Agouti | Appaloosa Spotting | Appaloosa Pattern-1 | Camarillo White | Champagne | Cream | Dominant White | Dun | Gray | Lethal White Overo | Pearl | Red Factor | Roan | Sabino 1 | Silver | Splashed White | Tobiano |
The basic coat colors of chestnut, bay, brown and black horses are controlled by the interaction between two genes: Extension (gene symbol E) and Agouti (gene symbol A). The Extension gene (red factor) controls the production of red and black pigment. Agouti controls the distribution of black pigment either to a points pattern (mane, tail, lower legs, ear rims) or uniformly over the body. The effects of approximately 10 other genes may modify these pigments to provide an array of colors in the domestic horse ranging from white to black.
The basic colors can be diluted by at least five genes: Cream, Champagne, Dun, Pearl and Silver. The Cream gene has a dosage effect in that a single copy of Cream produces palominos, buckskins and smoky blacks. Two doses of Cream produce cremellos, perlinos and smoky creams. Pearl is recessive; two copies of the gene or one copy of Pearl and one of Cream, are needed to see the dilution effect on the coat color. Champagne, Dun and Silver do not show a dosage effect.
There are several genes responsible for white patterns on horses. White spotting patterns on the base coat color are produced by the Dominant White, Appaloosa, Tobiano and Overo genes or as mixed white and colored hair patterns produced by the Grey (progressive whitening with age) and Roan genes. Several genes are involved in the production of white spotting patterns known as overo. Among those, the gene responsible for the frame-overo pattern is associated with a lethal disease of newborn foals called Lethal White Overo foal syndrome.
Most color assignments can be correctly made based on physical appearance or phenotype alone. However, genetic testing may be necessary to define phenotypes that are visually ambiguous or the color possibilities for offspring. Researchers at the Veterinary Genetics Laboratory and other institutions are working towards the identification of the specific genes and mutations responsible for coat color traits in the horse. In the future, diagnostic tests may be available for all the major coat color genes. Currently, genetic tests for specific mutations are available for: Red Factor (Extension), Agouti, Appaloosa Spotting, Appaloosa Pattern-1, Camarillo White, Cream Dilution, Dominant White, Dun Zygosity, Pearl Dilution, Champagne Dilution, Silver Dilution, Gray, Sabino 1, Splashed White, Tobiano, and Lethal White Overo.