Dominant white (W) is a group of genetically related coat color alleles on the KIT gene of the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting, as well as bold white markings. Prior to the discovery of the W allelic series, many of these patterns were described by the term sabino, which is still used by some breed registries.
White-colored horses are born with unpigmented pink skin and white hair, usually with dark eyes. Under normal conditions, at least one parent must be dominant white to produce dominant white offspring. However, most of the currently-known alleles can be linked to a documented spontaneous mutation that began with a single ancestor born of non-dominant white parents. Horses that exhibit white spotting will have pink skin under the white markings, but usually have dark skin beneath any dark hair.
There are many different alleles that produce dominant white or white spotting; they are labeled W1 through W28 and W30 through W35, plus the first W allele discovered was named Sabino 1 (SB-1) instead of W1. They are associated with the KIT gene. The white spotting produced can range from white markings like those made by W20, to the irregularly-shaped or roaning patterns previously described as Sabino, to a fully white or almost fully white horse.
For many of the W alleles, the white coats are, as the name suggests, inherited dominantly, meaning that a horse only needs one copy of the allele to have a white or white spotted coat. In fact, some such alleles may be embryonic lethal when homozygous. Others, such as SB-1 and W20, are incomplete dominants, capable of producing viable offspring with two copies of the gene, and who generally have more white than horses with only one copy. In addition, different alleles which on their own give a white-spotted but not completely white horse, such as W5 and W10, can combine to make a horse completely white.
White can occur in any breed, and has been studied in many different breeds. Because of the wide range of patterns produced, some suggest the family be called "white spotting" rather than "white." Other researchers suggest the term "dominant white" be used only for the W alleles thought to be embryonic lethal when homozygous.
White is both genetically and visually distinct from gray and cremello. Dominant white is not the same as lethal white syndrome, nor are white horses "albinos"—Tyrosinase negative albinism has never been documented in horses.
Description
Although the term "dominant white" is typically associated with a pure white coat, such horses may be all-white, near-white, partially white, or exhibit an irregular spotting pattern similar to that of sabino horses. To add to the confusion, at least some horses in each of those groups might be referred to as "dominant white", "white spotted", or "sabino". The amount of white hair depends on which KIT alleles are involved. At birth, most of the white hair is rooted in unpigmented pink skin. The pink skin lacks melanocytes, and appears pink from the underlying network of capillaries. White spotting is not known to affect eye color, and most white horses have brown eyes.
White or near-white White horses are born with pink skin and a white coat, which they retain throughout their lives. The genetic factors that produce an all-white horse are often also capable of producing a near-white horse, which is mostly white but has some areas that are pigmented normally. Near-white horses most commonly have color in the hair and skin along the topline (dorsal midline) of the horse, in the mane, and on the ears. The color is often interspersed as specks or spots on a white background. In addition, the hooves are usually white, but may have striping if there is pigmented skin on the coronary band just above the hoof. In some cases, foals born with residual non-white hair may lose some or all of this pigment with age, without the help of the gray factor.
White spotting White spotting from a W allele is difficult to identify visually, as it can range from small white markings in the case of a heterozygous W20 horse all the way to an obvious pinto pattern. In addition, even completely white horses can have genes which by themselves would only give white spotting, such as W20 combined with W22 or W5 combined with W10. As such, the only reliable way to find out whether a horse has one of the known white spotting patterns from an allele on KIT is to have it genetically tested.
Prevalence Dominant white is one of several potential genetic causes for horses with near-white or completely white coats; it may occur through spontaneous mutation, and thus may be found unexpectedly in any breed, even those that discourage excessive white markings. To date, forms of dominant white have been identified in Thoroughbreds, Standardbreds, American Quarter Horses, Frederiksborg horses, Icelandic horses, Shetland ponies, Franches-Montagnes horses, South German Draft horses, and the Arabian horse. The Camarillo White Horse is noted for its completely white coat.
Inheritance The W locus was mapped to the KIT gene in 2007. KIT is short for "KIT proto-oncogene receptor tyrosine kinase". White spotting is caused by multiple forms, or alleles, of the KIT gene. All horses possess the KIT gene, as it is necessary for survival even at the earliest stages of development. The presence or absence of dominant white is based on the presence of certain altered variants of KIT. Each unique form is called an allele, and for every trait, all animals inherit one allele from each parent. The original or "normal" form of KIT, which is expected in horses without dominant white spotting, is called the "wild type" allele. Thus, a dominant white horse has at least one KIT allele with a mutation associated with dominant white spotting.
Allelic series The KIT gene contains over 2000 base pairs, and a change in any of those base pairs results in a mutant allele. Over forty seven such alleles have been identified by sequencing the KIT genes of various horses. The resultant phenotype of many of these alleles is not yet known, but over 30 have been linked to white spotting. DNA tests can identify if a horse carries the identified W alleles. SB-1 (Sabino 1) was first identified in 2005. The allele was designated with "SB" in an attempt to align with traditional equine coat color terminology. It is located on the KIT gene and is a single nucleotide polymorphism designated KI16+1037A. The mutation results in the skipping of exon 17.When heterozygous, it creates a distinctive white spotting pattern of irregular, rough-edged white patches that usually include two or more white feet or legs, a blaze, spots or roaning on the belly or flanks, and jagged margins to white markings. Homozygous foals are typically at least 90% white-coated at birth, and sometimes termed "Sabino-White."
W1 was the second KIT allele found to be involved in horse coat color. The researchers who discovered W1-4 decided to name them "W" following the convention in mice, rather than continuing the "SB" series. W1 is found in Franches-Montagnes horses descended from a white mare named Cigale born in 1957. Cigale's parents' coats were not extensively marked. A single nucleotide polymorphism (SNP), a type of mutation in which a single nucleotide is accidentally exchanged for another, is thought to have occurred with Cigale. This mutation (c.2151C>G) is predicted to truncate the protein in the middle of the tyrosine kinase domain, which would severely affect the function of KIT. It is a nonsense mutation located on exon 15 of KIT. Some horses with the W1 mutation are born pure white, but many have residual pigment along the topline, which they may then lose over time. Based on studies of KIT mutations in mice, the severity of this mutation suggests that it may be nonviable in the homozygous state. However, horses with the W1 mutation have been found to have normal blood parameters and do not suffer from anemia.
W2 is found in Thoroughbred horses descended from KY Colonel, a stallion born in 1946. While KY Colonel was described as a chestnut with extensive white markings, he is known for siring a family of pure white horses through his white daughter, White Beauty, born in 1963. His son War Colors was registered as roan because he had some spots of color, but later became white. The W2 allele is linked to a single nucleotide polymorphism (c.1960G>A), a missense mutation where a glycine is replaced with arginine (p.G654R) in the protein kinase domain, located on exon 17. W3 is found in Arabian horses descended from R Khasper, a near-white stallion born in 1996. Neither of his parents were white, and the causative mutation (c.706A>T) is thought to have originated with this horse. It is a nonsense mutation on exon 4, predicted to truncate the protein in the extracellular domain. Horses with the W3 allele often retain interspersed flecks or regions of pigmented skin and hair, which may fade with time. Some members of this family possess blue eyes, but these are thought to be inherited separately from the white coat. Based on similar studies in mice, researchers have named W3 as potentially homozygous nonviable. W4 is found in Camarillo White Horses, a breed characterized by a white coat, beginning with a spontaneous white stallion born in 1912 named Sultan. Like W1 and W3, these horses may be pure white or near-white, with pigmented areas along the topline that fade with time. This mutation is an SNP (c.1805C>T) which produces a missense mutation replacing alanine with valine in the kinase domain, on exon 12.
W5 is found in Thoroughbreds descending from Puchilingui, a 1984 stallion with white spotting and roaning. Horses with the W5 allele exhibit a huge range in white phenotype: a few have been pure white or near-white, while others have sabino-like spotting limited to high, irregular stockings and blazes that covered the face. Twenty-two members of this family were studied, and the 12 with some degree of white spotting were found to have a deletion in exon 15 (p.T732QfsX9), in the form of a frameshift mutation. A later study found that the members of this family with the greatest depigmentation were compound heterozygotes who also carried the W20 allele. W6 is found in one near-white Thoroughbred named Marumatsu Live born to non-white parents in 2004. The potential range of expressivity, therefore, is not yet known. The mutation (c.856G>A) is thought to have occurred spontaneously in this horse. It is a missense mutation on exon 5. W7 is found in another near-white Thoroughbred named Turf Club born in 2005 to a dam that had nine other offspring, all non-white. The dam did not possess the W7 allele, which results from a splice site mutation (c.338-1G>C), located on intron 2 of KIT. W8 was found in an Icelandic horse with sabino-like white spotting, mottling, and roaning, named Þokkadís vom Rosenhof. Both parents and four maternal half-siblings, all non-white, were found without the W8 allele. The W8 allele is also a splice site mutation (c.2222-1G>A), located on intron 15. W9 was found in an all-white Holsteiner horse with a single nucleotide polymorphism (c.1789G>A). No relatives were studied, but both parents are non-white. It is a missense mutation on exon 12. W10 was found in a study of 27 horses in a family of American Quarter Horses, 10 of which were white or spotted and 17 that were solid and non-white. The 10 family members with W10 had a frameshifting deletion in exon 7 (c.1126_1129delGAAC). Like W5, a wide range of phenotypes were observed. The most modestly marked had large amounts of white on the face and legs and some medium-sized belly spots, while another was nearly all-white. The founder of this line was GQ Santana, foaled in 2000.