Labrador Retriever Coat Colour Genetics - Genotypes

Genotypes

In order to analyse 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 hypothesised 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):

• 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 dominant 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 (dominant 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.