E-Locus Horse (Chestnut)

E-Locus Horse (Chestnut)

One of the most fundamental genes involved in the formation of a horse’s coat colour is the Extension Locus (E-Locus). The E-Locus corresponds to the gene for melanocortin 1 receptor (MC1R), which is involved in the regulation of pigment production. Active MC1R causes a cell to “switch” from the production of phaeomelanin (yellow/red pigment) to eumelanin (black/brown pigment). Mutations affecting the E-locus often result in a lightening of the coat, from black/brown to red/yellow/white. In horses, this results in the common reddish-brown coat colour known as chestnut or red.

Inheritance

The wild-type allele for the E-Locus is designated as ‘E’. Horses with at least one E allele are able to produce dark coat pigment. However, horses carrying two copies of the recessive mutation ‘e’ have a chestnut base colour instead.

Chestnut can be further modified by other coat colour loci. For example, a chestnut horse affected by Dun dilution becomes red dun, and a horse carrying one copy of the Cr-mutation becomes palomino.

Relevant tests

• P904