GENETICS OF COAT COLOUR IN RETRIEVERS

by Sue Taylor, DVM, Saskatoon, Saskatchewan

The traditional unit of heredity is the gene, and there are 50,000 to 100,000 genes in dogs which determine the inheritance of many normal traits as well as the inheritance of genetic defects. Approximately 10 of these genes influence coat colour. Some of these 10 genes determine colour itself while others influence the distribution of colour or the intensity of colour in individual hairs or in the entire coat.

        Within each cell of the body (except for the sperm and egg cells) there are 39 pairs of chromosomes. One member of each pair of chromosomes is supplied by the sire, and the other is supplied by the dam. Each chromosome is made up of genes, the units of inheritance. Each gene is a sequence of base pairs (on a DNA strand) that codes for a particular trait (or set of traits). While the 2 chromosomes in a pair are not identical, they contain genes for all the same traits. These genes are lined up in a sequence, so that each one occupies a specific site on the two paired chromosomes.

        This means that each dog has two versions of every gene, on inherited from its sire and one from its dam. They may be identical or they may be different alleles of the gene (any of the variations on a gene). If both members of a gene pair (alleles) are identical, then the animal is said to be homozygous for that gene (the dog got identical genes at that particular site from its sire and its dam). If the alleles are different, then the dog is heterozygous for that gene.

        Some alleles are dominant to others, masking the presence of the recessive gene in the pair. For example if both A and a are present (Aa) and A is dominant to a, the dog will look just as if it was genetically AA. That dog will, however, pass on the dominant A allele to approximately half of its offspring and it will pass the recessive a on to the other half of its offspring. This is only one way in which dogs and bitches can pass of hidden genetic traits to their offspring.

        There are many different ways in which the genetic makeup of a dog can determine its colouration. We know that there are at least ten sites on the chromosome where a gene pair occurs which can influence coat colour. Some of the genes govern colour itself (black, brown, yellow) while others influence the distribution of colour, resulting in light coloured points (as in Dobermans), dark muzzles (as in Mastiffs and Great Danes), brindling (as seen in Boxers and Whippets), ticking (as seen in some hounds) or spots (as seen in Beagles). In addition, genes at many of the sites on the chromosome interact with genes at other sites so that together they produce a different result than if they were present in isolation.

        The topic of coat colour genetics in dogs is very complex, but if we restrict our discussion to retrievers we are fortunate because there are essentially only 3 genes that vary between these dogs that influence coat colour. The other 7 sites are constant so that retrievers don’t have dark masks, points, ticking, spots or brindling.

        In retrievers, the important variable regions are known as B, E and C regions. At each of these locations on the chromosome, the pair of alleles that are present will significantly determine the colour of the coat (black vs. chocolate vs. yellow), while genes at C determine the shade or the intensity of the lighter pigments.

Site C: Rich vs. Dilute pigment

        The gene pair at this site determines the depth of pigmentation in dogs that are not black. The allele C allows for full rich colour of the tan, red or golden coat, while the allele c^ch lightens or dilutes the pigmentation. C is incompletely dominant to c^ch.

In chocolate or liver dogs:

C C         dark brown

C c^ch        medium brown

c^ch c^ch              light brown

In yellow or red dogs:

C C         dark or bronze or red

C c^ch        medium shades

c^ch c^ch       nearly white

Site B: Black vs. Brown

        The gene pair at this site determines the amount and quality of dark (black) pigment in hairs. Black coat colour is dominant so dogs that are genetically BB or Bb will be black with black noses (unless they are yellow – see below).

Labrador and Flat-Coated Retrievers can be BB (black), Bb (black) or bb (chocolate/ liver).

BB           black dog not carrying chocolate

Bb           black dog carrying chocolate

bb           chocolate dog

Chesapeake Bay Retrievers are all bb.

Golden Retrievers are all BB (black pigment) – their red/ yellow colour results from the recessive gene pair ee at site E.

Site E: Non-yellow vs. Yellow

        Yellow is on an entirely different spot on the chromosome so is independent of black/ chocolate status. The gene pair at site E determines the ability to distribute dark pigment in the coat. The dominant allele (E) allows for a solid dark colour (as in black or chocolate dogs), so that dogs that are genetically EE or Ee will be black or chocolate depending on the gene pair present at site B. Only dogs homozygous for the recessive allele (ee) will have yellow or red hair – they will be yellow or red regardless of which gene pairing occurs at the B location.

Labrador Colours

Black dogs

BB EE     black, no recessives

Bb EE      black, carries chocolate

BB Ee      black, carries yellow

Bb Ee      black, carries yellow and chocolate

Yellow dogs

BB ee       yellow, carries black

Bb ee       yellow, carries black and chocolate

bb ee       yellow, carries chocolate

Chocolate dogs

bb EE      chocolate

bb Ee       chocolate, carries yellow

Combinations

1.     Yellow X Yellow is always yellow.

2.     Chocolate X Chocolate is usually all chocolate. If both the sire and dam carry yellow (bbEe) then you could get ¾ chocolate pups, ¼ yellow pups.

3.     Black X Black has many possibilities depending on the recessives carried – but realize that most black dogs are BBEE (carrying neither chocolate nor yellow).

All Golden Retrievers are ee.

All Yellow Labrador Retrievers are ee.

All Flat-coated Retrievers and Chesapeake Bay Retrievers are EE.