Grizzle Coat Color – EG locus
Grizzle coat color, also known as domino, is a coat pattern which is a mix of eumelanin and pheomelanin. This phenotype is caused by Eg gene on E locus, also called Extension. Whether it is called domino or grizzle depends on dog breed in which it occurs. Like that, this phenotype is Saluki dog breed is known as grizzle, but it is called domino in the Afghan Hound breed. The name was derived from a famous Afghan Hound show dog named Tanjored Domino, born in 1954. He gained his popularity in show circles in United Stated due to his rare coat color pattern at the time. Prior to Tanjored Domino’s popularity, the pattern was called reverse mask by fanciers of Afghan Hounds.
Malamutes and Siberia Huskies are considered to be old breeds relatively closely related to the Afghan Hound and Saluki, and a similar facial pattern can be found in them. However, neither in Malamutes nor in Siberian Huskies the same causative mutation as in Afghan Hound and Saluki can be found.
E locus is responsible for almost all non-agouti eumelanin or phaeomelanin patterning in dogs. E stands for ”extension”. Alleles in E series are Em (masked), Eg (grizzle/domino), E (normal extension-has no effect on phenotype) and e (recessive red). The dominance hierarchy is following: Em> Eg>E>e.
Grizzle coat color appears as mixed hair on the dog with no discernible pattern. The colors appear as blended together, so when until it is not examined closes, it looks like just one color. Grizzle coloring includes different coat colors, so black hairs may be mixed with some tan or brown hairs, or white with black, which results in looking as grey coat color. Official grizzle definition given by Casey and Margot is: ”A pattern that is present at birth and remains throughout life, which comprises a dark overlay covering the top and sides of the body and outside of the limbs, from the top of the head to the tip of the tail. The darker color on the head gives the impression of a ‘widow’s peak’ between the eyes where it contrasts with the lighter color of the face. The underside of the dog and inside the legs comprises the lighter base color. The overlay color may be Black, Grey/Silver, red or Chocolate and can vary in intensity with the seasons and age. The base color may be various shades of red, fawn, golden, cream or silver.”
Eg gene (grizzle) is dominant over all alleles of E locus except Em (eumelanistic mask), which means a grizzle dog can never have a mask. For example, a heterozygous dog with EmEg genotype will have a mask but would not show grizzle pattern. Also, expression of the grizzle pattern depends on alleles of A locus. In order for a dog to be grizzle, except having grizzle alleles, dog has to have tan point alleles on A locus (atat). Dog carrying Eg gene but no tan points will not be grizzle and its A locus will be expressed normally. In other words, a dog can be carrier of the grizzle gene but will never have it expressed due to genes of its A locus.
Grizzle coat pattern is caused by a point mutation within the MC1R gene (Melanocortin 1 Receptor), which is also known as Extension (E locus). Its mode of inheritance is considered as complex. The mutation appears to affect binding of ASIP (Agouti Signaling Peptide) to MC1R which results in eumelanin hairs with phaeomelanin pigmented portions.