K-Locus (Dominant Black)

Description

Coat color in dogs is controlled by several different genes. The K-Locus, also known as the dominant black gene, is a Beta-defensin gene (CBD103). The protein it makes interacts with the pigment cells, and its variants produce the different K-Locus colorations. This gene occurs in certain breeds and has an important impact on many types of coloration.

The K-Locus depends on the E-Locus. If the E-Locus genotype is e/e (recessive), the K-Locus is not expressed. If the E-Locus is E/E or E/e, the K-Locus is expressed.

The “K^B” Allele

The K-Locus has three different alleles, or variants. The first allele, which is dominant, is written “K^B,” or dominant black. The dominant black allele is a mutation that reduces or eliminates the expression of the agouti gene (A-Locus). Because this mutation is dominant, a dog needs only one copy to affect the A-Locus. A dog with one or two copies of K^B will not show its agouti pattern and will be solid black, unless other genes such as the B-Locus or E-Locus change the color.

The “k^br” Allele

The second allele is known as the “brindling” allele and is written “k^br.” The k^br allele is a separate mutation that allows the A-Locus to be expressed, but with brindling of the agouti pattern. The A-Locus produces several different colors, such as fawn/sable, tricolor, tan points or recessive black. The k^br allele is recessive to the K^B allele, so a dog that is K^B/k^br will still be black. It is, however, dominant over the third allele, k^y.

The “k^y” Allele

The third allele is written “k^y.” This allele allows the agouti gene to be expressed without brindling. If a dog is k^y/k^y at the K-Locus, the A-Locus determines the dog’s coat color. The k^y allele is recessive to both K^B and k^br. A dog that is K^B/k^y will be black, and a dog that is k^br/k^y will show a brindled A-Locus pattern.

How the K-Locus and A-Locus work together

For example, a dog that is a^y/a^y at the A-Locus will be fawn/sable if it is k^y/k^y at the K-Locus. If that same dog is K^B/K^B at the K-Locus, the A-Locus expression will be hidden. Its color will then be determined at the B and E loci, and there is a good chance the dog will be black.

At this time, there is no direct test for the k^br allele. The allele can generally be inferred by testing for the dominant black allele and by looking at the phenotypes of the parents and offspring. Testing for the dominant black mutation determines whether the dog is able to express the agouti phenotypes (A-Locus). However, the test is limited in that it will not tell you whether the dog will pass on brindle.

Possible results

Reference

A β-defensin mutation causes black coat color in domestic dogs.

[PMID: 17947548]