Most of us are familiar with gray horses. Horses that are born a solid color but then slowly (or sometimes not so slowly) turn white as they age. We've seen them as the majestic “white” Lipizzaners and the “white” horses in many Hollywood productions including the Lord of the Rings movies. Recently however, it has been discovered that not all gray horses turn white as they age. In fact, some don't look “gray” at all even when at an advanced age. Often these horses have been labeled as Rabicano or even roan but at least one of the more famous has been confirmed to carry the Gray mutation. This horse is Comico IV.
This unique coloration appears to be concentrated primary in the Spanish breeds with examples found in Connemaras, Mangalarga Marchadors, and PRE horses. However, at least two pony's have been located that appear to share the same suppressed gray. Unfortunately full pedigree for either of these pony's is unavailable (please see our forum posts linked at the bottom of the page).
A 10 yr old Pure Spanish Stallion Exhibiting the Gray Inhibitor Phenotype Photo Courtesy of Tiago92 and http://www.special-horses.de
Visually these suppressed gray horses do gray as they age, just much more slowly than a normal gray. Often, even at the age of 20 they will still have a much of their natural coloring, and while normal gray horses often darken before they begin to gray or have a darker hue to their coats these gray horses do not appear to do so. They seem to retain what would be considered “normal” or even lightened coloring. As these gray inhibited horses age, the gray seems to want to locate in “patches”. The most noticeable of which can be found at the tail head. This often leads these horses to be confused with Rabicano. However, the white hair concentrated at the flank which is so common in Rabicano horses is noticeably absent. Also, as apposed to Rabicano, the head, neck, legs, and sometimes mane and tail tend to be more heavily affected than the rest of the horse with dapples often predominating.
Another image of the 10 yr old PRE Stallion that show his full body Photo Courtesy of Tiago92 and http://www.special-horses.de
Because of the small sample of horses, it is impossible to determine with confidence the method of inheritance. However, it is quite possible that this mutation is inherited as a dominant that is only expressed if gray is also present. This would lead to appropriately 25% of the offspring of a gray inhibited horse (when bred to a nongray non-carrier) also being gray inhibited with another 25% being regular gray and 50% being non-gray. This has been inline with what has been found this far. Gray however, is extremely common in the breeds where the gray inhibitor has been found and it is not known how being homozgyous gray may affect expression. Since homozygous gray horses are known to gray faster than heterozygous gray, it might be assumed that the gray inhibitor may not function fully if a horse is homozgyous for gray thus hampering pedigree research in these breeds where gray is a dominant color.
Similarly Acting mutations
A mutation that modifies the expression of another mutation is not unheard of in the animal world. In Great Danes, Harlequin modifies Merle to create the Harlequin pattern that is so popular. Also, in Dalmatians, the flecking modifier actually requires two other mutations, white spotting, and ticking to express. In cats, there is a Dilutions Modifier which modifies the expression of Dilution and in mice there is Dilution Inhibitor, which almost completely reverses the expression of Dilution. Similarly, in horses Pattern 1 and Pattern 2 modify the appearance of Leopard Complex. All of these examples give a precedence for mutations that modify other mutations.
How you can help!
If anyone knows of family of gray horses, preferably from a breed that is not predominately gray, that expresses this delayed graying characteristic please contact me. Studying the pedigrees of such horses could enable us to better understand the mode of inheritance.
Many thanks to Threnody and Third Peppermint from our forums for their research on this subject.
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