Post by Carole on Jan 18, 2006 16:03:25 GMT -5
The Colorful Thoroughbred
The Thoroughbred has a remarkably colorful genetic palette, ranging from ordinary bays to dazzling extreme sabinos. This page will delve into the mechanisms behind the colors as well as dispel some common misunderstandings about Thoroughbred color
Along with the two base colors, chestnut and black, the Thoroughbred gene pool also includes agouti (bay), brown, grey, cream (palomino, cremello, buckskin, perlino, smoky black, and smoky cream), frame overo, splash white, sabino, manchado, and rabicano.
Black is a rare color for Thoroughbreds, even though it is dominant to chestnut, the other base color. Pictured at right is the Aussie Champion Lonhro, a magnificent true black Thoroughbred stallion. True black horses will not have any brown hairs in their coat, unlike brown or dark bay horses, who may look black, but who usually have brown hairs on their muzzles, flanks, and inner forearms and thighs. Some black horses will fade with sun exposure---usually observable as brown hairs in the mane and tail---but it is only temporary, much like human hair will lighten in the summer. (Photo
www.whitehorseproductions.com/tbcolor1.html
Chestnut is recessive to all colors. Chestnuts vary in shade in from a light golden color to red to liver. Some even have have flaxen manes and tails.
The Modifiers
Bay is the most common color of Thoroughbreds, and it is actually a modification of the black base color. The agouti gene acts to lighten the hairs on the body, but it does not affect the legs, mane or tail, which is why they remain black. Agouti acts only on black hairs, so a chestnut horse can carry the gene and pass it along to its offspring, but because chestnuts have no black hairs, the agouti gene has no observable effect on the coat
Bay comes in many shades, including a lovely reddish shade often referred to as blood bay or bright bay
Brown is also a common color in Thoroughbreds, though the genetics of it are not entirely understood yet. It is known to be a black based color that is modified by an allele located at the same locus where agouti would be present. (Horses that are bay carry the agouti allele in either the homozygous dominant form [AA] or the heterzygous dominant form [Aa]. Horses that are not bay are considered 'aa.' Brown horses have been genetically tested to determine that they do not carry 'A' or 'a' but an allele present at the same locus known as 'At.' Thus they are neither bay or just black, but rather something else entirely, namely "brown.") Seattle Slew was great example of this color. Brown horses have a black coat except for telltale brown hairs on their muzzles, flanks, and inner forearms and thighs.
The Cream Dilution
Unlike agouti, which can only act on the black gene, cream can modify any color it acts in conjunction with, though it is most commonly seen combined with chestnut, bay, and black. The cream gene is an incomplete dominant, meaning it is always expressed when it's present, but it acts differently in its heterozygous (1 copy of the gene) and homozygous (2 copies of the gene) states. Simply put, horses with one copy of the cream gene will have a diluted coat; horses with 2 copies will have a doubly diluted coat. Double dilutes always have blue eyes. The cream gene does not effect black hairs in it's single form, only in it's double form.
When chestnut is combined with one cream gene, the resulting color is palomino. Just like chestnut, palomino ranges in intensity and shade. King's Ransom, pictured at left, is an example of a pale palomino.
The late Glitter Please, was a lovely darker palomino. He was the only palomino colored TB stallion in the world with an impressive show resume in FEI dressage.
RFF The Alchemist, a cremello son of King's Ransom. Cremello occurs when a chestnut coat is combined with 2 cream genes (one from each parent). Though he looks white, he is actually a very pale cream color. White markings are discernable on double dilutes.
When the cream gene is combined with bay, the result is buckskin. Like the agouti gene, the cream gene does not affect black hairs, so only the brown hairs of the coat are diluted to the golden color.
Like palomino, buckskin can vary in shade. This mare, Golden Belle, looks like a light bay, but is in truth a sooty buckskin. She is pictured here with her cremello colt by King's Ransom.
www.whitehorseproductions.com/tbcolor1.html
Perlino occurs when a bay coat is combined with 2 cream genes (one from each parent). Two cream genes do dilute the black points of the horse, often leaving a reddish cast to the points.
As is mentioned above, a single creme gene has very little, if any, effect on a black coat, so smoky blacks are hard to identify unless their pedigree is known or unless they produce cream dilute foals
The Grey Modifier
(or, why TBs can NOT be roan, with one bizarre exception)
Many grey Thoroughbreds are registered as grey/roan or even just as roan due to confusion that still exists regarding the differences between these two distinct colors. Grey is a gene that modifies the horse's existing coat color (bay, chestnut, etc) by slowly de-pigmenting all of the hairs of the coat as the horse ages. It's a process akin to the greying of human hair. All grey horses are born the color of their base coat, but most will have a few grey hairs around the eyes and muzzle, indicating the presence of the grey gene. As the horse ages, the grey hairs will spread from the face all through the coat. Many grey horses dapple as they age, and eventually end up a very light grey or "white" shade.
Some greys retain odd patches of color known as "bloody shoulder marks" though the marks can occur on the neck, barrel, and elsewhere as well.
Unlike greys, roan horses remain the same color throughout their lives. Roan acts similarly to agouti and cream in that it only affects the body hairs and not the hairs of the legs, mane, and tail. Thus, roan horses will always have dark heads, legs, manes, and tails, and their bodies will have white hairs mixed in with the coat color no matter what age they are. The roan gene DOES NOT exist in the Thoroughbred breed.* Any Thoroughbred that is registered as roan as actually grey and will lighten with age. The confusion seems to have arisen from bay and chestnut horses that turned grey because they often go through a stage where their coat color is very muddy and looks somewhat similar to roan.
The Catch A Bird Exception: The only exception to the statement that there are no true, dark-headed roan TBs has cropped up very recently. In 1982, a very unusually marked Thoroughbred was born in Australia named Catch A Bird. He looks like a bay horse with white brindling, the opposite of the dark lines seen on "normal" brindles. Stranger still, as a stallion, Catch A Bird has produced four offspring that appear to be true, dark-headed roans, indicating that Catch A Bird carries a one-time genetic mutation that has produced roan. As far as I know, none of the 4 "roan" foals, Odd Colours (1992 mare), Slip Catch (1993 mare), Goldhill Park (1994 horse), and Red Noble (1996 gelding), have been tested to see if they carry the roan gene, but they certainly exhibit the typical roan phenotype. Please note that these four horses appear to be the only true roan TBs in existence. All other horses regsitered as roan or grey/roan worldwide are actually greys.
www.whitehorseproductions.com/tbcolor2.html
Rabicano: More Confusion with Roan
Colorful Tour is a chestnut rabicano, a pattern that is often confused with true dark-headed roan. The rabicano gene, however, is entirely seperate from the roan gene, though it does create a pattern of white hairs that is similar to roan. It is usually manifested as a sprinkling of white hairs radiating out from the horses flank. Rabicanos also have a white-topped tail, known as a coon tail or skunk tail. Other notable rabicano TBs are Cox's Ridge, his dam Our Martha, and Carrier Pigeon. There is some thought that rabicano may be tied to the sabino gene.
The Sabino Pattern and The Myth of the True-Breeding White/Albino Horse
Sabino (or sabino overo) is a pinto pattern of white hairs imposed on any coat color. In its most minimal form, it is often manifested as a blaze and stockings. More boldly marked sabinos will have white splashes on their bellies and sides, often with roany edges, and the most extremely marked sabinos can be all white or nearly so. Marquetry, is an excellent example of moderate form of this pattern, as is Northern Dancer. Note the left hind stocking which seems to come to a point as it rises up Marquetry's leg---this is very typical of the sabino pattern. Also notice the odd white spot on his left knee. Sabinos tend to have odd spots like this one on their bellies as well. The sabino pattern tends to be more loudly expressed on chestnut and chestnut-based colors like palomino.
www.whitehorseproductions.com/tbcolor3.html
Frame Overo
Though it is very rare, the frame overo pattern does exist in the Thoroughbred gene pool. (Several suspected frame overo TBs have been tested for LWO and came back positive, so that is concrete evidence of the pattern in the breed.) Frame is almost always seen in conjunction with the sabino pattern which makes for some spectacularly patterned horses. Horses carrying only the overo gene almost always have four dark legs. White legs on a horse also carrying frame overo usually indicates sabino at work as well
Splash White
The splash white overo pattern has only recently been recognized in the Thoroughbred. Though there have been a few isolated reports of potential splash whites, no good examples were found until a mare named Hey What The in New Zealand produced two unique fillies in 2003 and 2004. Hey What The, a 1998 mare by Hey Baba Riba (NZ) out of Nagol Lass (NZ), by Americus (IRE), pictured with her 2004 filly Bubba by Go Corp (NZ). Both horses appear to be buckskin, but there are no indications in their pedigrees as to where the cream dilution came from.
Seeing Spots: Birdcatcher Spots, Chubari Spots (aka Tetrarch Spots), Bend Or Spots, and Manchado
These odd patterns of dark and light spots do occur in other breeds, but they seem to be particularly prolific in the Throughbred, which is, of course, why most of them are named after TBs. The genes responsible for these spots have not yet been identified, so not a great deal is known about them. They are, however, entirely seperate from the genes responsible for Appaloosa coloration.
Birdcatcher spots or ticks are patterns of small white spots on a dark coat. Usually, these spots appear once a horse has reached maturity and eventually disappear. Sometimes, however, they do seem to be permanent. This is Willspynow, a 1991 mare by Well Selected out of Spy Gail, by Father Hogan. This mare's spots are bigger and more highly concentrated than most Birdcatcher spots.
www.whitehorseproductions.com/tbcolor3.html
Chubari spots (also sometimes called Tetrarch spots) are similar to Birdcatcher spots except that they tend to be much larger. They are usually egg-shaped and egg-sized, as seen here on The Tetrarch. They seem to be tied to the grey color, but they are different from dapples. Dapples can change and fade as the horse ages, but chubari spots don't disappear until the horse has completely greyed out. It is a fairly unusual pattern, and though it does occur on non-grey horses, it is most often seen on greys, most of whom descend from The Tetrarch.
Bend Or spots are random dark spots on a horse's coat. They seem to be tied to the chestnut color. Man O' War, a descendant of Bend Or, is said to have had these spots. . Bend Or spots seem to show up most frequently on chestnuts and chestnut-based colors, like palomino, but they can occur on other colors.
The Thoroughbred has a remarkably colorful genetic palette, ranging from ordinary bays to dazzling extreme sabinos. This page will delve into the mechanisms behind the colors as well as dispel some common misunderstandings about Thoroughbred color
Along with the two base colors, chestnut and black, the Thoroughbred gene pool also includes agouti (bay), brown, grey, cream (palomino, cremello, buckskin, perlino, smoky black, and smoky cream), frame overo, splash white, sabino, manchado, and rabicano.
Black is a rare color for Thoroughbreds, even though it is dominant to chestnut, the other base color. Pictured at right is the Aussie Champion Lonhro, a magnificent true black Thoroughbred stallion. True black horses will not have any brown hairs in their coat, unlike brown or dark bay horses, who may look black, but who usually have brown hairs on their muzzles, flanks, and inner forearms and thighs. Some black horses will fade with sun exposure---usually observable as brown hairs in the mane and tail---but it is only temporary, much like human hair will lighten in the summer. (Photo
www.whitehorseproductions.com/tbcolor1.html
Chestnut is recessive to all colors. Chestnuts vary in shade in from a light golden color to red to liver. Some even have have flaxen manes and tails.
The Modifiers
Bay is the most common color of Thoroughbreds, and it is actually a modification of the black base color. The agouti gene acts to lighten the hairs on the body, but it does not affect the legs, mane or tail, which is why they remain black. Agouti acts only on black hairs, so a chestnut horse can carry the gene and pass it along to its offspring, but because chestnuts have no black hairs, the agouti gene has no observable effect on the coat
Bay comes in many shades, including a lovely reddish shade often referred to as blood bay or bright bay
Brown is also a common color in Thoroughbreds, though the genetics of it are not entirely understood yet. It is known to be a black based color that is modified by an allele located at the same locus where agouti would be present. (Horses that are bay carry the agouti allele in either the homozygous dominant form [AA] or the heterzygous dominant form [Aa]. Horses that are not bay are considered 'aa.' Brown horses have been genetically tested to determine that they do not carry 'A' or 'a' but an allele present at the same locus known as 'At.' Thus they are neither bay or just black, but rather something else entirely, namely "brown.") Seattle Slew was great example of this color. Brown horses have a black coat except for telltale brown hairs on their muzzles, flanks, and inner forearms and thighs.
The Cream Dilution
Unlike agouti, which can only act on the black gene, cream can modify any color it acts in conjunction with, though it is most commonly seen combined with chestnut, bay, and black. The cream gene is an incomplete dominant, meaning it is always expressed when it's present, but it acts differently in its heterozygous (1 copy of the gene) and homozygous (2 copies of the gene) states. Simply put, horses with one copy of the cream gene will have a diluted coat; horses with 2 copies will have a doubly diluted coat. Double dilutes always have blue eyes. The cream gene does not effect black hairs in it's single form, only in it's double form.
When chestnut is combined with one cream gene, the resulting color is palomino. Just like chestnut, palomino ranges in intensity and shade. King's Ransom, pictured at left, is an example of a pale palomino.
The late Glitter Please, was a lovely darker palomino. He was the only palomino colored TB stallion in the world with an impressive show resume in FEI dressage.
RFF The Alchemist, a cremello son of King's Ransom. Cremello occurs when a chestnut coat is combined with 2 cream genes (one from each parent). Though he looks white, he is actually a very pale cream color. White markings are discernable on double dilutes.
When the cream gene is combined with bay, the result is buckskin. Like the agouti gene, the cream gene does not affect black hairs, so only the brown hairs of the coat are diluted to the golden color.
Like palomino, buckskin can vary in shade. This mare, Golden Belle, looks like a light bay, but is in truth a sooty buckskin. She is pictured here with her cremello colt by King's Ransom.
www.whitehorseproductions.com/tbcolor1.html
Perlino occurs when a bay coat is combined with 2 cream genes (one from each parent). Two cream genes do dilute the black points of the horse, often leaving a reddish cast to the points.
As is mentioned above, a single creme gene has very little, if any, effect on a black coat, so smoky blacks are hard to identify unless their pedigree is known or unless they produce cream dilute foals
The Grey Modifier
(or, why TBs can NOT be roan, with one bizarre exception)
Many grey Thoroughbreds are registered as grey/roan or even just as roan due to confusion that still exists regarding the differences between these two distinct colors. Grey is a gene that modifies the horse's existing coat color (bay, chestnut, etc) by slowly de-pigmenting all of the hairs of the coat as the horse ages. It's a process akin to the greying of human hair. All grey horses are born the color of their base coat, but most will have a few grey hairs around the eyes and muzzle, indicating the presence of the grey gene. As the horse ages, the grey hairs will spread from the face all through the coat. Many grey horses dapple as they age, and eventually end up a very light grey or "white" shade.
Some greys retain odd patches of color known as "bloody shoulder marks" though the marks can occur on the neck, barrel, and elsewhere as well.
Unlike greys, roan horses remain the same color throughout their lives. Roan acts similarly to agouti and cream in that it only affects the body hairs and not the hairs of the legs, mane, and tail. Thus, roan horses will always have dark heads, legs, manes, and tails, and their bodies will have white hairs mixed in with the coat color no matter what age they are. The roan gene DOES NOT exist in the Thoroughbred breed.* Any Thoroughbred that is registered as roan as actually grey and will lighten with age. The confusion seems to have arisen from bay and chestnut horses that turned grey because they often go through a stage where their coat color is very muddy and looks somewhat similar to roan.
The Catch A Bird Exception: The only exception to the statement that there are no true, dark-headed roan TBs has cropped up very recently. In 1982, a very unusually marked Thoroughbred was born in Australia named Catch A Bird. He looks like a bay horse with white brindling, the opposite of the dark lines seen on "normal" brindles. Stranger still, as a stallion, Catch A Bird has produced four offspring that appear to be true, dark-headed roans, indicating that Catch A Bird carries a one-time genetic mutation that has produced roan. As far as I know, none of the 4 "roan" foals, Odd Colours (1992 mare), Slip Catch (1993 mare), Goldhill Park (1994 horse), and Red Noble (1996 gelding), have been tested to see if they carry the roan gene, but they certainly exhibit the typical roan phenotype. Please note that these four horses appear to be the only true roan TBs in existence. All other horses regsitered as roan or grey/roan worldwide are actually greys.
www.whitehorseproductions.com/tbcolor2.html
Rabicano: More Confusion with Roan
Colorful Tour is a chestnut rabicano, a pattern that is often confused with true dark-headed roan. The rabicano gene, however, is entirely seperate from the roan gene, though it does create a pattern of white hairs that is similar to roan. It is usually manifested as a sprinkling of white hairs radiating out from the horses flank. Rabicanos also have a white-topped tail, known as a coon tail or skunk tail. Other notable rabicano TBs are Cox's Ridge, his dam Our Martha, and Carrier Pigeon. There is some thought that rabicano may be tied to the sabino gene.
The Sabino Pattern and The Myth of the True-Breeding White/Albino Horse
Sabino (or sabino overo) is a pinto pattern of white hairs imposed on any coat color. In its most minimal form, it is often manifested as a blaze and stockings. More boldly marked sabinos will have white splashes on their bellies and sides, often with roany edges, and the most extremely marked sabinos can be all white or nearly so. Marquetry, is an excellent example of moderate form of this pattern, as is Northern Dancer. Note the left hind stocking which seems to come to a point as it rises up Marquetry's leg---this is very typical of the sabino pattern. Also notice the odd white spot on his left knee. Sabinos tend to have odd spots like this one on their bellies as well. The sabino pattern tends to be more loudly expressed on chestnut and chestnut-based colors like palomino.
www.whitehorseproductions.com/tbcolor3.html
Frame Overo
Though it is very rare, the frame overo pattern does exist in the Thoroughbred gene pool. (Several suspected frame overo TBs have been tested for LWO and came back positive, so that is concrete evidence of the pattern in the breed.) Frame is almost always seen in conjunction with the sabino pattern which makes for some spectacularly patterned horses. Horses carrying only the overo gene almost always have four dark legs. White legs on a horse also carrying frame overo usually indicates sabino at work as well
Splash White
The splash white overo pattern has only recently been recognized in the Thoroughbred. Though there have been a few isolated reports of potential splash whites, no good examples were found until a mare named Hey What The in New Zealand produced two unique fillies in 2003 and 2004. Hey What The, a 1998 mare by Hey Baba Riba (NZ) out of Nagol Lass (NZ), by Americus (IRE), pictured with her 2004 filly Bubba by Go Corp (NZ). Both horses appear to be buckskin, but there are no indications in their pedigrees as to where the cream dilution came from.
Seeing Spots: Birdcatcher Spots, Chubari Spots (aka Tetrarch Spots), Bend Or Spots, and Manchado
These odd patterns of dark and light spots do occur in other breeds, but they seem to be particularly prolific in the Throughbred, which is, of course, why most of them are named after TBs. The genes responsible for these spots have not yet been identified, so not a great deal is known about them. They are, however, entirely seperate from the genes responsible for Appaloosa coloration.
Birdcatcher spots or ticks are patterns of small white spots on a dark coat. Usually, these spots appear once a horse has reached maturity and eventually disappear. Sometimes, however, they do seem to be permanent. This is Willspynow, a 1991 mare by Well Selected out of Spy Gail, by Father Hogan. This mare's spots are bigger and more highly concentrated than most Birdcatcher spots.
www.whitehorseproductions.com/tbcolor3.html
Chubari spots (also sometimes called Tetrarch spots) are similar to Birdcatcher spots except that they tend to be much larger. They are usually egg-shaped and egg-sized, as seen here on The Tetrarch. They seem to be tied to the grey color, but they are different from dapples. Dapples can change and fade as the horse ages, but chubari spots don't disappear until the horse has completely greyed out. It is a fairly unusual pattern, and though it does occur on non-grey horses, it is most often seen on greys, most of whom descend from The Tetrarch.
Bend Or spots are random dark spots on a horse's coat. They seem to be tied to the chestnut color. Man O' War, a descendant of Bend Or, is said to have had these spots. . Bend Or spots seem to show up most frequently on chestnuts and chestnut-based colors, like palomino, but they can occur on other colors.