A heritable form of brindle discovered in horses

Figure 1

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.

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3 Responses to A heritable form of brindle discovered in horses

  1. Nicole April 4, 2014 at 3:41 am #

    so why do they keep breeding them?

  2. LaBella May 1, 2014 at 2:52 am #

    I have known for quit a while there were inheritable brindles that were not chimeric in nature. While the breeder states that her brindles have textured coats like appaloosas, I have not seen or noticed anything about hyper-pigmentation and alopecia in the Sharon Batteate brindles, but then I might be looking at them and it’s there in front of my face, and I just don’t know what I was talking about.
    It could be that this is in a different type of brindle? As we know phenotype doesn’t equal genotype.

  3. Dr. Leonardo Murgiano July 16, 2014 at 8:36 am #

    Hello, I am Dr. Leonardo Murgiano I am one of the authors of the paper.

    I write for a clarification: the Incontinentia Pigmenti is typical only of these few mares. Other brindle horses are brindled for completely different reasons.

    Many of them are chimeric, and there are cases of brindle horses with an inherited phenotype which is not pathological at all (no health issue for the horses, only brindle mantle), is supposedly not there because the horse is chimeric but because of a gene mutation, but the molecular mechanism has not been clarified.

    The offspring of Ima Star Moon Bar are an example of this. The trait is inherited, the horses are fine, but so far is not clear the mechanism.

    For any clarification, question, or even to ask for an investigation, write me.