Facts About Equine Genetics

5. Horses have three base coat colours that are controlled by the interaction of two genes. The coat colours chestnut, bay, and palomino are determined by horses’ genotypes at the MC1R gene (extension (E) locus), which controls the production of red and black pigment, and the ASIP gene (agouti (A) locus), which controls the distribution of black pigment to the mane, tail, lower legs, and ear rims (points), or uniformly over the body.

. Dilution factors modify base coat colours. These include cream, champagne, dun, pearl, silver, and mushroom. The resulting coat colour depends on the combination of the base colour genotype and the dilution factor genotype. For example, a horse with one copy of the cream allele on a chestnut base colour will be palomino whereas on a bay base colour it will be buckskin. Modes of inheritance and causative mutations have been identified for these dilution factors, and genetic testing is available. 

7. Horse size is highly heritable. Height in horses is determined by the interaction of genetic and environmental factors (such as nutrition). Four genetic variants have been identified that account for more than 80 percent of horse height. One variant has a particular influence on height in warmbloods and a different variant has a strong influence on height in Shetland ponies and Miniature horses. Mutations related to dwarfism have also been identified in some breeds (dwarfism in Friesians, skeletal atavism or the reappearance of an ancestral genetic trait in Shetland ponies and American Miniature horses). Additional unidentified genetic variants that influence height in horses are likely.