Is that your natural color? What a horse’s hair color means for their health

horse in stall 2_blogKnown as the most exciting two-minutes in sports, the Kentucky Derby returns this weekend marking the 141st year of the race held annually in Louisville, Kentucky. Whether you are an avid watcher and horse race aficionado or simply along for the big hats and mint juleps, you probably have never stopped to think about how horses get their coloring, or what exactly it means.

A recent article delves into this topic and explains that the world of equine coat colors is really an equation that is only solved by digging deep into the genes that make up equine DNA. Understanding how a horse inherits a particular coat color is not only essential if you’re planning to breed, but it can also mean the difference between health and sickness. In fact, sometimes colors can even determine whether a new foal can survive. This is because the gene mutations responsible for producing specific color patterns can also cause some serious health problems. More on that later.

First, it’s important to understand that all equine coat colors and patterns stem from three basic coat colors: red, black, and bay, and that two genes control how these colors are expressed. The Extension (or E) locus gene determines whether black pigment will be expressed somewhere in the coat, and the Agouti (or A) locus gene defines where the black will be expressed.

All bay and black horses have at least one copy of the E allele and because the A gene determines the black’s location, it’s the determining factor of whether a horse is completely black or just has black points (legs, tail, mane). The chestnut color, on the other hand, is a recessive trait, so horses must have two alleles for it to be apparent. All other coat colors are variations on these three colors.

As stated in the article, It wasn’t until 1997 that a group of scientists sequenced the equine genome and made breeding for a specific color more than just a roll of the dice. Known as The Horse Genome Project, the scientists created a genetic map of the horse, an immediate boon for scientists and veterinarians who could now more easily use genetic information to understand common hereditary diseases.

The completion of The Horse Genome Project also made it easier for equine geneticists to predict coat colors and avoid those combinations that have serious health implications, Kathryn Graves, PhD, director of the Animal Genetic Testing and Research Laboratory at the University of Kentucky, in Lexington, said in the article.

For example, one area of study involves the cream and pearl dilution of color—a mutation that dilutes a base coat color (which can be black, bay, or chestnut) into a variety of different colors, such as cremello, buckskin, or perlino. Cecilia Penedo, PhD is one equine geneticist studying this effect and the mutations that cause white coat patterning or white spotting.

Horses with flashy white markings or spots are oftentimes desirable but, Penedo said it the article, it’s also risky because the mutation that produces the white patches is the same mutation that produces the deadly overo lethal white syndrome (OLWS).

Foals born with OLWS lack the ability to pass food through the gut and excrete waste, causing them to become impacted and die within a few days.  Because of the fatal consequences now  associated with breeding two overo-pattern horses, most owners of horses that clearly carry the overo gene test their stock before breeding.

Overo lethal white syndrome is the only known fatal disease associated with a coat color gene mutation, but that doesn’t mean other coat types don’t also cause problems.

The same genes that cause Appaloosas’ distinctive spotted coat pattern, the LP (leopard complex) mutation, can cause adverse health effects in horses. A similar problem is also associated with the silver gene, which produces a very attractive color dilution on black-based horses, but can also result in multiple congenital ocular defects that impair vision.

Color preference certainly varies among the disciplines, but Penedo and Graves both emphasize the importance of breeding for healthy animals first, with priorities such as color falling second. They also caution that while genetic testing has greatly improved the accuracy of color prediction, there is still an element of genetic variability.

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