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Description
The D-Locus test looks for the variants that cause a diluted coat color in dogs, such as blue, lilac and champagne. The MLPH gene codes for a protein called melanophilin, which is responsible for transporting and fixing melanin-containing cells. A mutation in this gene leads to improper distribution of these cells, causing a diluted coat color. Dilution is recessive, so two copies of a mutated allele (a “d” allele at the D-Locus) are needed to produce the dilute coat color.
Dilution affects both eumelanin and phaeomelanin, the pigments that control the color of the dog. Black, brown and yellow dogs are all affected by the D-Locus, although the dilution is more pronounced in black dogs. A diluted black dog is known as a blue dog. Names for this color vary across breeds, with charcoal, slate and grey being common. A diluted chocolate dog is often referred to as lilac, and a diluted yellow dog is known as champagne. Dogs that show these diluted colors have two dilute alleles (d/d) genotype. At the B and E loci, which determine the base coat color, a blue dog is B/B or B/b, a lilac dog is b/b, and a champagne dog is e/e.
Because the mutation responsible for the dilution phenotype is recessive, a dog can be a carrier of the dilution gene and still appear to have a normal coat color. These dogs can pass on either the full-color allele or the dilute allele to any offspring. This means that two dogs that appear full-colored can have a dilute puppy. This makes DNA testing for the D-Locus an important breeding tool, whether breeding for a dilute coat or to avoid it.
The three dilute mutations: d1, d2 and d3
Animal Genetics tests for three recessive MLPH mutations that dilute coat color: d (also called d1), d2 and d3. All three are tested whenever a dilute test is requested. The d1 mutation is the most common and is found in many breeds. The d2 and d3 mutations were identified later, in breeds where individual dogs had a diluted coat color yet tested non-dilute for d1. The d2 mutation has been reported in the Sloughi, Chow Chow and Thai Ridgeback. The d3 mutation is rare and has been reported in breeds including the Chihuahua, Italian Greyhound, Pekingese, Shih Tzu, Tibetan Mastiff, Yorkshire Terrier, Shetland Sheepdog, Pumi and Mudi.
A dog with two copies of the same mutation will be dilute. A dog with one copy each of two different mutations is also expected to be dilute, although for the rare d3 mutation this is based on a small number of dogs. In rare cases a dog can inherit two different mutations together from the same parent. That dog still has one full-color allele from its other parent and is a non-dilute carrier, even though the test detects two mutations.
A few dilute dogs in some breeds are not explained by any of the three known mutations, so other dilution variants are likely to exist.
Possible results
| Genotype | Description |
|---|---|
| D/D | Non-dilute: Dog is negative for the mutations associated with a diluted coat color. |
| D/d | Carrier: Dog carries one copy of the d1 mutation associated with a diluted coat color and may pass the mutation to offspring. |
| D/d2 | Carrier: Dog carries one copy of the d2 mutation associated with a diluted coat color and may pass the mutation to offspring. |
| D/d3 | Carrier: Dog carries one copy of the d3 mutation associated with a diluted coat color and may pass the mutation to offspring. |
| d/d | Dilute: Dog has two copies of the d1 mutation associated with a diluted coat color. The dog’s base coat will be diluted. |
| d/d2 | Dilute (expected): Dog has one copy of d1 and one copy of d2. The dog’s base coat is expected to be diluted. |
| d/d3 | Dilute (expected): Dog has one copy of d1 and one copy of d3. Most dogs with this result are dilute. In rare cases both mutations are inherited together from one parent and the dog is a non-dilute carrier. |
| d2/d2 | Dilute: Dog has two copies of the d2 mutation associated with a diluted coat color. The dog’s base coat will be diluted. |
| d2/d3 | Dilute (expected): Dog has one copy of d2 and one copy of d3. This combination is very rare and is expected to dilute the coat, but it has not been confirmed in published studies. |
| d3/d3 | Dilute: Dog has two copies of the d3 mutation. The dog’s base coat will be diluted. |
Reference
Polymorphisms within the canine MLPH gene are associated with dilute coat color in dogs.
[PMID: 15960853]
[PMID: 17519392]
A novel MLPH variant in dogs with coat colour dilution.
A Third MLPH Variant Causing Coat Color Dilution in Dogs.
[PMID: 32531980]