Coat Colour Genetics
Chromosomes (genes) have various locations or what are called locus (loci plural) which are responsible for the coat colour patterns. Each of this loci is responsible for one or more trait either to work independently or in combination with another locus. These interactions affect the distribution of two major types of melanin - Phaeomelanin and Eumelanin.
Enter Epistasis. Epistasis occurs when genes at two different loci interact to affect the expression of a single trait. A gene can either mask or modify the phenotype controlled by the other gene.
I found this wonderful website called Dog Genetics and it is full of information www.doggenetics.co.uk. I suggest starting from basic concepts to the more complex (e.g. agouti, merle, dilution and tan points). Another good website to simulate coat color inheritance is www.blueknightlabs.com/color/coatcolor.html by Dian Welle of Blue Knight Labradors.
The loci that are of interest in Labrador Retriever breed are the E and B loci.
The B locus (Black or Brown allele) - In the dominant form of this gene, or the "B" allele, normal eumelanin is produced in the coat, and the dog's coat appears black in color. A recessive form of the gene lessens the intensity of the black color pigment turning it to a brown color pigment. This recessive gene is known as the "b" allele. Only when a dog has two copies of the recessive "b" allele, its coat will be brown in color. The brown color is termed as liver/chocolate in Labrador Retrievers.
E-Locus (Recessive Yellow, Melanistic Mask Allele) - The dominant form of the gene, the "E" allele, allows the dog to produce eumelanin, which is a black pigment. A mutation in the gene causes the pigment-producing cells to only produce phaeomelanin, which is a yellow pigment. This form of the gene is represented as the "e" allele. The "e" allele is recessive, meaning that a dog must have two copies of the "e" allele to express the yellow coat color.
To illustrate the interplay between the E and B genes and how they affect the resulting coat colour, below are diagrams with the possible gene combinations and their phenotype (outward appearance).
So it is easy to see why blacks dogs are statistically more common because of the combination required to produce the black phenotype.
Phaeomelanin & Eumelanin
In humans, melanin is responsible for skin, hair, and eye colour. People with dark skin have more melanin than people with light coloured skin.
In dogs, there are two forms of melanin that interact to create coat colour—eumelanin and phaeomelanin.
Eumelanin is black pigment, while Phaeomelanin is red/yellow/cream pigment.
How do I TEST FOR COAT COLOUR?
Coat colour DNA test may be included in a full breed profile test or can be purchased as a stand-alone test. A swab sample is taken from the puppy/dog and matched with its microchip. The DNA laboratory will run the test for B & E locus and you will get a certificate of the results.
WHY DO BREEDERS test for Coat colour?
Breeders test for coat colour and other traits so that they have an idea of the probability of a trait being passed on from the parents to their offsprings.
Aside from coat colour, what other DNA tests do breeders test for?
Conscientious breeders test for breed-specific hereditary diseases to minimise the risk of those diseases being passed on from parents to their offsprings. Examples of DNA tests are: PRA-PRCD, EIC.