Genetic basis of E locus - corresponds to the MC1R gene (melanocortin 1 receptor), which controls pigment production in melanocytes. This gene has multiple alleles, with a dominance hierarchy: Em (melanistic mask) > E (normal black pigment) > e (recessive red/yellow) The recessive e allele is a loss-of-function mutation, causing melanocytes to produce only phaeomelanin (red/yellow pigment) instead of eumelanin (black pigment). Dogs homozygous for e (e/e) will express cream, red, or yellow coat colors regardless of other coat color genes. Common recessive red variants are designated as e1, e2, and e3, with e1 being most widespread (found in Golden Retrievers, Labrador Retrievers, Irish Setters, etc.).
Pathophysiology - In dogs with the e/e genotype, the MC1R receptor is non-functional, signaling melanocytes to produce only phaeomelanin. This results in a coat color spectrum from pale cream up to rich red or yellow. This recessive red/yellow color masks other patterns or colors that require black pigment, making eumelanin-based patterns invisible on e/e dogs.
Complications - There are no health complications directly related to the recessive e allele causing cream/red/yellow coloration. However, masking of eumelanin-based colors can complicate the identification of other coat color traits or patterns.
Clinical Presentation - The coat colors include cream, yellow, red, apricot, lemon, gold, and orange hues. Seen in breeds like the Golden Retriever and Labrador Retriever, with color intensity varying among individuals. These dogs may still carry other coat color genes "hidden" beneath the uniform red/yellow pigmentation.
Inheritance - The recessive e allele must be inherited from both parents (homozygous e/e) for the cream/red/yellow coat color to be expressed. Dogs with one copy (E/e) have black/eumelanin pigmentation and can be carriers of the recessive red allele. Carriers do not show the recessive red phenotype but can produce cream, red, or yellow offspring if mated with other carriers.
Why This Matters to Breeders and Vets - Understanding the E locus mutations helps breeders predict and manage coat colors in litters. Genetic testing for the MC1R gene, including all known recessive red variants, informs breeding decisions for coat color and helps avoid unexpected color outcomes. The recessive red color masks other coat patterns, so underlying genetic diversity might be hidden.