Curly Genes and Mutations

by Diane Mitchell

Studies involving curly horses and their genetics have shown us many things, but it also left some questions we didn’t know we would have. It seems there is no single curly gene which causes curly coats in horses, instead there are mutations on existing horse coat genes which cause the curly coats. A ‘mutation’ is simply a version of an existing gene which differs from the more common version known as ‘wild type’. Mutations can be bad, but they are just as likely to be advantageous and a good thing for the animal. Because there are many causative mutations for curly coats which can be inherited independently, it is possible for horses to inherit two curly mutations simultaneously. These horse are known as ‘dual gene curly horses’. Dual gene inheritance can lead to some partially or totally bald horses, which many owners are afraid of producing when they breed curly horses. There may be other causes of baldness, but incompatible gene combinations is thought to be one such cause.
In this article, when I speak of KRT25 or SP6, I am speaking of mutations. All horses have these genes, but curly horses have mutations of these genes. Homozygous refers to having 2 mutations of one type (+/+), heterozygous, just one (+/-).

A mutation on the hair keratin gene KRT25, is the most common cause of curly coats in North America. It is the most common causative curly coat mutation found both in wild herds and domestic curly horses. The Missouri Foxtrotter lines of curly horses have a transcription factor mutation which affects the hair follicle during fetal development. It is known as SP6. KRT25 affects the hair shaft causing brittle hair versus SP6 which affects the shape of the hair follicle. There are 3 other known curly coat causative mutations found in North America. These are known as Sulphur, Cook, and Unknown Curly Gene a (UCGa). Through a combination of testing for KRT25 and SP6, in conjunction with pedigree and inheritance patterns, these curly coat producing gene mutations are known to exist. The specific horse genes where these mutations reside have not yet been isolated, so currently testing for these specific gene mutations is unavailable.
The newly discovered UCGa mutation has been found, but the specific gene where it resides has not been isolated yet. It’s home is probably in a keratin gene family like KRT25. So far, UCGa horses have a good curly coat in the winter, but a very sparse summer coat. These horses have long been thought to be a version of ‘extreme’, which they are not. One characteristic found with UCGa horses is a thinning of hair at the top of the tail, and a ‘poof’ at the end of the tail. This has been termed a ‘paint brush tail’. This unusual tail morphology is found winter and summer. By itself, it isn’t a problem. However, when combined with other types can lead to partial baldness or even total baldness. It is also not known what a homozygous UCGa horse looks like.
It is unknown if mixing UCGa with SP6 will lead to baldness. It is known that mixing UCGa with KRT25 leads to baldness.
Note- An extreme horse (KRT25 +/+) is not bald, but merely a homozygous KRT25. If a horse is bald, UCGa is involved.
There are acceptable combinations, e.g. KRT25 with SP6. See possible combinations below.

(+/-) KRT25 & (+/-) Sulphur = unknown
(+/-) KRT25 & (+/-) Cook = unknown
(+/-) KRT25 & (+/-) SP6 = acceptable combination
(+/-) SP6 & (+/-) Sulphur = unknown
(+/-) SP6 & (+/-) Cook = unknown
(+/-) KRT25 & (+/-) UCGa = bald patches
(+/-) Cook & (+/-) UCGa = unknown
(+/-) SP6 & UCGa = unknown
(+/+) KRT25 & (+/-) UCGa = total baldness

From the above combinations, you can see there are many possible outcomes for dual gene horses. However, until the genes for these mutations can be isolated so testing can be available, we do not know the physical appearances (phenotype) for most of these coat types. We do know that the KRT25 and SP6 combination produces a very good coat.
In the meantime, many wonder about testing. If you breed at any level, testing might be a good decision. Do you want nice curl on your foal? Do you care if it’s bald? Do you want it for future breeding prospects? Or for others to breed to?
Knowing the mutations that your horse has may be of benefit. It may also help identify the genes that we don’t know now. Whatever you decide to do, there are places to test. Etalon (www.etalondx.com) is the main one.
Straight haired, or smooth-coated horses are a good match to those that are  homozygous. They also may be a really good match to the UCGa. Again, more testing will show, but for now, don’t be afraid to use those smooth coated horses for breeding, because they could be a really good asset to breeding programs.
As time goes on, we hope to have more answers, especially in identifying UCGa. As it and others are found, I’m sure we’ll have more answers. Will we have more questions? Just need to wait and see.