Question

Labradors come in three colors-- black, brown and yellow. What is the genetic basis for these different coat colors? One gene produces melanin, a pigment which is deposited in the dog's fur and makes the color dark. With this gene, allele B (black) is dominant to allele b. Only in the case of a recessive homozygote (bb) will the dog's phenotype be brown. The regulatory gene is separate from the melanin gene but it acts as a switch, either turning the melanin gene on or turning it off. Allele E is dominant and allows for the melanin to be deposited in the dog's fur ("on" switch), but if the switch gene is a recessive homozygote, the melanin is blocked ("off" switch) and a yellow dog is the result!

Review the information on Labrador retrievers above. What are the phenotypic ratios of the F1 generation offspring of two dihybrids? Make sure to match the numbers with coat colors (e.g, which number in the ration goes with which color). Use a Punnett square to determine the ratios and show your work. Make sure to state the phenotypic ratios. Finally, what color are the parents?

Answer 1

P YELLOW X BLACK

F1 = BLACK

F2

BE Be   bE be  

BE BBEE BBEe BbEE BbEe

Be BBEe BBee BbEe Bbee

bE BbEE BbEe bbEE bbEe

be BbEe Bbee bbEe bbee

9 black: 3 Brown : 4 Yellow

9 B-E -: 3bbE-: 3B-ee -: 1bbee

Phenotypic ratios are 9:3:4 in F2.

Labrador Retrievers are a popular dog breed in many countries. There are three recognised colours, black, brown, and yellow, that result from the interplay among genes that direct production and expression of two pigments, eumelanin (brown or black pigment) and pheomelanin (yellow to red pigment), in the fur and skin of the dog. The recognized colours are due to two genes, while a third gene affects the range of colouration observed within the yellow Labrador. These individual genes do not act independently of each other, and their interaction in affecting the trait of coat colour is used to demonstrate the genetic principle of epistasis, where multiple genes react synergistically to affect a single trait.

Genes for black, brown, and yellow colouration

The three recognised colours of Labrador Retrievers result from differences in two genetic loci that affect pigment expression. The first of these affects the colour of the dark pigment, eumelanin, and is referred to as the B (brown) locus. The variation displayed by this locus is observed in many mammals, reflecting a so-called 'dilution', a lightening, of black eumelanin to a brown colour. Initial genetic research excluded a role for the melanocortin 1 receptor and the Agouti locus as being the cause of the black dilution trait in dogs. Instead, TYRP1 (tyrosinase related protein 1) was found to be responsible.This enzyme is localised to melanosomes, the cellular organelles that produce and store pigments, and serves to catalyze oxidation of eumelanin precursors.

In dogs, three mutations in the TYRP1 gene have been identified, one resulting in a truncation of the protein, the other two leading to an amino acid deletion or a single amino acid substitution in the sequence of the protein. All of these mutations are found across the range of dogs, and hence are thought to have preceded the divergence of distinct breeds, and all three are found within Labrador Retrievers. Each of the mutations appears to eliminate or significantly reduce enzymatic activity, and the colouration phenotypes (the visible traits) produced by the three mutations are indistinguishable.

These represent recessive mutations in the TYRP1 gene, and since mammals have two copies of each gene, one from each parent, an animal with at least one copy of the fully functioning TYRP1 protein (represented as 'B') will display the dominant trait, black pigmentation, while to display brown pigmentation, both copies of this gene must be mutant alleles (collectively represented as 'b'). Thus a dog with the genotypes BB or Bb will express black eumelanin, while brown eumelanin will be seen in dogs with the bb genotype.

A recessive mutation in this E gene truncates the protein, producing a non-functional receptor incapable of directing eumelanin deposition in the fur.Among dogs, this mutation is unique to yellow Labrador Retrievers and Golden Retrievers and is thought to have arisen in the retriever population before these individual breeds became distinct. The exact mutation has also been found to underlie the colouration of white coyotes found around Newfoundland, having apparently passed into that population through interbreeding with a Golden Retriever.

As with the B locus, presence of a single copy of the functional receptor gene ('E') will result in the dominant phenotype: presence of eumelanin in the fur. If both copies of this gene are the recessive mutated variant ('e'), the dog will have no eumelanin in its fur. Such a dog will appear yellow, with eumelanin evident only in the skin of the nose, lips, eye rims and foot pads, of a colour determined by the B locus. A variant of the functional MC1R allele that produces a facial 'mask' in other breeds of dogs (E^m) 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.

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.
• Brown 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.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 assort independently, so a single genetic cross involving two black Labradors each with a recessive allele at both the B and the E locus (BbEe) has the potential of producing all of the possible colour combinations, while crosses involving chocolate dogs can never produce black (there being no dominant B allele in either parent) but can give rise to yellow. Yellow Labradors will breed true with regard to fur colour but those with black skin can potentially produce a Dudley. Dudleys breed true for both fur and skin. The ability of the E locus to override the coat colour directed by the B locus is a classical example of epistasis, where multiple genetic loci affect the same observed trait.