Equine Curly Coat
Gene or region: KRT25 and SP6
Reference allele: G, C
Mutant allele: A, T
Affected Breeds: Bashkir Curly, American Curly Horse, Mustang, Missouri Fox Trotter
Research Confidence: Moderate confidence, strong correlation in larger studies.
What it does:
Described in horses as early as the 1800’s, the curly coat phenotype can be observed in a wide range of “curliness” phenotypes, ranging from wavy to extreme permanent curled hairs. This variation in phenotype can also be seasonal, as some individuals shed their mane and tail during summer. Genetically, the curly coat is associated with two variants in the keratin 25 (KRT25) and transcription factor Sp6 (SP6) genes.
Some curly coated horses develop persistent or complete hypotrichosis, a form of follicular dysplasia, an effect associated with homozygosity of the dominant A allele in the keratin 25 (KRT25) gene. Heterozygotes (G/A) at the KRT25 gene may have incomplete hypotrichosis and curly coat. Meanwhile, the SP6 T allele is associated with a dominant curly phenotype, without hypotrichosis. The curly variant at KRT25 is epistatic over the SP6 variant, causing the individual to demonstrate hypotrichosis characteristics.
Interestingly, the curly coat is suggested as hypoallergenic and curly horses may be a suitable alternative for horse allergic riders. The curly coat trait has also been observed in dogs, cats, mice/rats, cattle, rabbits, pigs and humans!
Morgenthaler, C., Diribarne, M., Capitan, A., Legendre, R., Saintilan, R., Gilles, M., Esquerré, D., Juras, R., Khanshour, A., Schibler, L., & Cothran, G. (2017). A missense variant in the coil1A domain of the keratin 25 gene is associated with the dominant curly hair coat trait (Crd) in horse. Genetics, selection, evolution : GSE, 49(1), 85. https://doi.org/10.1186/s12711-017-0359-5
Thomer, A., Gottschalk, M., Christmann, A., Naccache, F., Jung, K., Hewicker-Trautwein, M., Distl, O., & Metzger, J. (2018). An epistatic effect of KRT25 on SP6 is involved in curly coat in horses. Scientific reports, 8(1), 6374. https://doi.org/10.1038/s41598-018-24865-3
Mitlehner, W., Mitlehner, H. C., & Niggemann, B. (2015). Horse Allergy: Curly Horses Allow Horse Allergic Riders To Ride Again. Pferdeallergie: Curly Horses erlauben es pferdeallergischen Reitern wieder zu reiten. Pneumologie (Stuttgart, Germany), 69(12), 711–718. https://doi.org/10.1055/s-0034-1393396
G/G - No curly alleles, horse likely has straight coat characteristics.
G/A - One dominant curly allele, horse has curly coat characteristics and is affected by incomplete hypotrichosis
A/A - Two dominant curly alleles, horse has curly coat characteristics and is affected by complete/persistent hypotrichosis
C/C - No curly alleles, horse likely has straight coat characteristics.
C/T - One dominant curly allele, horse has curly coat characteristics.
T/T - Two dominant curly alleles, horse has curly coat characteristics.