Most brindle horses are chimeras, formed when fraternal twins fuse early in development. As the fused fetus develops, the pigment of the two individuals migrate along the Lines of Blashko. If the two individuals were destined to have different colors, then those lines take on the two-toned pattern associated with brindle. The results can be very striking, but because the color comes from an accident of development rather than the DNA itself, chimeric brindles cannot reproduce their color.
Breeders interested in brindle have long sought a true-breeding brindle. This past winter, the first heritable form of brindling was identified in a single family of horses. Incontinentia Pigmenti (IP) causes areas of darker pigmentation that follow the Lines of Blaschko, producing a pattern much like that of the chimeras. The mutation is dominant and sex-linked, so all the living examples are female descendants of the founder. Females that inherit the mutation are born brindle, while the males die in utero.
Unfortunately while Incontinentia Pigmenti (IP) is heritable, it is not something any breeder would actually want. A close look at the mare above shows that the affected horses have areas of hairlessness as well as hypopigmentation. The disease – and that is what this is – causes lesions in the skin, and sometimes the hair does not return after the area heals. A comparable disease in human beings causes problems with teeth, nails and eyes. According to the researchers, the affected mares also had abnormalities of the teeth, hooves and eyes. Obviously this is not the hoped-for true breeding brindle.
So like the recently identified Lavender Foal Syndrome, this is not really something that would be classified as a color. It is a genetic disease where the altered color is one of the symptoms. Breeders looking for a genetic form of brindling will have to continue looking.
The above photo is of a third-generation mare from this line, taken from the original paper. That paper is available by open access at PLOS One. The breed of the test family was not given.