masks the effects of the B alleles no matter what type of B allele (domiant or recessive) is present.

  • Labradors whose Phenotype is Yellow with pale pigment or chocolate pigment or an absence of pigment, will have the genotype bbee.

Because this dog is homozygous recessive for the E allele (ee), he cannot express dark pigment. This, combined with the double chocolate gene (bb), results in the yellow dog having very pale skin pigment (lips, nose, eye-rims, etc) usually seen in chocolate labradors. These dogs are often referred to as Dudleys; they are considered to be a disqualification under current standards.

The outcomes of any cross can be determined with a simple Punnett Square for 2 alleles. For example, a Black dog, with the genotype B/B;E/E bred to any other genotype will produce all Black puppies because the dominant black dog will always produce gametes with B and E. Anytime B and E are paired, no matter what the other allele is, the result will be a black dog. This phenomenon is referred to as recessive epistasis.This is due to the fact that the recessive allele e is epistatic to the ability to express pigment or coat colour.

The Biochemical Pathway

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Melanin

The pigmentation of Labradors is controlled by melanin, a pigment molecule. Melanin is controlled by the “B” locus and when its gene product is made, it is packaged into melanosomes, small organelles in each cell. Melanosomes are transferred by melanocytes into the cells that make up the structure for the eyes, hair, and skin. The colour of the cells is determined by the colour of the melanosome and the density of melanosomes within the cell. (Davol, 1999)

In coat colour of dogs, there are two types of melanin; Eumelanin and Pheomelanin. Eumelanin is responsible for black or brown pigment and Pheomelanin is responsible for red or yellow pigment.

Eumelanin, controlled by the B locus, is responsible for producing and packaging the melanin into the melanosome. Both the “B” and “b” alleles will produce eumelanin that contribute to pigmentation, but the dominant allele “B” is more efficient and packs more melanin into each melanosome, producing a black dog. The less efficient recessive allele “b” is responsible for chocolate colouration when the individual is homozygous recessive at the B locus. (Davol, 1999 and Hales, 2007)

Pheomelanin, controlled by the C locus, is responsible for the red/yellow pigment and is always produced, whether or not Eumelanin (black/brown) is produced. When the Eumelanin is not produced, due to epistasis, the pheomelanin is the only pigment in the cell, resulting in a yellow coated Labrador. A yellow dog can have any of these genotypes at the “C” locus: C/C, C/c, or c/c. The C/C produces more pigment, resulting in a darker yellow and the c/c genotype results in a lighter yellow dog. An individual heterozygous for the “C” allele (C/c) will be a medium shade of yellow. (Davol, 1999 and Hales, 2007).

The Phenomenon of Epistasis

Epistasis, as defined by iGenetics: a Molecular Approach, “is the interaction between two or more genes that controls a single phenotype. For instance, the expression of a gene at one locus can mask or suppress the expression of a gene at another locus.” (733). Labrador coat colour inheritance, the E (expression allele) masking B (pigment allele) is an example of recessive epistasis, where an individual homozygous for E (ee) masks the pigment coded for by the B allele, resulting in a yellow dog, despite the genotype at the B locus (BB, Bb, or bb). The homozygous e allele does not completely restrict the expression of the B locus alleles as they are responsible for the pigmentation of the iris, lips, nose, and eye rims of a yellow dog (Templeton 1997).

The Role of the E Locus

Melaninpathway.jpg 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.

Mosaics Labradors and other "mis-marks"

Sponenberg and Bigelow published their findings in the Journal of Heredity on a mosaic male Labrador Retriever who exhibited random, but distinct black and yellow patches throughout his coat. He was the result of a black female (carrying yellow) bred to a yellow male. This dog was mated to 2 yellow females, one black female, and 2 chocolate females. All puppies resulting from these breedings were either black or yellow and the percentages of each in each mating follow the inheritance pattern of a yellow Labrador (BBee) with black pigment. They explained that this pattern was not characteristic of a “merle” coat because merle develops from a transposable element and because all of the mosaic dog’s puppies did not exhibit mosaic colouring, but were normal black or yellow, therefore there is no evidence of a transposable element. Sponenberg acknowledges the likelihood of a recessive gene causing this mosaic pattern, but says it would be unique and is highly unlikely. The most probable cause was a somatic mutation early in development. (1987)

Other “mis-marks” such as brindling, tan points, white spots, and rings around the tails are common in Labradors. Each of these conditions have various underlying genetic as well as environmental causes. Examples of “mis-marked” Labradors can be found at [2].

References

Coode, C. "Colour Inheritance." Labrador Retrievers Today. Howell Book House: New York 1993. 28-32.

Davol, P. 1999. B/b, E/e, and Beyond: A Detailed Examination of Coat colour Genetics in the Labrador Retriever. [3]

Everts, R., J. Rothuizen, and B van Oost 2000. "Identification of a premature stop codon in the melanocyte-stimiulating hormone receptor gene (MC1R) in Labrador and Golden retrievers with yellow coat colour." Animal Genetics 31:194-199.

Hales, K. Davidson College: Biology 301 Lecture. September 26, 2007.

Russel, P.J. iGenetics: A Molecular Approach. Pearson Publishing: New York 2006. 733. Sponenberg, D.P. and B.J. Bigelow 1986. "An Extension Locus Mosaic Labrador Retriever Dog." The Journal of Heredity 78: 406

Templeton, J., A. Stewart, and W. Fletcher 1977. "Coat colour Genetics in the Labrador Retriever." The Journal of Heredity 68: 134-136

See also


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