A Locus
The agouti signalling peptide (ASIP) produces red pigment (phaeomelanin). Mutation(s) in the ASIP gene will result in the interference of phaeomelanin production. The four coat types controlled by ASIP are: Fawn (ay), Sable (aw), Black-and-Tan (at) and Recessive Black (a); the dominance level is described as follows: ay > aw > at > a.
B Locus
The B locus or Tyrosinase Related Protein TYRP1 gene is associated with eumelanin production. The bS, bC, bD and bA represent the recessive alleles, and bSbS, bCbC, bDbD and/or bA/bA genotypes are required to confer the liver/chocolate colour.
Cocoa
Several variants in the TYRP1 gene result in brown phenotypes in dogs, however, not all brown phenotypes are accounted for by these variants. In French Bulldogs, a variant associated with a visually distinct chocolate phenotype (cocoa) was identified in the Hermansky-Pudlak syndrome 3 (HPS3) gene.
D Locus
Melanophillin (MLPH) is a protein, required for the transportation of melanin between or within melanosomes. Mutation(s) in the MLPH gene reduces the distribution of eumelanin and phaeomelanin resulting in a dilute phenotype. “D” represents non-dilute and presents with intense colour. “d” is recessive, and “dd” represents dilute colour.
Hair Length
Fibroblast growth factor 5 is a protein encoded by the FGF5 gene. FGF5 acts as a key signalling molecule to initiate the transition from hair growth to regression phase. Disruption of this gene or protein function results in extension of the growth cycle and produces long hair.
E Locus
The Melanocortin 1 receptor (MC1R) or extension (E) locus is encoded by the MC1R gene, which determines the distribution of eumelanin (black to brown) within the coat of the canine and determines the colour of the dog’s nose. This locus has two described mutations that affect the distribution of eumelanin within the coat.
I Locus
Mammals produce two kinds of pigment: eumelanin (black and/or brown in colour) and phaeomelanin (red and/or yellow in colour). Many genes contribute to the type and amount of pigment produced; several breeds of dogs however have an extreme dilution phenotype that has been shown to only affect the red pigment phaeomelanin. The degree of dilution varies within and between breeds with coat colour ranging from cream to solid white.
K Locus
Beta defensin (CBD103) is a peptide involved in the production of eumelanin in coat colour; CBD103 and (Agouti Signalling Peptide) ASIP bind to Melanocortin 1 Receptor (MC1R). KB is inherited in an autosomal dominant manner and therefore only requires one mutant allele to exhibit dominant black.
S Locus
White spotting (locus S) in dogs is caused by mutations in the microphthalmia-associated transcription factor (MITF) gene. MITF is responsible for multiple developmental processes; the SP mutation is association with lack of pigmentation. Two copies of the mutation (SP) are required to display the Landseer phenotype. The SP locus exhibits incomplete penetrance, individuals with one copy of SP may exhibit some spotting or display solid colour.
M Locus
Merle is a coat colour pattern caused by the antisense insertion of a short-interspersed element (SINEC_Cf) in the pigmentation gene PMEL (AKA SILV). The SINE mutation is unstable and prone to dramatic mutations which results in four main merle variations: cryptic, dilute, standard, and harlequin. Merle is dominant and will likely be expressed in the coat colour of the dog. Multiple merle alleles have been identified which may have a compounded effect resulting in cryptic or mosaic phenotypes.
