Understanding Horse Color - The Weird Stuff
There are a lot of things that show up on horses that are odd, strange, and may not fit our current understanding of equine genetics. Some appear to be inherited through a color gene we haven't found yet, others may be the result of mutations. And some we just can't work out the inheritance on. Here are a few of the 'odd' or 'weird' horse colors and patterns that can show up.
Brindle in horses is highly controversial. Most experts say there is no such thing as a brindle horse, but there are certainly horses that look brindle.
This seems to result from four phenomenon:
1. Extreme dun factor. Very rarely a dun horse may have striping over the body. Genetic testing reveals that these horses are, in fact, dun, and most produce more 'normal' dun offspring.
2. Ribbing. Striping along the ribs is seen in some roans and spotted (lp) horses. This striping can be quite distinct.
3. Chimera. Although rare, equine 'fusion' chimeras, caused when twin foals fuse in the womb, do exist. For some reason, these chimeras tend to be brindle patterned. One particularly interesting case is the Thoroughbred stallion Catch A Bird who, in addition to being a chimera, sires roan offspring (the roan gene is otherwise unknown in the Thoroughbred breed), showing there is something very strange going on here genetically. Genetic testing has identified some chimeras. Chimeras, of course, do not breed true.
4. A possible 'brindle' gene. Pedigree analysis of horses descended from a mare called 'Brenda Batty Atty' indicates that there may be a subtle brindle that is inherited as a dominant trait. These brindles are nowhere near as spectacular as dun brindles or chimera brindles, but show distinct coat texturing and appear to breed true.
Brindle in horses is extremely rare and many still argue that genetic brindle does not exist in the species.
Based off of pedigree analysis, mealy (also called pangare) is a simple dominant. It is associated with primitive breeds and also carried by Przewalski's horses, suggesting that, like dun, it may be part of the equine 'wild type'.
Mealy causes a lightening of the horse's muzzle and flanks that can range from very subtle to spectacular. Haflingers are particularly well known for being 'two tone' with loud mealy all the way across their flanks and belly.
Mealy in mules, which is almost universal, comes from the donkey genetics and may or may not be directly connected to mealy in horses.
Also called countershading, the sooty factor causes the top of the horse to be darker, spreading down the flanks. This can produce wonderful dark bays with rich, warm, red flanks. It's theorized that many dark bays and chestnuts including so-called 'liver chestnuts' (which can range through chocolate to black) have sooty that covers their entire bodies. Sooty can also create a dorsal stripe which is sometimes confused with the dun stripe.
However, a dun's back stripe is narrow and very crisp. A stripe created by sooty is wider and shades smoothly into the coat.
The genetic mechanism behind sooty is as yet unknown, but it appears that it may be a polygenetic trait.
Anyone who has spent time around horses will have noticed that some red horses have yellow or blonde manes.
Called 'flaxen chestnut' or 'blonde chestnut', this appears to be caused by a gene that is both recessive and only visible on red horses. Some breeds, such as Haflinger and Avelignese ponies, are universally flaxen. Flaxen chestnut is also very popular in Arabians.
A very light flaxen chestnut may have a cream or white mane and an almost golden body - such horses are sometimes mistaken for palominos. The cream gene no longer exists in the Arabian breed (there is anecdotal evidence that it was bred out some time in the 19th century), but sometimes you will hear mention of a 'palomino' Arabian. These are either not pure or are very light flaxen chestnuts.
Sometimes horses are seen with a few spots that are either white or darker than the surrounding coat.
The white spots are named after the Thoroughbred stallion that was first recorded with them, making them 'Birdcatcher' spots. These spots appear on otherwise solid horses at some point in adulthood, and slowly increase in size and number. I have witnessed a 15 year old horse suddenly 'break out' in birdcatcher spots with a 'hey, that white spot wasn't there last week' effect. Sometimes, they can make a horse look almost pinto, but more commonly there are only half a dozen of them at all.
A different Thoroughbred stallion gave his name to 'Bend-Or' spots. These are spots of darker pigmentation that most commonly occur on red based horses but can also be seen on bays and bay variants. In most cases, like birdcatcher spots, a horse will only carry a few of these spots, but some have been seen that look almost like Appaloosas.
While I'm at it, I'm going to add a quick note on white markings.
By white markings I mean the apparently random white that shows up on a horse's face and lower legs. It used to be believed that these markings were genetic. Then the first equine clone was born - with white markings quite different from its donor. How did this happen?
It appears that there may be a genetic component to whether a horse has white markings, but that the extent of them is determined at least partially by uterine environment. This causes problems for registries that require, in their standard, no or very minimal white markings, such as the Friesian and Dales Cob registries.
However, if you breed a horse with a lot of white you are more likely to get a lot of white. It is sometimes hard to tell, however, whether a blaze and four white legs is created by 'white markings' or whether the horse actually carries sabino or splashed white.