Equine Coat Color Genetics

 

Lesson Six

 

Patterns of white: white hairs mixed in

 

 

 

Having wrapped up the various types of dilutions, it's time to move on to another category of color modifying genes – those that add white hairs to the coat.  There are two general categories of white-adding genes:  those that cause white hairs to be mixed into the coat, without pink skin underneath, i.e. the roans and greys, and those that cause white spotting of various types, which does have unpigmented skin underneath.  We will cover the white-hairs-mixed-in types of genes this time.

 

Grey

(Note: in America, the color is often spelled “gray”, but for whatever reason, when talking about horse color, the English spelling of “grey” has generally remained.  Some American registries do spell it Gray.  Either way is fine, but we will stick with the traditional spelling when talking about the gene.)  

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 several years old. 

Some of the most famous grey horses are the Lipizzaners found at the Spanish Riding School of Vienna.  Lipizzaners are born dark and turn white with age, with the occasional one staying dark, indicating it did not receive the Grey gene from either parent.

One interesting thing about grey is that it often causes the horse to be much darker, when young, than it would have been without the grey gene.  Black foals that will go grey are typically born jet-black, for example, while those that are not grey are usually more of a pewter-grey shade at first.  Chestnuts and other "e/e" colors that will go grey are typically born with black skin, rather than the pinkish skin that is more normal at birth on these colors, and they often go nearly black before greying out.

 

                                                                                                                                                               

This born-jet-black foal has a Grey gene.                                                               This foal is black, showing the usual greyish newborn color.

 

 

This sequence is of a palomino with a Grey gene at various ages.

 

Palomino foal that is also Grey.  Note the unusually dark skin for a palomino newborn.

 

 

Same foal a few months later; looks dark palomino, with no real sign of grey yet.

 

 

Same foal at age 2, could still be mistaken for a dark sooty palomino.

 

 

Same foal at age 3, and now the Grey has kicked in and he is so dark it’s hard to imagine he started out as palomino.

 

 

Same foal at age 4, looking like any other young Grey horse.

 

 

An interesting effect that occurs in many grey horses after they have gotten quite light is called "flea-bitten".  These are tiny flecks of the original base color reappearing on the coat after it has gone white.  Some horses have so many, and they are so dark, that they look downright speckled.  Recent studies have suggested that the heterozygous greys are much more likely to be fleabitten and the homozygous greys are much less likely to have it.

“Fleabitten” grey

 

 

Another interesting phenomenon that occurs on some grey horses is what's commonly called a "bloody-shoulder" marking.  It can actually occur on any part of the body, but the wither/neck/shoulder area is the most usual place.  This is a dark, irregularly shaped spot of the original base color that does not get lighter as the rest of the horse greys out.  The cause is unknown, but not thought to be genetic.  They appear randomly and are not very common.

“Fleabitten” grey with a “Bloody-shoulder” marking

 

 

Greys can sometimes have a very uniform color when young, which is often called “steel grey” or “iron grey”, and is sometimes mistaken for roan.  But the lighter head will show its true color.

                                                                                                                                                                                                      

                                                                                                                                 Dark uniform grey young horse                                          Young grey face

 

 

 

Greys in middle age often become dappled, like these examples.

                                     

 

 

And then lighter and lighter as the years go by.

                                                  

 

How it works

The genetics of grey are very simple.  Grey is a simple dominant gene.  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, and the test has recently become available in the USA. 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.  For some reason grey is also hard to see on champagnes.  Those who have grey horses with two grey parents are glad to have a way to know if their horse is homozygous for grey, too.  Previously there was no way to know that for sure until the horse had a large enough number of foals that were all grey.

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 greyish 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 is very common for greys to be registered as "roan", and sometimes vice versa.  Also, some registries (mainly the TB) actually use the term "roan" to mean a particular shade of grey, and not roan at all.  All this confusion makes it difficult to tell where the color came from in a pedigree at times.  Of course, in a breed that does not have any true roans, such as Arabian or Thoroughbred, you can assume that a horse registered as "roan" is a grey.  99% of the time or more, that will be correct, although there have been a very few loud sabinos or rabicanos registered as roan.  But if the horse has a grey parent and is producing greys, there is no doubt whatsoever that it is a grey.  In breeds such as the QH which does have both, it can be tricky to untangle the lines, especially in cases where one horse had both genes.  If you are looking at a horse's sire record and he has plenty of both "grey" and "roan" foals there is really no way of knowing whether all those "roans" were actually greys (fairly common), or whether there were some of each (less common, but it happens), or whether all the "greys" were really roans (much more rare, but possible), unless you can get pictures of enough of them to figure it out.  It is also sometimes the case that a horse is registered as its base color before the owner realized it was in fact grey, and nobody ever bothered to update the color in the registry.  Cases like that would be the cause of grey seeming to "skip a generation" in a pedigree.  Grey can also be hidden by a few colors, such as cremello.

 

 

 

 

 

 

Photos courtesy of Cedar Ridge Ranch

This is the same horse from foal to 2 year old stage.  Notice the drastic change in color from birth (born chestnut) to the grey stage as a yearling and again as a 2 year old.  This horse will continue to change until it is solid white when fully mature.

 

 

Roans

 

Roan is not as simple as Grey.  “Roaning” is a general term that means any type of white hairs mixed into the base coat that is not grey.  Roans may vary with the seasons but they do not get lighter and lighter over time like the Grey.  Roans do not have dapples like greys.  Roan shades are usually named for the base color, i.e. “Bay Roan”, “Palomino Roan”, “Dun Roan”, or whatever the case may be, but on a chestnut base the usual name is “Red Roan” and on a black base the usual name is “Blue Roan”.  There are also various descriptive terms such as “strawberry roan” for a pinkish shade, “lilac roan” for a purplish shade, and so on, but these do not have any specific genetic meaning.  Registries usually put all roans together no matter the base color, like greys, but some have more than one category of roan. 

There are several different genes that cause roaning in horses. 

The first is called True Roan.  When most people say “Roan” this is the one they mean.  This gene causes a roaned body (usually fairly uniform) and dark (base color) head and points.  They often change dramatically with the seasons, darker in winter coat and lighter in summer coat, or vice versa.  If you look at a True Roan from the front, the dark parts of their legs almost always have a characteristic “inverted V shape” at the top. (Roan breeders often call these “pointies”.)  When a roan horse gets an injury in the roaned part (scrapes, bites, etc.) the hair usually grows back dark in that area.  These marks are called “Corn Spots”. 

 

                                                                                                                                                                    

   Roan on a Black base (Blue Roan)                                                               Roan on a Chestnut base (Red Roan)

 

 

                                                                                                                                                                                                                          

Roan on a Bay base (Bay Roan)                                                                              Not tested, but looks to be Brown Roan

 

 

                                                                                                                                                                 

Front views showing the leg “pointies”.   

Red Roan, left; Buckskin Roan, right.

 

 

                                                                                                                                                                                                                        

Good example of “corn spots”.

 

 

 How it works

True Roan is a simple dominant gene, but there are a couple of things about it that are a little different than genes we have discussed previously.  Remember back in Lesson One when we touched on genes that are linked?  This is one of them.  The Roan gene is located on the same chromosome, and very close to, both the Extension (E) and Tobiano (T) genes.  This means that they are almost always passed on together.  They are so close that it is very, very rare for them to become separated.  When the R and E are on the same chromosome, the roan color is called “E-linked”, and when the R and e are on the same chromosome, the roan color is called “e-linked”.

Genetic testing does not tell you whether the genes it finds are on the same chromosome or not.  It can usually be figured out pretty easily by looking at the family tree and production records.  Let’s say you have a blue roan stallion who is E/e and R/r.  Breed him to several solid chestnut mares (so they don’t contribute any color genes that would confuse the results) and you will get either bay/brown/black solid foals and red roan foals, which indicates his Roan gene is “e-linked”, or else you will get bay/brown/black roans and solid chestnut foals, which indicates his Roan gene is “E-linked”. 

Another unusual thing about the Roan gene is that it appeared to be lethal when homozygous.  Dr. Van Fleck, one of the first researchers into coat color genetics, indicated that homozygous roan horses did not exist. His theory was that horses which received a double dose of the “R” gene obtained through the mating of two roan horses suffered early embryonic death.  The reason for this belief was: one, there had never been documented a true-breeding roan, one that sired 100% roans; and two, the ratio of roan to non-roan foals when breeding roan x roan was 2/3 roans, 1/3 non-roans, as opposed to the ¾ roan, ¼ non-roan that would normally be expected (remember our Punnett Square).  If no “R/R” foals were being born, then the remaining foals would be as expected, 2/3 “R/r” and 1/3 “r/r”.  These statistics still hold true today in several breeds which have a large enough number of True Roans to look at, and detailed production records.

However, in 1977, a homozygous roan stallion was reported. Today there are several Quarter Horse stallions that are reported to be homozygous roans.  We don’t really know yet if True Roan is in fact lethal when homozygous, or if there are exceptions in certain breeds, or if it’s not lethal at all and the statistics in other breeds were wrong.

The exact mutation that causes True Roan has not yet been isolated, but the UC Davis genetics lab has identified DNA markers in Quarter Horses and Paints associated with Roan that can be used to determine if a horse has the roan gene and how many copies.  

There is another possible answer to the Roan mystery.  There is another type of roan called “Frosty Roan” which occurs mainly in Quarter Horses.  It has not been studied yet, but is thought to be a different gene than True Roan.  What if these homozygous roans are in fact Frosty Roan rather than true roan?  Maybe it can be homozygous with no problem.  Or maybe these horses have both types of roan, and that’s why they are producing 100% roan foals.

Frosty Roan was described by D. Phillip Sponenberg in his book Equine Color Genetics (I am not certain if he coined the term but that was the earliest reference to it that I found).  It is similar to True Roan, but the roaning is more uneven and tends to be concentrated more over the horse’s topline, especially the bony prominences such as hips, spine, and shoulder.  There is often much more roaning in the mane and tail, and on the head and legs, than is usual with True Roan.  Heavily roaned manes and tails are sometimes called “squaw tails” and “squaw manes” in some parts of the country.  Frosty Roan has not really been studied, and its genetic mechanism is unknown.  So far it seems to occur in breeds which have True Roan, so it may be a variation of that gene, or maybe a different gene entirely; we don’t know at this time.  But from very limited breeding records it does appear possible for a horse to be homozygous for it (all foals look similar). 

Another type of roaning is called Rabicano.  This is one of the lesser-understood types of roaning.  Unlike the other types, it usually does not cause a large amount of white hairs; in fact, unless the base color is dark and the roaning fairly loud, it usually is not noticeable from a distance.  The vast majority of Rabicanos are registered as their base color, with no comment about the rabicano pattern except perhaps a brief comment in the "markings" section.  There have been some so loud that they were actually registered as "roan", though -- the famous Quarter Horse Go Man Go is the best-known example.  He and many of his descendants were registered as roans.  Another extensively roaned Rabicano is the famous sire Smart Chic O’Lena.  There have been a handful of Rabicano Arabians and Thoroughbreds registered as roan, as well (True Roan does not exist in these breeds).  But for the most part when it is less extensive than that, it goes unnoticed. 

The genetic mechanism for Rabicano is not known at this time.  It is thought to most likely be a simple dominant, because horses that have it seem to pass it on fairly consistently to many of their foals.  But it has never been formally studied, so no one really knows yet.  Horses with the pattern have popped up from two parents not known to have it.  It is thought that perhaps it can be very minimally expressed, so that it would go unnoticed.  It can also hide on certain colors; for example, a horse whose tail is already white from palomino or pinto genes. 

Rabicano is not common, but it is widespread.  It occurs on any base color, and in most breeds, and can occur in combination with any other pattern.  (By this we know that it is not linked to any other color gene.)  In some breeds it is fairly common to have horses which are true roan, rabicano and often frosty roan as well (QHs), or rabicano plus tobiano, splash, or any of the other patterns or combinations of patterns (Paint).  There are enough stallions that are known to be Rabicano plus one or more other patterns, and have a large enough number of foals to look for patterns, and tend to pass on either gene, or both, or neither - all at the same frequency.  So they do not appear to be related in any way. 

The defining characteristic of rabicano is the horizontal stripes on the tailhead.  Commonly called "skunk tail" or "coon tail", there may be just a few faint stripes or several big bright ones.  It is possible that it could express so minimally that there are only a few white hairs in this area, but this has not been proved yet.  There is some confusion out there about what a rabicano tail looks like.  White at the tip or end of the tailbone is a Splash characteristic, and lighter hair down the sides of the tail can be caused by various dilution genes or just baby-hair that is eventually outgrown.  Those things are not rabicano.  Rabicano stripes are white; they are horizontal, not vertical; and are at the very top of the tail. 

The other characteristic of Rabicano is, of course, the roaning.  It is quite variable in amount, but typically starts at the flank, spreading upward and forward onto the barrel, and often occurs on the chest and underside of the neck as well.  The interesting thing about rabicano roaning is that the white hairs, if they go as far as the barrel, often form a distinct vertical striping pattern over the ribs.  This is often called "rib barring" (not to be confused, however, with "leg barring" on duns, which are stripes of a darker shade).  Some extremely loud rabicanos have been called "brindle" but this is incorrect, as brindle would have darker stripes, not white ones.  The effect can be quite striking, but the really loud ones are rather rare.

 

                                                                                                                                                        

Rabicano on chestnut, side view.

 

 

                                                                                                                                                       

 

Rabicano tail markings

                                                                                                                                                            

 

                                                                                                                                                                                               

Rabicano chest and belly roaning                                       Rabicano flank roaning

 

 

                                                                                                                                                                                                                          

Rabicano “rib barring”

 

 

Some of the white-spotting genes can cause extensive roaning.  The pattern called Sabino is especially known for this.  Many of them have been registered as “roan” in some breeds.  But we will discuss that one next time. 

Additionally, random roaning sometimes just pops up out of nowhere, not associated with any of the known Roan type genes.  It is unknown at this time whether there is any genetic basis for such markings.  If extensive, they are sometimes called Roan, but it would be more correct to call it “roaning”.  

 

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