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THE GENETICS OF BRINDLE AND TAN POINT LABRADOR RETRIEVERS
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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.   
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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. 
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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.   
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Click here to visit the page of Sonny, my only brindle Labrador
 
 
 .The Genetics of Color in Labrador Retrievers
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White spot on Labrador chest
by Jack Vanderwyk
 
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