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Definition of "Roaning":
A mixture of white hairs scattered throughout the coat, like salt and pepper; usually there is a fairly uniform mixture of dark and white over the horse's body. Roan is not progressive -- it doesn't change over the years, as grey does. It often varies tremendously with the season, though -- usually lighter in winter and darker in summer.
"Roaning" has been used as a "catch-all" phrase in the horse world, to refer to several genetically different color effects:
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True Roan
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A horse that is a true roan will have white hairs scattered throughout its coat on the main parts of its body; but the head, legs, mane and tail will remain the base color of the horse.
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Generally, the dark to white ratio is about 50/50 and the mixture tends to be quite uniform overall. The horse often appears from a distance to be a light color, and it's only apparent on closer inspection that the light appearance is caused by a mixture of white and dark individual hairs.
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True roan is caused by the "R" gene, which is dominant; a horse must *be* a roan in order to have a roan offspring, and a roan can never come from two non-roan parents.
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The R gene has long been believed to be lethal in the homozygous state, with all RR embryos being resorbed very early in the pregnancy (usually before the owner knew the mare had conceived). This theory is based on two things: one, no true breeding roan has ever yet been found, even in breeds where roan is relatively common, and two, a study was once done in which roan horses were bred together and the results were a 2:1 ratio of roan to non-roan foals, which is different than the 3:1 ratio that would normally be expected. Thus, the theory that RR is lethal. This is generally believed to be the case, but has not been proven genetically, as the gene has not yet been isolated. In recent years there have been a few Quarter Horse stallions claimed to be homozygous for Roan, but as of this point in time their numbers of offspring from non-roan mares are not large enough to prove homozygosity.
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Rabicano "Roan"
| This form of roaning is typically expressed with a patch of white hairs at the flanks, which may spread out onto the barrel (and if it does, it commonly appears "stripey" or "brindled" looking), and white hairs at the base of the tail, usually in a few horizontal bands, commonly called "coon tail" or "skunk tail".
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Sabino "Roan"
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It is believed that the gene that produces the white markings of sabino can also cause, in some horses that have it, a distribution of white hairs throughout the horse's body, including the face, legs, mane and tail (sabino roaning does not leave the points dark like true roan).
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Most of the roaning tends to be much more "patchy" and irregular than the uniform mixture of a true roan.
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It's possible that this characteristic may be the cause of the pinkish, mottled skin seen on some palominos (not to be confused with the pink skin of champagnes).
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Appaloosa "Roaning" aka "Varnish Roan"
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This form of roaning is found in Appaloosas, caused by the LP gene that gives the Appaloosa color. The roaning increases as the horse ages. Eventually the horse is completely "roaned out" and at this stage, this form of roaning can be somewhat hard to tell apart from other types of roaning. There have been registered Quarter Horses with this pattern.
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See the Appaloosa section for more information on this type of roaning.
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