Equine Coat Color Genetics - Extension

Extension controls whether or not true black pigment (eumelanin) can be formed in the hair. True black pigment may be restricted to the points, as in a bay, or uniformly distributed in a black coat. Horses capable of producing eumelanin in the hair may have a genotype of either E/E or E/e. Horses without the ability to produce eumelanin in the hair always have the genotype e/e, and are most often chestnut or "red". The e allele is also sometimes called "red factor" and can be identified through DNA testing. Horses homozygous E/E are sometimes called "homozygous black", however depending on the color of the mate, E/E status confers no guarantee of black-coated offspring; only that no offspring will be "red".

The Extension locus is occupied by the Melanocortin-1-receptor (Mc1r) gene, which encodes the eponymous protein. The MC1R protein straddles the membrane of pigment cells (melanocytes). MC1R picks up a chemical called alpha-melanocyte-stimulating hormone (α-MSH), which is produced by the body, from outside the cell. When MC1R comes into contact with α-MSH, a complex reaction is triggered inside the cell, and the melanocyte begins to produce black-brown pigment (eumelanin). Without the stimulation of α-MSH, the melanocyte produces red-yellow pigment (pheomelanin) by default.

Various mutations in the human Mc1r gene result in red hair, blond hair, fair skin, and susceptibility to sundamaged skin and melanoma. Polymorphisms of Mc1r also lead to light or red coats in mice, cattle, and dogs, among others. The Extension locus was first suggested to have a role in horse coat color determination in 1974 by Stefan Adalsteinsson. Researchers at Uppsala University, Sweden, identified a missense mutation in the Mc1r gene that resulted in a loss-of-function of the MC1R protein. Without the ability to produce a functional MC1R protein, eumelanin production could not be initiated in the melanocyte, resulting in coats devoid of true black pigment. Since horses with only one copy of the defective gene were normal, the mutation was labeled e or sometimes Ee. A single copy of the wildtype allele, which encodes a fully functional MC1R protein, is protective against the loss-of-function. The normal or wildtype allele is labeled E, or sometimes E+ or EE.