Genetic basis of D locus - is primarily associated with coat color dilution in dogs and is linked to mutations in the MLPH (melanophilin) gene. Mutations at this locus are recessive, involving alleles designated as D (dominant, full color) and d (recessive, dilute). Three different variants of the d allele are known: d1 (most common across many breeds), d2 (found in breeds like Chow Chow, Thai Ridgeback, and Sloughi), and d3 (seen in breeds such as Chihuahua, Italian Greyhound, and Pumi). Dogs must inherit two copies of the dilute allele (d/d) to exhibit coat color dilution.
Pathophysiology - The MLPH protein plays a key role in the transport and proper distribution of melanin-containing pigment granules (melanosomes) within hair follicles. Mutations disrupt this process, causing pigment granules to clump instead of spreading evenly. This results in the coat color appearing "diluted" or lighter: black pigment becomes gray or blue, brown pigment becomes pale brown (e.g., lilac or isabella), and red pigment becomes cream or buff. The dilution affects both eumelanin (black/brown pigment) and phaeomelanin (red/yellow pigment).
Complications - Dilute coat colors can sometimes be associated with color dilution alopecia (CDA), a condition causing hair thinning, hair breakage, and follicular dysplasia, especially in certain breeds. Not all dilute-coated dogs develop CDA, but breeds like Doberman Pinschers and Dachshunds are particularly predisposed.
Why This Matters to Breeders and Vets - Understanding the D locus allows breeders to predict and select for coat colors, including dilute variations, in their breeding programs.Genetic testing for the various d alleles aids in identifying carriers and planning matings to avoid undesirable or health-related issues associated with dilution. Vets benefit from this knowledge when diagnosing alopecia or coat quality issues in dilute dogs.
