D Locus (D-LOCUS)
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 dilute coat color. This mutation is recessive so two copies of the mutated gene (or “d” allele) are needed to produce the dilute coat color. This mutation affects both Eumelanin and Pheomelanin pigments, so black, brown and yellow dogs are all affected by the dilution with the effect being more pronounced in black dogs. The mutation responsible for the dilution phenotype is recessive so a dog can be a carrier of the dilution gene and still appear to have a normal coat color. A diluted yellow dog is often referred to as a champagne.
It is important to note that certain breeds have exhibited an association between carrying two copies of the D Locus mutation and a dermatological condition called color dilution alopecia (CDA) or black hair follicular dysplasia (BHFD). These conditions are characterized by hair loss and potential recurring skin infections. The effect is variable within and between breeds so not all dogs that carry two copies of the D Locus mutation will exhibit symptoms. It is likely that additional mutations or environmental factors are involved so the D Locus status of a particular dog can be used as a guide in determining potential disease susceptibility.
It is also important to note that a newly discovered second mutation in the MLPH gene (MLPH:c.705G>C) has recently been described for Chow Chows, Sloughis and Thai Ridgeback breeds. Although rare, this mutation can also lead to dilute coat color in these breeds. This newly discovered mutation will not be detected by the current D Locus test.
A (CLEAR/NORMAL): These dogs have two copies of the normal gene, will have an undiluted coat and will not pass the mutation to their offspring.
B (CARRIER/NOT AFFECTED): These dogs have one copy of the normal gene and one copy of the mutation associated with dilute coat coloring. They will have an undiluted coat but will, if bred, pass the mutation to 50% of its offspring, on average.
C (AT RISK/AFFECTED): These dogs have two copies of the MLPH mutation associated with a diluted coat color which results in blue, charcoal, grey, lilac or champagne coat dependent on other coat color loci.