Genetic basis of Amber and Russet Coat Colour E Locus - The E Locus (also called the Extension Locus) in cats corresponds to the melanocortin 1 receptor gene (MC1R), which plays a crucial role in regulating the type of pigment produced by melanocytes in the hair follicles. The gene influences whether eumelanin (black or brown pigment) or phaeomelanin (yellow, orange, or red pigment) is produced. The wild-type dominant allele (E) enables normal function of the receptor, allowing production of dark eumelanin pigment. The recessive mutations at the E locus (notably e, er, ea) reduce or block MC1R function, resulting in lighter coats due to increased phaeomelanin production. Two important recessive alleles at the E locus have been identified which produce the distinctive coat colours known as Amber and Russet: The ea allele (found in Norwegian Forest Cats) causes the Amber coat colour (a lightened yellow/orange tone). The er allele (found in Burmese cats) results in the Russet coat colour (a reddish-brown shade). Cats must inherit two copies of the recessive allele (e.g., ea/ea or er/er) to express these colours. Heterozygous cats (E/ea or E/er) will have standard eumelanin pigmentation depending on their other alleles.
Pathophysiology - The MC1R protein is a receptor on melanocytes that, when active, signals switching from phaeomelanin (light pigment) to eumelanin (dark pigment) synthesis. In cats with the wild-type E allele, eumelanin is produced normally, resulting in black or brown pigmentation. In cats homozygous for the ea or er mutations, MC1R function is reduced or inactive, so melanocytes fail to switch to eumelanin synthesis, leading to lighter, yellow/red pigment predominance. This molecular dysfunction results in the Amber or Russet coat colours, which are variants of red/yellow shade but genetically distinct from the common Orange (O) locus-based red colour, which is X-linked. The exact coat tone depends on interaction with other loci (such as the B locus for brown shades and D locus for dilution).
Complications - There are no known health complications associated with the Amber or Russet mutations at the E locus. These are purely cosmetic coat colour variations. However, because these recessive alleles must be inherited from both parents to appear, breeders should be aware of genetic testing options to manage coat colour outcomes according to breed standards or preferences.
Why This Matters to Breeders and Vets - Breeders benefit from understanding the E locus mutations to predict and select for Amber, Russet, or related coat colours in breeds such as Norwegian Forest Cats (Amber) or Burmese (Russet). DNA tests are available for these alleles, which aids in breeding decisions to achieve desired coat colours or avoid unexpected outcomes. Veterinarians may use knowledge of the E locus genetics when advising breeders or owners about coat colour inheritance, or when differentiating between coat colours in clinical or show contexts. Understanding these genetic mechanisms assists in genetic counseling, supporting breed standards, and maintaining genetic diversity while managing aesthetic traits.
Summary - The E Locus in cats corresponds to the MC1R gene, which regulates pigment production switching between black/brown eumelanin and yellow/red phaeomelanin. Recessive mutations such as ea (Amber) and er (Russet) reduce MC1R activity, causing distinct amber or russet coat colours in specific breeds like Norwegian Forest Cats and Burmese. These alleles are inherited recessively, requiring two copies for expression. These coat colours arise from altered pigment production and have no health implications. Genetic testing facilitates informed breeding and accurate prediction of coat colours related to the E locus.