Last year I began to question if my seemingly dun horse was actually turning gray (click here to learn what initially triggered my suspicions).
At the time I wasn’t even sure it was possible, but after doing a little research, and enlisting the help of the good folks at the UC Davis genetic testing lab, I was finally able to come up with with answer. I stepped into the Horse Nation Laboratory to devise a plan to share the answer to this mystery. As always, when in the lab, or on a horse…
Before we get to the answer, time for a quick lesson in genetics! C’mon, what did you expect?! Science nerdery is kind of my scene.
Horses, much like humans are diploid organisms, meaning they get one set of chromosomes from each parent for a total of two sets of chromosomes. Within those chromosomes are a set of molecular instructions to build your horse in the form of genes. There are variations of each gene known as alleles. Typically alleles are either dominant or recessive, with the dominant allele expressing a trait, and a recessive allele not expressing a trait, or only expressing a different trait in the absence of a dominant allele. Because your horse has two sets of chromosomes, there will be two alleles present for each gene, with one dominant allele generally being adequate to express the trait. The combination of alleles present is known as the genotype, whereas the physical expression of the genes is known as the phenotype.
So what I’d like to do now is go through Helix’s genotype to figure out how we get to his particular phenotype. Let’s start by looking at the Extension (E) and Agouti (A) genes. A dominant E allele indicates the ability to produce black pigment. A recessive e allele indicates the ability to produce red pigment. A dominant A allele limits expression of the black pigment to the mane, tail, and points. A horse with a dominant E allele and two recessive a alleles will be all black. Since Helix has a black mane and tail, we can infer he has one dominant E allele producing the black pigment, and at least one dominant A allele limiting the production of black pigment to his mane, tail, and points.
Next we’re going to talk about the dun dilution gene. A dominant D allele dilutes the pigment on the body of the horse, while leaving the mane, tail, and expressing pigment causing primitive markings such as a dorsal stripe and leg/shoulder barring. There are two recessive variations of the dun gene, nd1: not dun diluted, but primitive markings may be present, and nd2: not dun diluted with primitive markings absent. I had Helix tested for the Dun gene and found out he is D/nd1 with the D allele being expressed over nd1.
Lastly we’re going to talk about the gray gene (G). The presence of a gray gene will cause a horse’s pigment producing cells to initially over produce pigment leading them to eventually burn out, so over time the amount of pigment produced in the coat decreases until pigment production virtually stops. I learned from Helix’s test results that his genotype is G/N, meaning he has one copy of the gray gene and will in fact turn gray in years to come.
Like any good gray, Helix is fond of long rolls in the mud and has developed the ability to artfully place his manure in his stall to create delightful pattern on his coat when he lays down.
I’d like to point out that I only touched on the few genes that were most relevant to Helix’s color, and there are many more genes that influence coat color and pattern. There is a lot to equine color genetics and more being discovered all the time. I would refer people to the UC Davis website and also Equine Tapestry, a really interesting blog focusing on color genetics, if they are interested in learning more about equine color genetics.
Do you have questions regarding equine science? Leave it in the comments! HN Labs would love to help you out. We may not have all the answers, but what we lack in knowledge we make up for in our willingness to ask experts stupid questions!
And Go Riding!
Biz Stamm is a horse trainer/mad scientist (plant pathologist) who enjoys spending her free time running like a gentle breeze in the foothills of the Oregon coast range. Specializing in starting young horses under saddle at Stamm Sport Horse LLC, she brings the analytical approach she has acquired while working in laboratory to her training. She currently owns two horses: the Kalvin Cycle (Kalvin), an 11-year-old half-Arabian gelding, and DB’s Alpha Helix (Helix), a 6-year-old Kiger mustang gelding. While she is currently pursuing competitive goals, her main goal is to enjoy her horses, and for her horses to enjoy her.