If you have a horse with a coat, odds are it has a unique color or pattern. And if that coat is spotted or speckled with white, you likely have a paint or pinto pony. These are some of the most popular and easily recognizable of the horse colors and patterns, but many more exist. Geneticists have largely solved the basic outline of equine coat color genetics, and DNA tests to determine the likelihood that a horse will produce offspring with specific colors are available. But discussions, research and even controversy continue to surround specific spotting patterns and color sub-shades (like “brown” which, in some breed registries, is not considered a distinct color).
Graves began her presentation with the basics of equine coat pigmentation, explaining that there are two main colors produced by a horse’s hair cells: pheomelanin, which produces reddish brown, and eumelanin, which produces black. From these two genes, horses come in three basic shades: chestnut, bay and black.
From there, other modifying genes create different spotting patterns and color sub-shades. For example, a silver dapple occurs when a horse’s base coat of chestnut or black is interrupted with lighter rings of grey hair. These spots, called dapples, are scattered throughout the horse’s coat and will fade as the horse ages to produce a “fleabitten” grey or pure white color.
Other spotting patterns include roan, which is a solid color with white over the neck and legs and a mixture of colored and white hairs on the body. Graves said the exact mutation responsible for roan coloring has yet to be identified, but it’s linked to both the MC1R and KIT genes.
Then there are the dilution genes, which lighten a base color by either eliminating eumelanin or adding pheomelanin to the cell. Thus, a grullo is a diluted version of the black gene. A seal brown is a darker dark bay that almost looks like a black horse, while a sooty shade of dark bay is a mealier appearance.
As Graves discussed these spotting and color patterns, she stressed that the key to understanding a horse’s genetics is to know whether it’s dominant or recessive. Dominant genes are those that must be present for the trait to be expressed, while recessive genes require two copies of the allele, or mutation, to be expressed.
After she discussed a number of spotting patterns and color sub-shades, Graves addressed the issue of the word paint. She explained that while the terms paint and pinto are often used interchangeably, it is important to differentiate between the two, as each has its own unique genetics. She said that pinto horses, for example, have a semi-dominant genotype, which means the foal is homozygous (E/E) for the MC1R gene, but heterozygous for the KIT gene—meaning the foal has one copy of the dominant MC1R gene and one copy of the recessive KIT gene. Therefore, the foal will have the spotting pattern of a pinto but won’t be affected by the deadly overo lethal white syndrome (OLWS).