Colour dilution associated with the D locus is caused by variants in the MLPH gene (melanophilin). The recessive allele d gives rise to lightened coats (blue, grey, lilac, etc.) and, in a subset of affected dogs, predisposes to colour dilution alopecia (CDA), a dermatosis with hair loss and folliculitis. The effect on colour is seen in many breeds; susceptibility to CDA varies greatly between breeds.

The d1 allele is widely distributed in many breeds; d2 is relatively rare and concentrated in Chow Chow, Sloughi and Thai Ridgeback; d3 appears at low frequency across a broad spectrum of breeds and canids. There is no single reliable frequency for the species as a whole.

- Lightened or diluted coat (blue, grey, lilac or similar)
- In predisposed dogs, colour dilution alopecia (CDA): progressive hair loss in pigmented areas
- Skin with folliculitis, scaling and secondary infections in severe cases
- Dogs with white markings are affected only in the pigmented areas

In 2007 the non-coding variant c.-22G>A of the d1 allele was proposed as the causal mutation (Drögemüller et al.). A second allele, d2 (c.705G>C), was later described segregating in Chow Chow, Sloughi and Thai Ridgeback (Bauer et al., 2018). In 2020, Van Buren et al. identified a third allele, d3 (c.667_668insC), present at low frequency in multiple breeds, wolves, wolf hybrids and indigenous dogs.

- Do not mate two diluted dogs if diluted offspring and the risk of CDA are to be avoided
- Before introducing dilution alleles into a breed, take into account the risk of CDA, which can be severe
- In breeds with a high predisposition to CDA, consider diluted colour as a selection criterion
- Remember that the D-locus test alone does not predict the onset of CDA

Do not confuse MLPH dilution (D locus) with other causes of dilution, such as the MYO5A variants described in the Miniature Dachshund (dilution with Griscelli-type neurological deficit) or alleles of other loci that lighten the coat. The diagnosis of CDA is clinical-dermatological and is supported by MLPH genotyping; expression is highly variable between breeds, so advice should be cautious.

1. Drögemüller C et al. 2007. A noncoding melanophilin gene (MLPH) SNP at the splice donor of exon 1 represents a candidate causal mutation for coat color dilution in dogs. J Hered. PMID: 17519392
2. Bauer A et al. 2018. A novel MLPH variant in dogs with coat colour dilution. Anim Genet. PMID: 29349785
3. Van Buren SL et al. 2020. A Third MLPH Variant Causing Coat Color Dilution in Dogs. Genes (Basel). PMID: 32531980
OMIA000031-9615.