In Labrador retriever breed of dogs, coat colour is determined by several genes.

Question

In Labrador retriever breed of dogs, coat colour is determined by several genes. One gene determines the black or chocolate coat colour, with the black colour (B) dominant over chocolate (b). Your neighbour owns two Labradors from the same litter, both female, one black, one chocolate and he paid 5000 dollars to send his black Labrador female to breed with a champion black Labrador owned by a disreputable character. He is not a witness to the mating. Last weekend the neighbour’s dog gave birth to 12 puppies, 7 of which are chocolate colour. What are the most likely and the second most likely explanation for this? Show your working.

Explanation / Answer

If a Labrador Retriever dog possesses dominant phenotype for extension allele (genotype EE or Ee), then it will display the fur colouration determined by its brown locus genotype, while a dog with recessive extension trait (ee) will have a yellow coat with either black (BB, Bb) or brown (bb) exposed skin.

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 genes assort independently, so a single genetic cross involving two black Labradors each with a recessive allele at both the B and E locus (BbEe) has the potential of producing all of the possible colour combinations, while crosses involving chocolate dogs can never produce black but can give rise to yellow, while yellow Labradors will breed true with regard to fur colour but those with black skin can produce a Dudley. Dudleys breed true for both fur and skin. Ability of E locus to override coat colour directed by B locus is a classical example of epistasis, where multiple genetic loci affect the same observed trait.

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