β-defensin 103 (CBD103) gene on K locus has a major role in coat colour expression in dogs. Its protein binds to MC1R which results in pigment switching. Recently mutation in CBD103 gene was associated with black coat colour in many dog breeds. K locus is dominant over A locus and therefore determines if colours coded on A locus will be expressed. Expression of K locus depends on E locus genotypes. Three different alleles are described on K locus with following hierarchy: KB (dominant black) > kbr (brindle) > ky (normal).
KB allele (dominant black) codes for uniform coat coloration. Mutation does not allow expression of A locus, which results in solid pigmented regions. Because of dominant mode of inheritance, only one allele is necessary for suppression of A locus. Homozygous or heterozygous dog will express only basic coat colour determined by genotypes on B and E locus.
Allele kbr codes for brindle coat pattern. It allows expression of A locus, but it affects it in a way, that dog has brindle agouti patterns. Brindle coat consists of dark eumelanin stripes on pheomelanin base (fawn). The colour of eumelanin stripes is defined by B and D loci. Allele kbr is recessive to KB allele, but dominates over ky. Genotypes, which express brindle coat pattern, are kbr/kbr or kbr/ky.
Allele ky is normal allele without a mutation and has no effect on coat colour. It allows full expression of colours defined by A locus.
With available genetic test, it is possible to detect KB allele. Reliable differentiation of kbr and ky alleles is not possible at the moment therefore alleles where KB is not present are labelled as n. Genetic test for K locus determines if dog is homozygous or heterozygous for KB allele allowing expression of A locus, but cannot differentiate which of the recessive allele is present in tested dog. With a combined analysis of parent’s and offspring’s coat colour and K locus genetic test, it is possible to predict whether dog carries kbr allele. In breeds, which do not express brindle colour pattern genetic test for K locus is sufficient for full analysis.
|KB/KB:||dog is homozygous for uniform coat coloration in pigmented areas, and transfers KB alleles to all of its puppies; agouti and brindle coat patterns are not expressed.|
|KB/n:||dog is heterozygous for uniform coat coloration in pigmented areas, and transfers KB alleles to half of its puppies; agouti and brindle coat patterns are not expressed due to dominant KB allele.|
|n/n:||colour is defined by A locus; in breeds that express brindle pattern the colour can also depend on kbr allele (kbr allele can be determined by a combined analysis of K locus genetic test and coat colour).|