White (horse)
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True "white" horses, especially those that carry the White or "W" gene, are rare. Most horses that are commonly referred to as "white" are actually gray horses whose hair coats are completely white.[1]
White horses are born white, always have a white coat with no other color present, and stay white throughout their life. White horses may have blue, brown or hazel eyes.
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[edit] Horses that appear white, but are not
- See also: Equine coat color genetics, Gray (horse), Cremello, Pinto horse, Sabino horse, and Creme gene
Gray horses are often mistakenly called "white." However, the most noticeable difference between a gray horse whose hair coat is completely white and a white horse is skin color: gray horses have black skin and dark eyes, white horses have pink skin, and most have blue eyes. (Some gray horses may have pink skin under white markings, which, if they had a dark color coat, are visible while the horse still shows its base color). Gray horses are born with a dark base coat, usually bay, chestnut or black that gets lighter as the horse ages. On the other hand, white horses are born white, always have a white coat with no other color present, and stay white throughout their life.
Horses who are a light cream color are technically known as "Cream," cremello, or perlino horses. Although a white horse may look similar to a very light cremello, the colors come from different genes and produce different colors in their offspring. Cremello and perlino coat colors are produced by a dilution gene sometimes called the cream gene. A cremello always passes on a dilution gene in some form, and so produces a palomino if bred to a chestnut or a buckskin if bred to a bay. On the other hand, a white horse will pass on its white color to some foals but not at all others, and there is no in-between dilution factor involved.
[edit] Sabino white
Some white horses may actually be a type of pinto horse whose white markings are "fully" or "maximally" expressed, meaning, essentially, that the entire horse is one big white spot. It is thought this can occur with the Sabino 1 gene (SB1).[2] However, some breeds that exhibit sabino patterns, such as the Arabian horse or the Clydesdale horse rarely or never produce true white horses, and have not been found to carry the SB1 gene. So far, "sabino white" has been most closely linked to the SB1 gene, but the sabino gene complex is not fully understood and some sabino horses that are over 90% white may have other genes involved.
Some argue that that all white horses are simply fully expressed pintos who exhibit some form of leucism, going so far as to argue that the "W" gene does not exist.[3] However, the white WNT1 gene was mapped in 2004.[4] Although both sabino-1 and the white "W" gene both have been mapped to the equine KIT gene, they are in independent regions.[5] (see "Genetics of white horses," below)
[edit] Famous White Horses
Most white horses used in movies are actually grays, in part because they are easier to find. However, there are a few truly white horses who were used in film. One of the best-known examples was "Silver," ridden by the Lone Ranger, a role actually played by two different white horses. At least one horse who played "Topper," ridden by Hopalong Cassidy, was also white.
[edit] Genetics of white horses
The WNT 1 or "W" gene is neither a dilution gene nor a graying gene. It is a dominant gene, and thus statistically, a heterozygous white horse (Ww) bred to a horse of any other color will produce white offspring 50% of the time. Further, all "true" white horses are heterozygous for the color. This means that they have only one copy of the gene (Ww). A Ww horse will appear white, because the white (W) allele "dominates" the non-white (w) allele.
The W gene has been mapped to the equine KIT gene and may be produced in different horse breeds by different mutations on assorted exons. The roan, sabino-1 (SB1, which may also produce entirely white horses, but by a different genetic pathway), and tobiano genes also trace to the KIT gene, but in independent regions and DNA tests can distinguish between these colors.[5]
If a horse has two white genes (homozygous white or WW), it is generally considered a lethal gene and the ensuing foal will die in the womb.[6] It is not known at present why this happens, but the end result is that there are no true albinos in the horse world.
Because the chance of breeding a live white foal is the same (50% probability) whether a white horse is bred to a white horse or to a colored horse, a breeder breeding for white coloration is advised to breed to a non-white horse to eliminate the risk of fatal homozygous white.
- see also "Lethal White" below
Statistically, if you breed...
- Colored Stallion X Colored Mare: Colored foal
- Color Stallion X White Mare (or vice versa): 50% of foals will be white, 50% will be colored
- White Stallion X White Mare: 50% foals will be white (Ww), 25% will be another color (ww), 25% will have the lethal homozygous trait (WW) and die in the womb.
These combinations can be demonstrated using Punnett squares. Here, W denotes a Dominant White allele, and w denotes a non-white allele:
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Because WW is lethal, there is no Punnett square shown for breeding WW horses.
[edit] Albinism
True albino animals have a white coat, mane, and tail, with pink skin and pink eyes. However, there is no reported case of a true albino horse. Albinism in horses appears to be linked to lethal traits and albino foals do not survive. Therefore, all living "albino" horses actually have blue or brown eyes and thus are white horses, not true albinos.
In horses, the white or "W" gene is known to be responsible for the absence of pigment which is usually referred to as albinism.[7] The theory that "dominant white" is lethal in horses--when the W gene is homozygous (WW)--was first proposed as a theory in 1953 and after 15 years of test breeding, was confirmed in 1969.[8] The same group of researchers found that the WW gene complex was also lethal in Dexter cattle, platinum foxes, and bluefrost minks.
The American Albino Registry was originally formed to register white and cream-colored horses. However, living white horses are never true albinos, so as the genetics of dominant white and lethal white became better understood, the registry renamed itself the American White Horse and American Creme Horse Registry.[9]
[edit] "Lethal White"
- See also: Lethal white syndrome
The issue of "lethal white," whether via the homozygous white gene (WW) or via the (OO) gene that is linked to lethal white syndrome (LWS), is controversial. Researchers now have evidence for both these conditions, though other individuals, usually breeding organizations, argue with equal vehemence that one or the other condition does not exist.
Lethal white syndrome (LWS) has been verified through genetic testing and healthy horses can be tested to determine if they carry the gene. LWS, also called overo lethal white syndrome (OLWS), is represented by the allele (O). There is a DNA test for this gene, available from the Veterinary Genetics Lab at the University of California, Davis. Foals homozygous for this gene (OO), are born with incomplete development of the colon and die within a short period after birth. In spite of the name and the association of the condition with the "frame overo" coat color pattern,[10] the gene has appeared in both overo and non-overo patterned horses, and even in solid-colored animals.[11] While not all horses with overo coloring carry the recessive LWS gene, to date all horses known to have produced an LWS foal do have overo ancestry in their pedigrees.[12] the recessive nature of the gene means even that having a carrier ancestor cannot be way to know if a horse is a carrier or not, because statistically a recessive is only passed on 50& of the time. Therefore, it is not possible to tell if a horse carries this gene by looking at its color or its pedigree, only a blood test will detect a carrier. Though most commonly linked to American Paint Horses, it may also appear in other breeds that have overo ancestors; it even was found in one case involving a Miniature horse.[13] [14] There is a gene in humans that causes a similar condition known as Hirschsprung disease.
Dominant white (W) appears to be lethal when homozygous (WW). The mutation responsible for the white phenotype is lethal in a very early stage of gestation.[6] The embryo is either reabsorbed or the fetus dies in utero. The W allele has not been precisely pinpointed but has been localized to horse chromosome 3q22.[6]
There do not appear to be lethal genes affiliated with the Sabino SB1 gene or with other gene-complexes that create Sabino coloring.[15][16]
[edit] References
- ^ "Introduction to Coat Color Genetics" from Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008
- ^ "Horse coat color tests" from the UC Davis Veterinary Genetics Lab, Web Site accessed January 12, 2008
- ^ "The Myth of True-Breeding White Horses"
- ^ Bailey, Ernest, et.al. "Proposal to sequence the genome of the domestic horse, Equus caballus." Web page, accessed March 8, 2007
- ^ a b Haase B, Brooks SA, Schlumbaum A, Azor PJ, Bailey E, et al. (2007) "Allelic Heterogeneity at the Equine KIT Locus in Dominant White (W) Horses." PLoS Genet 3(11): e195 doi:10.1371/journal.pgen.0030195
- ^ a b c Mau, C., Poncet, P. A., Bucher, B., Stranzinger, G. & Rieder, S. (2004) "Genetic mapping of dominant white (W), a homozygous lethal condition in the horse (Equus caballus)." Journal of Animal Breeding and Genetics 121 (6), 374-383. doi: 10.1111/ j.1439-0388.2004.00481.x. Accessed September 6, 2006 at [1]
- ^ "Introduction to Coat Color Genetics" from Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008
- ^ Householder, D. Douglas. "The Genetics of Equine Coat Color" Texas A&M University Department of Animal Science Equine Sciences Program, publication HRG-003 (undated)
- ^ American White Horse and American Creme Horse Registry
- ^ "The Genetic Equation: The overo patterns." American Paint Horse Association, web site accessed December 1, 2007
- ^ Overton, Rebecca. "By a Hair." Paint Horse Journal, March 2004.
- ^ " Overo-Lethal White Foal Syndrome (OLW)", University of Queensland, web page accessed December 1, 2007
- ^ Stalking the Lethal White Syndrome
- ^ "Horse coat color tests" from the UC Davis Veterinary Genetics Lab, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008
- ^ Horse coat color test
- ^ Lethal White Overo FAQ
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