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.
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.
This website called Dog Genetics is full of information if you are interested in coat colour genetics in particular (www.doggenetics.co.uk). It suggested that the reader start from basic to the more complex concepts (e.g. agouti, merle, dilution and tan points).
Dian Welle of Blue Knight Labradors (US) created a website to determine the probability of producing coat colours based on the coat color genes of both parents. See www.blueknightlabs.com/color/coatcolor.html. Her website is easy to navigate, and allows the user to see the possible coat colours of the based on breeding combinations.
The loci that are of interest in Labrador Retrievers are the B and E 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 labelled 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.
A black lab is a dog that has at least one "E" and one "B" allele
A yellow lab is a dog that carries two copies of the “e” allele and can carry either the “B” or “b” alleles.
Shades of yellow: from light cream to fox red
A chocolate lab is a dog that carries two copies of the “b” allele with at least one copy of the “E” allele
Shades of chocolate: from milk chocolate to dark chocolate