"When a horse has white hairs mixed into the coat (of whatever color), it falls into one of two categories.
Grey is caused by a single, dominant gene called "G". It causes the horse to have progressively more white hairs every year until it is virtually all white. The rate of greying can be fast or slow. Some horses go from very dark to nearly white in just a couple of years, while others progress much more slowly and take many years to change color.
Roan is a general term meaning a mixture of white hairs in the coat that is not progressive, that is, it does not increase every year like grey does (although they can vary with the seasons). There are several different genes that can cause roaning, some of which are well understood, some not so much.
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:
True Roan
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.
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.
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.
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.
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".
Sabino "Roan"
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).
Most of the roaning tends to be much more "patchy" and irregular than the uniform mixture of a true roan.
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).
Appaloosa "Roaning" aka "Varnish Roan"
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.
The most important thing to keep in mind about Grey is that it is not a color in and of itself, but rather is a modifying effect that is "overlaid" on top of whatever color the horse already was, genetically. A grey horse will be born looking like an ordinary colored horse -- whatever color it would have been without the grey gene. Gradually over time, more and more white hairs will appear, until eventually the horse is all white. The speed of the greying process varies from horse to horse, but typically there will be enough white hairs appearing at the time the foal-coat is shed (around 3-4 months) to be able to tell the horse will be grey, then the dark dappled grey stage is usually around age 2-5 or so, progressing through lighter and lighter stages each year until reaching the white or nearly white stage around age 15-20. Again, this does vary dramatically in some individuals, though. Rarely, some greys are already very light by 6 months old, while others show no sign of greying until they are a few years old.
The genetics of grey are very simple. At the "G" locus, there are two possible alleles -- G and g. All non-grey horses are gg, and all grey horses have at least one G. Since grey is dominant to non-grey, all grey horses must have at least one grey parent. Grey cannot skip generations. A horse with two grey parents may inherit a G allele from each of them, and be GG -- homozygous for grey. If that happens, then every one of that horse's foals will always be grey.
Researchers in England have now pinpointed the location of the G gene. Presumably, there will soon be a test available for the gene. Although most of the time it's plain to see whether a horse is grey or not, there are some occasions where a test would be needed to tell if the G gene was there -- on an all-white pinto, for instance, or a cremello, or a very roaned-out Appaloosa.
Breeding grey horses can give some unexpected results. If a horse is purchased as a mature stallion, already grey, and there is no record of what color he was at birth, you have no way of knowing what other color genes he has. He may surprise you with dilute colored foals, or pinto spots.
Grey is sometimes confused with other colors, most often with blue roan. Some greys do go through a stage in their greying that looks rather like a blue roan, but it's easy enough to tell the difference. Roans have dark heads and points (the white hair is mixed in on the body only) and they do not get lighter and lighter each year. Greys are usually lighter on the face than the rest of the body, and they change color every year until they are nearly white. Occasionally grey might be confused with grulla. But a grulla's grey-looking body color is caused by each hair being a grey color, not a mixture of white and dark hairs like a grey or roan has. Also, grullas, like roans, have dark points, and do not change from year to year.
It should be noted that some registries actually use the term "roan" to mean a particular shade of grey, and not roan at all. This frustrating practice causes much confusion.
Silver (dapple)
The Silver gene is abbreviated "Z" in genetic symbols. The gene for absence of Silver is abbreviated "z".
The Silver gene is a SIMPLE DOMINANT gene. This means that if one gene is present it will manifest fully, and if two are present it manifests no differently than with one.
The Silver gene ONLY AFFECTS BLACK PIGMENT
The Silver gene dilutes the mane & tail on a horse with black pigment to a flaxen or silvery color.
The Silver gene dilutes the black pigment on the body to a chocolate-brown shade."
Next topic will be Heredity!
:o)
--- In haflingerfriends@yahoogroups.com, "rivervalleyph" <rivervalleyph@...> wrote:
>
> "Pangare, also called Mealy, or sometimes "donkey type shading", or just "lighter shading"
> This term refers to a considerably lighter shade of the body color on the muzzle, underbelly, the inner part of the upper hind legs, and so on. It can be quite variable in amount. It is most commonly seen on draft and pony breeds, and on almost all mules and donkeys. (The effect may have an entirely different genetic cause in donkeys and mules, though. Nobody really knows yet.) It is very common in Haflingers and Belgians, and similar breeds. In the past it was suggested that it was caused by a single dominant gene. However, we now have photos of pangare-marked horses from parents that had none, and horses with no pangare from parents which were both so marked. This rules out a simple, single gene, either dominant or recessive. In this respect it is similar to flaxen. Some have suggested that it may simply be one end of the lighter-to-darker "shade" spectrum, but some horses are fairly dark shades of their color yet still show pangare shading. At this time there has been no research into the genetic mechanism of this coloring, yet.
>
> Sooty, occasionally called Smutty, or Countershading, Coloring
> This term refers to a layer of black or other dark hairs which appears to be "overlaid" on the base color, typically over the center line of the back, the hips, the shoulders and sometimes other parts of body, and may include the mane and tail. It can be highly variable from just a dusting of black hairs over the wither area, to an extreme amount nearly covering the entire horse. Some horses show a dorsal stripe and no other sootiness - it is thought to be a different variation of the same thing, but nobody really knows yet if they have the same cause or are entirely different. It has also been hotly debated whether or not sootiness actually adds black hairs to a red-based color, i.e. chestnut or palomino. The hairs appear to be black, but the argument goes that since a red-based horse is "ee", which blocks production of black pigment leaving only red, then there could be no black hairs made, and therefore they must be very dark red that just looks black. Others feel that the sooty gene may cancel out the black-pigment-blocking effect of "ee" and allow the sooty hairs to be, in fact, black. The presence of black-looking sooty hairs on a palomino would suggest that this may be the case, since if they were some shade of red they would be diluted to yellow by the Cream gene, which does not dilute black pigment when heterozygous. Until the actual sooty gene is found and studied, we simply do not know exactly how it works or how it is inherited. It does seem to be readily passed on so is likely dominant to some degree at least.
>
> Flaxen
>
> It has been believed until this past year that flaxen is a simple recessive gene. However, since two flaxen chestnuts do not always produce a flaxen foal, this cannot be the case.
>
> At the time of this writing, the mechanism by which flaxen is inherited is unknown.
>
> Flaxen only affects red pigment in manes and tails."
>
> More to come on other color genes, ex: silver dapple, grey, and roaning...
>
>
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