Excerpt from Chapter 6, “Peculiarities of Hair Growth”, in Equine Color Genetics by D. Phillip Sponenberg, DVM, PhD, Iowa State University Press/Ames, 1996.
"A rare variant in hair growth is the curly trait. Curly horses have curled body hair, and also curly mane and tail. The winter coat of most curly horses is very, very curled. The summer coat is frequently only subtly curled, so horses could be misidentified. Another peculiarity of some curly horses is the shedding of the mane and tail along with the winter body coat each spring. The curly trait is the basis of a few registries, even though it occurs in several different breeds.
"At least two independent genetic mechanisms can lead to curly horses. One of these is due to a recessive mutation, and is symbolized as CrRc for curly at the Curly recessive locus. This mechanism accounts for most curly horses that crop out of straight-haired breeds. This occurs in the Missouri Fox Trotter and the Percheron, among others. The other mechanism is dominant, and is symbolized as CrDc for curly at the Curly dominant locus. Horses homozygous for CrDc are reputed to be more curled than heterozygous horses. This allele is responsible for most Asiatic curly horses (notably the Lokai breed), and also most of those in the Western Hemisphere. Most curly horses in the Western Hemisphere descended from Spanish horses brought over during the conquest, and these occur in both North America and South America. Although the curly trait is dominant in these horses, it does have variable expression. Occasional horses with the CrDc allele are minimally curly, do not shed the mane and tail, and could be easily missed as being curly. They are, however, fully capable of passing the allele along to offspring, some of which will be fully curly. The two genetic mechanisms for curliness in horses are totally unrelated except that they cause a similar appearance in horses."
Dr. Gus Cothran at Texas A&M is currently conducting a study sponsored by ICHO to isolate the curly gene in horses.