Equine coat color genetics
The dun gene lightens some areas of the horse's coat, while leaving a darker dorsal stripe, mane, tail, face, and legs.
Horses with a gray gene can be born any color and their hair coat will lighten and change with age.
[3] Leopard complex patterns also predate domestication, having been found in horse remains from 20,000 years ago.
[1] The mutations causing chestnut, sabino 1, and tobiano appeared shortly after horse domestication, roughly 5000 years ago.
Wild type alleles can be represented as + or n, so Zz, Zz+, Z/+, and Z/n are all valid ways to describe a horse heterozygous for silver.
There are two chemically distinct types of melanin: pheomelanin, which is a red to yellow color, and eumelanin, which is brown to black.
The genes extension and agouti together affect the placement of the two types of pigment, black eumelanin and "red" (coppery brown) pheomelanin.
[8] A recessive mutation to extension removes this functionality, causing the solid red color of chestnut horses.
This molecule interacts with MC1R, the receptor coded by extension, to block the signal for black pigment production.
[14] Extension is found on equine chromosome 3 as part of a linkage group with roan, tobiano, and the KIT gene.
The Extension locus was first suggested to have a role in horse coat color determination in 1974 by Stefan Adalsteinsson.
[21] 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.
In mice, two mutations on Agouti are responsible for yellow coats and marked obesity, with other health defects.
[3][28] The molecular cause behind the dun coat colors is not entirely understood, but the dilution effect comes from the placement of pigment in only part of the hair.
It creates primitive markings but does not dilute the base color, and is co-dominant with the more common non-dun 2 but recessive to dun.
Such horses, sometimes called "single-dilutes", exhibit dilution red pigment in the coat, eyes, and skin to yellow or gold, while eumelanin is largely unaffected.
[32] The Matp gene encodes a protein illustrated to have roles in melanogenesis in humans, mice, and medaka, though the specific action is not known.
[32] Mutations in the human Matp gene result in several distinct forms of Oculocutaneous albinism, Type IV as well as normal variations in skin and hair color.
[33] Mice affected by a condition homologous to cream, called underwhite, exhibit irregularly shaped melanosomes, which are the organelles within melanocytes that directly produce pigment.
[35] However, the distinction between Dun and Cream remained poorly understood until Stefan Adalsteinsson wrote Inheritance of the palomino color in Icelandic horses in 1974.
[21] The mutation responsible, a single nucleotide polymorphism in Exon 2 resulting in an aspartic acid-to-asparagine substitution (N153D), was located and described in 2003 by a research team in France.
[36] Champagne differs from Dun in that it affects the color of the coat, skin, and eyes, and in that the unaffected condition is the wildtype.
Affected horses are born with blue eyes which darken to amber, green, or light brown, and bright pink skin which acquires darker freckling with maturity.
[36] The difference in phenotype between the homozygous (CH/CH) and heterozygous (CH/ch) horse may be subtle, in that the coat of the homozygote may be a shade lighter, with less mottling.
[31][37] The Champagne locus is occupied by the Solute carrier family 36, member 1 (SLC36A1) gene, which encodes the Proton-coupled amino acid transporter 1 (PAT1) protein.
