Extension (E) Gene

The Extension Gene, located at the Extension Loci (3p12), is controlled by two different alleles: “E” and it’s mutated counterpart “e”. This gene works in cooperation with the Agouti (A) Gene, and encodes for either the production of black Eumelanin, or reddish-yellow Pheomelanin.  

    The dominant Extension allele “E” codes for the production of equine melanocortin-1-receptor (MC1R) on the melanocytes of the skin. This protein, when functional, binds to alpha-melanocyte-stimulating hormone (a-MSH) signaling the melanocyte to produce a black pigment called Eumelanin.  The production of black pigment allows for dark skin color and black hair production. When AS1P, produced by the Agouti Gene,  is partially functional, it nullifies a portion of the a-MSH, allowing only a small amount of Eumelanin to be produced. When AS1P is non-functional (a/a), Eumelanin is fully expressed and only black pigment is produced.

    The recessive mutated Extension allele “e” codes for the production of a non-functional equine melanocortin-1-receptor (MC1R). A single nucleotide polymorphism (SNP) at codon 83 located at position C901T (TCC -> TTC), results in the production of non-functional MC1R receptors. Because the receptors are non-functional, the a-MSH hormone cannot bind, and therefore, cannot signal the melanocyte to produce Eumelanin. The melanocyte instead produces larger amounts of a reddish-yellow pigment known as Pheomelanin. If the horse is homozygous for the mutated Extension allele (e/e), it cannot produce any black pigment and AS1P function is insignificant.