Post by Haley on Jan 12, 2010 16:28:33 GMT -6
From here.
There is no such thing as an albino horse! Albinism is caused by a recessive gene, meaning an animal must carry 2 copies of the gene, one from each parent, in order to express the color (or rather, lack there of). One copy of the gene has no effect on coat color. Because of this, 2 albinos bred together will always produce an albino.*** As you will read below, this simply does not happen in horses because no white horse's coat is caused by a recessive gene, and none of these white horses breed true as an albino would.
So where do "white" horses come from? From several different genetic patterns, as a matter of fact: the grey gene, the cream gene, the sabino gene, the splash white gene, the so-called "dominant white (W) gene," and occasionally from extreme overo, tobiano, or tovero patterns. The key idea to keep in mind, however, is that all of these genes act only to modify the horse's existing coat color---essentially they serve to block pigment from otherwise dark hairs---they are not really a color in the same sense as bay or chestnut.
Grey: Many people see a horse like the one pictured above and assume he is white. He is, however, a grey. Grey is a gene that modifies a horse's coat color, no matter what it is, and causes it to de-pigment with age, much like human hair changes color as we age, whether we're blond, brunette, or red-headed. All grey horses are likewise born "dark," be it black, chestnut, bay appaloosa, or palomino. As they age, their coat changes, often going through a dappled phase which is usually quite lovely. Some grey horses will never turn entirely "white," retaining traces of color in their manes and tails or on their knees and hocks. Some even develop little flecks of color throughout their coats, known as flea-bitten grey. But most importantly, no matter how greyed-out they are, grey horses are genetically grey, never white.
Cream Gene: Another color frequently confused as being white or albino is cremello, like this handsome TB stallion, RFF The Alchemist. This color is the result of two cream genes, one from each parent, acting on a chestnut base coat. The cream gene is an incomplete dominant, meaning that two copies of the gene will dilute the coat color more than just one. One cream gene combined with chestnut will dilute the coat to create palomino. Two creams genes dilute the coat to cremello, a very pale, golden shade often mistaken for white. Cremellos always have blue eyes which can lead to confusion with albino, a color that simply doesn't exist in any horse breed. True albinism is a recessive condition characterized by red-eyes, pink skin, and a total lack of dark pigment. Cremellos do still have some pale golden pigment to their coats and skin as well as blue pigment in the eyes.
Several pinto patterns are responsible for creating "white" or nearly white horses. Pinto, like grey, is really not a color, but rather a color modifier. It acts by removing pigment from areas of hair on an otherwise solid colored horse, creating the splashy patterns we see from Paint horses to Clydesdales. All pinto patterns are dominant! The most common one is sabino, a pattern of white that is expressed in its minimal form as stockings and a blaze. Moderate sabinos frequently have belly spots and white patches on their sides and shoulders. Extreme sabinos can be almost entirely enveloped by the pattern and thus look white. Most extreme sabinos retain a little color along their toplines, especially in the mane or on the ears. Breeding sabinos together may result in an offspring with even more white in their coat than the parents, but not always. Producing extreme sabino foals from extreme sabino mares or stallions has been a fairly unusual phenomonon until just recently. As more breeders today are breeding for color as well as function, extreme sabino families are slowly becoming more common. By no means do extreme sabinos throw that color on a regular basis---another nail in the coffin of the idea of true-breeding white horses---many produce moderately expressed sabinos.
Breeding for wildly patterned sabinos has become more popular in Thoroughbreds in the last few years. A few families have produced an unusual number of extreme sabino foals, the most notable of which is Patchen Beauty and her kin. The family began with White Beauty in 1963, an extreme sabino mare who, out of 6 foals, produced 1 extreme sabino foal and 3 chestnut foals with slightly less extensive sabino markings. One of these, a mare named World O'Beauty (born 1972), produced 2 extreme sabino fillies including Precious Beauty in 1981. And she, in turn, is the dam of the famous Patchen Beauty who has produced 3 whites foals to date. The White Fox is the only one of her foals to race so far.
Another pinto pattern that can cause completely white horses is called splash white, shown below. It is similar in appearance to sabino, in that the white pattern seems to "creep" up the horse starting with the legs and then belly. Most splash whites also have extreme face markings, sometimes even entirely white heads like the horse on the right. In its most extreme form, splash white leaves little or no color along the topline. Many splash whites are deaf.
Occasionally, tobianos and overos, or combinations of both,will exhibit such an extreme pattern that they are almost entirely white, as seen below, PPQ Spirit of Romance. He is an extreme tovero, and his color is entriely masked by the pattern. Though he looks entirely white, he is technically a pinto, and his offspring could be both pinto and solid-colored, as is true for the other horses pictured here.
A lethal white overo (LWO) is produced when a foal is homozygous for the frame overo gene (meaning it carries 2 copies of the gene, one from each parent). It causes foals to be born with so extreme an overo pattern that they are entirely white or nearly so. Frame is dominant, like all other pinto patterns, but it is only lethal in its homozygous state. When this happens, the resulting foal is born white, but sadly, it has an incomplete digestive track, and it dies within 72 hours of birth. This is the only white-pattern known to be lethal. (Horses that carry only one frame gene are heterozygous (and thus not lethal) for the trait but may pass it on to their offspring.) . Obviously, not all overos are lethal in the homozygous state---only frames, not sabinos or splash whites. However, patterns can easily be combined in an individual, and frame may not be obvious if the horse is minimally marked or carries other patterns as well. For this reason, a test has been devised to determine which horses do carry frame as a safety precaution for breeders
There is no such thing as an albino horse! Albinism is caused by a recessive gene, meaning an animal must carry 2 copies of the gene, one from each parent, in order to express the color (or rather, lack there of). One copy of the gene has no effect on coat color. Because of this, 2 albinos bred together will always produce an albino.*** As you will read below, this simply does not happen in horses because no white horse's coat is caused by a recessive gene, and none of these white horses breed true as an albino would.
So where do "white" horses come from? From several different genetic patterns, as a matter of fact: the grey gene, the cream gene, the sabino gene, the splash white gene, the so-called "dominant white (W) gene," and occasionally from extreme overo, tobiano, or tovero patterns. The key idea to keep in mind, however, is that all of these genes act only to modify the horse's existing coat color---essentially they serve to block pigment from otherwise dark hairs---they are not really a color in the same sense as bay or chestnut.
Grey: Many people see a horse like the one pictured above and assume he is white. He is, however, a grey. Grey is a gene that modifies a horse's coat color, no matter what it is, and causes it to de-pigment with age, much like human hair changes color as we age, whether we're blond, brunette, or red-headed. All grey horses are likewise born "dark," be it black, chestnut, bay appaloosa, or palomino. As they age, their coat changes, often going through a dappled phase which is usually quite lovely. Some grey horses will never turn entirely "white," retaining traces of color in their manes and tails or on their knees and hocks. Some even develop little flecks of color throughout their coats, known as flea-bitten grey. But most importantly, no matter how greyed-out they are, grey horses are genetically grey, never white.
Cream Gene: Another color frequently confused as being white or albino is cremello, like this handsome TB stallion, RFF The Alchemist. This color is the result of two cream genes, one from each parent, acting on a chestnut base coat. The cream gene is an incomplete dominant, meaning that two copies of the gene will dilute the coat color more than just one. One cream gene combined with chestnut will dilute the coat to create palomino. Two creams genes dilute the coat to cremello, a very pale, golden shade often mistaken for white. Cremellos always have blue eyes which can lead to confusion with albino, a color that simply doesn't exist in any horse breed. True albinism is a recessive condition characterized by red-eyes, pink skin, and a total lack of dark pigment. Cremellos do still have some pale golden pigment to their coats and skin as well as blue pigment in the eyes.
Several pinto patterns are responsible for creating "white" or nearly white horses. Pinto, like grey, is really not a color, but rather a color modifier. It acts by removing pigment from areas of hair on an otherwise solid colored horse, creating the splashy patterns we see from Paint horses to Clydesdales. All pinto patterns are dominant! The most common one is sabino, a pattern of white that is expressed in its minimal form as stockings and a blaze. Moderate sabinos frequently have belly spots and white patches on their sides and shoulders. Extreme sabinos can be almost entirely enveloped by the pattern and thus look white. Most extreme sabinos retain a little color along their toplines, especially in the mane or on the ears. Breeding sabinos together may result in an offspring with even more white in their coat than the parents, but not always. Producing extreme sabino foals from extreme sabino mares or stallions has been a fairly unusual phenomonon until just recently. As more breeders today are breeding for color as well as function, extreme sabino families are slowly becoming more common. By no means do extreme sabinos throw that color on a regular basis---another nail in the coffin of the idea of true-breeding white horses---many produce moderately expressed sabinos.
Breeding for wildly patterned sabinos has become more popular in Thoroughbreds in the last few years. A few families have produced an unusual number of extreme sabino foals, the most notable of which is Patchen Beauty and her kin. The family began with White Beauty in 1963, an extreme sabino mare who, out of 6 foals, produced 1 extreme sabino foal and 3 chestnut foals with slightly less extensive sabino markings. One of these, a mare named World O'Beauty (born 1972), produced 2 extreme sabino fillies including Precious Beauty in 1981. And she, in turn, is the dam of the famous Patchen Beauty who has produced 3 whites foals to date. The White Fox is the only one of her foals to race so far.
Another pinto pattern that can cause completely white horses is called splash white, shown below. It is similar in appearance to sabino, in that the white pattern seems to "creep" up the horse starting with the legs and then belly. Most splash whites also have extreme face markings, sometimes even entirely white heads like the horse on the right. In its most extreme form, splash white leaves little or no color along the topline. Many splash whites are deaf.
Occasionally, tobianos and overos, or combinations of both,will exhibit such an extreme pattern that they are almost entirely white, as seen below, PPQ Spirit of Romance. He is an extreme tovero, and his color is entriely masked by the pattern. Though he looks entirely white, he is technically a pinto, and his offspring could be both pinto and solid-colored, as is true for the other horses pictured here.
A lethal white overo (LWO) is produced when a foal is homozygous for the frame overo gene (meaning it carries 2 copies of the gene, one from each parent). It causes foals to be born with so extreme an overo pattern that they are entirely white or nearly so. Frame is dominant, like all other pinto patterns, but it is only lethal in its homozygous state. When this happens, the resulting foal is born white, but sadly, it has an incomplete digestive track, and it dies within 72 hours of birth. This is the only white-pattern known to be lethal. (Horses that carry only one frame gene are heterozygous (and thus not lethal) for the trait but may pass it on to their offspring.) . Obviously, not all overos are lethal in the homozygous state---only frames, not sabinos or splash whites. However, patterns can easily be combined in an individual, and frame may not be obvious if the horse is minimally marked or carries other patterns as well. For this reason, a test has been devised to determine which horses do carry frame as a safety precaution for breeders
