White (horse)

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A truly white horse has pink skin under their white hair coat

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][2]

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.

[edit] Horses that appear white, but are not

See also Equine coat color genetics

This horse is gray, not white. Its hair coat is completely white, but its underlying skin, seen around the eye and muzzle, is black.

This light palomino foal has dark skin and a creme-colored coat that can be mistaken for white. This same shade may be considered cremello if the horse had light skin and eyes, but still would not be white.

Gray horses are often mistakenly called "white." However, the most noticable difference between a gray horse whose hair coat is completely white and a white horse is skin color: gray horses have black skin, white horses have pink skin. (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 creme color are technically known as Creme, 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 others, and there is no in-between dilution factor involved.

Some white horses may actually be Paints or Pintos 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 gene complex, Overo or Tobiano genes. However, some breeds that exihibit sabino patterns, such as the Arabian horse or the Clydesdale horse have not been known to ever produce true white horses, which means that "maximum sabino" may be the result of a different gene. Some argue that that all white horses are simply fully expressed pintos. However, there is also strong evidence that the "W" gene exists independent of any gene for spotting patterns.^[3]

see also Gray (horse), Cremello, Pinto, Sabino and Creme gene.

[edit] Famous White Horses

The Lone Ranger's Horse, Silver, was a white horse

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. The most famous of these were the two horses named "Silver," who were ridden by the Lone Ranger.

[edit] Genetics of White horses

The W gene is neither a dilution gene nor a graying gene. Statistically, a white horse (Ww) bred to a horse of another color will simply produce white offspring about 50% of the time, the rest of the time the offspring will be the color of the other parent.

All "true" white horses are heterozygous in their white gene. This means that they have only one copy of the gene (Ww). If a horse gets two white genes (homozygous white or WW), it is a lethal gene and the ensuing foal will die in the womb.^[4]

see "Lethal White" within this article, 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. The practical result is that about 2/3 of living foals from two white parents will be white.

These combinations can be demonstrated using Punnett squares. Here, W denotes a Dominant White allele, and w denotes a non-white allele:

A Ww horse will appear white, because the Dominant White W allele "dominates" the non-white w allele. A WW horse, one with two Dominant White alleles, known as "lethal white" will not survive to birth (this is why there is no Punnett square shown for breeding WW horses).

Because the chance of breeding a live white foal is the same whether a white horse is bred to a white horse or a colored horse, 50%, a breeder breeding for white coloration should breed to a non-white horse, so that there is not a risk of the fatal homozygous white.

[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. ^[5] 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. ^[6] 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 formed to register white and creme 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.^[7]

[edit] "Lethal White"

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. Some researchers have strong evidence for both these conditions, others, usually breeding organizations, argue with equal vehemence that one or the other condition does not exist.

LWS, sometimes called overo lethal white syndrome (OLWS), is represented by the allele (O). Foals homozygous for this gene (OO) die within a short period after birth. In spite of the name, the gene has appeared in non-Overo patterned horses, and even in solid-colored animals.^[8] It is not possible to tell if a horse carries this gene by looking at its color, only a blood test will detect a carrier. Though most commonly linked to American Paint Horses, it may also appear in other breeds, and was found in one case involving a Miniature horse.^{[9] [10]} There is a gene in humans that causes a similar condition known as Hirschsprung disease. However, this condition does not appear to be connected to any of the potential lethal white alleles in horses.

There do not appear to be lethal genes affiliated with the Sabino SB1 gene or with other gene-complexes that create Sabino coloring in breeds such as the Arabian or Clydesdale.^[11]

for additional information, see Lethal white syndrome

[edit] References

1. ^ [http://www.vgl.ucdavis.edu/~lvmillon/coatcolor/coatclr3.html#genew description of the White gene
2. ^ Photos of white horses
3. ^ "The Myth of True-Breeding White Horses"
4. ^ 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]
5. ^ Bowling, Ann. [http://www.vgl.ucdavis.edu/~lvmillon/coatcolor/coatclr3.html "Coat Color Genetics: Positive Horse Identification." Accessed June 6, 2006
6. ^ Householder, D. Douglas. "The Genetics of Equine Coat Color" Texas A&M University Department of Animal Science Equine Sciences Program, publication HRG-003 (undated)
7. ^ American White Horse and American Creme Horse Registry
8. ^ Overton, Rebecca. "By a Hair." Paint Horse Journal, March 2004.
9. ^ Stalking the Lethal White Syndrome
10. ^ Lethal White Overo
11. ^ Horse coat color test<ref></ref>Lethal White Overo FAQ

[edit] External links

• Stalking the Lethal White Syndrome
• "The Myth of True-Breeding White Horses"
• Overton, Rebecca. "By a Hair." Paint Horse Journal, March 2004.