K-Locus (Dominant Black)
Coat colouration is controlled by several different genes in dogs. Dominant black is due to a mutation in a Beta-defensin gene (CBD103). This gene produces dominant black vs. brindle vs. fawn colours in certain breeds.
The dominant black gene consists of three different alleles, or variants. The first allele, which is dominant, is notated as "KB," or dominant black. The dominant black allele is actually a mutation that does not allow the agouti gene to be expressed. Because this mutation is dominant, a dog only needs to have one copy of the mutation to suppress the agouti locus. A dog that has one or two copies of the dominant black allele will only express his base coat colour, which is determined by the B and E loci. He will not express any colours that occur from the agouti gene, such as "black-and-tan" or "tricolour."
The second allele is known as the "brindling" allele, and is represented as "Kbr." The Kbr allele is a separate mutation that still allows the agouti gene to be expressed, but causes brindling of the agouti patterns. The agouti gene represents several different colours, such as fawn/sable, tricolour, tan points, or recessive black. The Kbr allele is recessive to the KB allele, however, it is dominant over a third allele, Ky. Thus, for a dog to express the brindle pattern, he must be either Kbr/Kbr or Kbr/Ky. Dogs that are KB/Kbr will not appear brindle, but can still pass on that allele and potentially produce brindle offspring.
The third allele is represented as "Ky". This allele allows the agouti gene to be expressed without brindling. When a dog is Ky/Ky at the K-locus, the agouti locus determines the dog's coat colour. For example, a dog that is Ay/Ay at the agouti locus could be fawn/sable. If that same dog is KB/KB at the K-locus, the agouti locus will be hidden, and his colouration will be determined at the B and E loci. However, if that same dog is Ky/Ky at the K locus, he will then be able to express agouti, and will be fawn/sable.
At this time, there is no direct test for the "Kbr" allele, although it can generally be inferred through testing for the presence of the dominant black allele, as well as through phenotypes of the parents and offspring. Testing for the dominant black mutation determines if the dog is able to express agouti phenotypes. However, it is limited in that it will not tell you if the dog will be brindled.
K Locus Testing:
Animal Genetics currently offers a test for the K Locus to determine how many copies of the dominant "KB" allele a dog carries. Dogs can be DNA tested at ANY age.
Animal Genetics accepts buccal swab, blood, and dewclaw samples for testing. Sample collection kits are available and can be ordered at Canine Test Now.
Testing Is Relevant for the Following Breeds:
KB is a critical allele in the formation of black pigment in at least 50 breeds.
Animal Genetics offers DNA testing for the dominant KB allele. The genetic test verifies the presence of the dominant mutation and presents results as one of the following:
|KB/KB||The dog carries two copies of the dominant "KB" allele. The dog will not have brindle or fawn offspring. The dog will always pass on a copy of the "KB" allele to all offspring.|
|KB/n||One copy of the dominant black allele was detected. The dog will be dominant black, and will not express his agouti phenotype. The dog could pass on this allele, or either the brindle or fawn allele, to any offspring.|
|n/n||The dog does not carry the dominant black mutation. The dog's coat colour will be determined by the agouti gene, and may be brindled or not brindled.|