The following information is provided by Jack Vanderwyk, The LabradorNet Data Base.
THE GENETICS OF BRINDLE AND TAN POINT LABRADOR RETRIEVERS
Nearly every breeder of Labrador Retrievers is familiar with the genetics behind the three recognized coat colors in the breed: black, chocolate, and yellow. They result from the interaction of two genes commonly referred to as the E (yellow) gene and the B (brown) gene. Occasionally, a litter of Labradors will appear that contain some animals with tan points either with or without a brindle pattern displayed in the tan areas. Due to recent advances in our understanding of canine coat color, we can now easily explain the genetics associated with these litters.
While the B and E genes are the only two genes that Labrador breeders normally pay any attention, there are actually many coat color genes at work in every dog regardless of breed. Their presence is often masked by the variations of other genes in what geneticists refer to as epistasis. A simple example of this phenomenon is found in yellow Labs. Every yellow Lab has two copies of the recessive “e” for the gene found at the E locus, and when this happens the dog is yellow and the genetic information about black or brown encoded by the B gene is hidden until the next generation.
Recently, a gene was identified at a locus called the K locus. There are three versions, or alleles, of this gene. The most dominant version, KB, is responsible for dogs having solid coloring throughout as we see in nearly all Labradors. The most recessive version of this gene is ky. Two copies of ky will allow another gene, A(agouti) to express itself in any of a number of patterns commonly seen in other breeds. One such pattern is tan points. The third version of the K gene is Kbr, which is responsible for brindle. It is intermediate in strength, so when paired with a second Kbr, or a ky, creates an animal that has a brindle pattern, but when paired with a KB is recessive. A single copy of KB is epistatic to, or sufficient to hide, all the genetic information of the A gene. Nearly every Labrador Retriever has two copies of KB. In a small sampling of 200 random Labradors about 4% were found to have only a single copy of KB. In that small percentage of dogs that had only one copy of KB, the second copy of the K gene was either Kbr or ky.
In the rare instances where two such dogs are bred to one another, the probability is that 25% of the pups will inherit the non-KB version from each parent. Any of these dogs that are not yellow (ee) will have tan points if they are kyky, and brindle points if they are kbrky or kbrkbr. This is because these versions of K allow expression of the Agouti gene. While there are many versions of Agouti, it appears that nearly every Labrador is atat, which is why the anomalous colors appear primarily on the muzzle, legs and chest.
If you want to know if your Labrador carries brindle or tan point (or any other color anomaly), you can have its DNA (K-locus) tested by VetGen, and probably by other firms as well. Just go to your vet and let him send your dog's blood sample to VetGen.
The Labrador was carefully bred up over time by the British sporting gentry from dogs brought back to England in the 19th century. These dogs were found to have unique characteristics which made them exceptional retrievers both on land and in the water.
Careful breeding also brought us English dual Champion Banchory Bolo (1915 - 1927).
Bolo marks are named after English dual Champion Banchory Bolo, who produced this mark in many of his puppies and future generations. The mark often goes away or is hidden by black hairs when the puppy grows up. Bolo marks are not considered a mismark. Many Labrador breeders see them as a sign of quality. Bolo marks are also linked to an excellent coat structure.
A small white spot, stripe or patch on the chest or Bolo pads (under the feet) are very common and do not lessen the quality of a Labrador, nor indicate it is not pure bred. To the contrary. A small white spot on the chest or under the feet is NOT considered a mismark and should not be penalised. (In most yellow Labradors the white spot is hardly visible, or not visible at all.)
The chances of white spots, (chest etc.) are much greater within the fox red shade than normal. This is due to several reasons:
White is very common within the yellow variation of the color. Because the white blends in with the normal yellow shade, it is not seen or noticed therefore ignored. With a red, because it is so dark, the white tends to stand out. In black Labradors, because the gene pool is so large, the tendency to throw white has been largely bred out. To throw white, it takes both parents so in either a black X black litter or black X yellow litter; white spots are not extremely common even though it does happen. In fox reds where the gene pool is so small, one can expect white spots in about half the puppies of the litter.
Labradors and Newfoundlands both trace their ancestry back to the now extinct breed known as a St. Johns Dog which usually had white markings on their feet, muzzle, and chests. Dogs with white markings are basically a genetic "throwback" to these ancestor's coloring. It often also means that they are closer to these ancestors.
An example: two of my foundation bitches came from a farm. Their mothers were companion dogs, only used for breeding when a next generation was needed. These farmers had always done it that way. Because of this much larger time span than in "commercial" or active breeding, my two bitches were generations closer to their "sources" (Donalbain Suede and Wetherlam Nutcracker) than their compeers from active breeders. The white spot was bred out in one generation, while I was able to build on my chocolate bloodlines.
For all the information you are looking for on these amazing true colors go to http://labradornet.com/index_e.html