Sabino describes a distinct pattern of white spotting in horses. In general, Sabino patterning is visually recognized by roaning or irregular edges of white markings, belly spots, white extending past the eyes or onto the chin, white above the knees or hocks, and "splash" or "lacy" marks anywhere on the body. Some sabinos have patches of roan patterning on part of the body, especially the barrel and flanks. Some sabinos may have a dark leg or two, but many have four white legs. Sabino patterns may range from slightly bold face or leg white markings—as little as white on the chin or lower lip—to horses that are fully white.
The known causes of the sabino patterns are the SB-1 allele and several other dominant white (W) alleles on the KIT gene. The genetics behind some types of sabino, such as that found in Clydesdales, have not yet been identified. Some genes, such as Sabino-1, are incomplete dominants, producing irregular spotting when heterozygous, but when homozygous, can produce a horse that is almost completely white. Some forms of sabino are thought to be a reason for solid-colored horses with bold white markings on the face and high white leg markings.
Prior to the development and widespread use of DNA testing to determine equine coat color, the term broadly encompassed pinto patterns that did not clearly appear to be tobiano or frame overo. Some breed registries still use the term to describe spotting phenotypes that include roaning or irregular spotting, regardless of the precise genetics involved.
Description Horses described as Sabino can have varying amounts of white, from bold white face and leg markings all the way up to fully white coats. Sabino patterns often have speckling and roaning. The different white spotting patterns usually have some consistency in which areas of the horse are white and which are pigmented. At the minimal end, there will typically be white on the legs and face, and the horse may also be lightly roaned. In horses with a bit more white, it usually extends above the knees and hocks onto the belly. At the other end of the spectrum, nearly white sabino horses may still have scattered flecks of color, though it is also fairly common to have color on the ears, lower neck and chest, flanks, tailhead, and back.
Unlike white markings on non-sabino horses, the white leg markings of the sabino pattern often have a strip of white extending up the front or the back of the legs toward the belly. Some people call these "lightning strikes".
Terminology The term "sabino" was in use prior to the mapping of the horse genome to describe a wide range of pinto spotting patterns that are characterized by irregular edges, and is used by some breed registries. For example, the American Paint Horse Association still groups sabino patterns in its "overo" family of color patterns, even though sabino, splashed white, and frame overo are produced by different genetic mechanisms and are visually distinct. Since the APHA classification system was created prior to modern genetic studies, it was based on physical description and not genetics.
In Welsh ponies and some other UK breeds, sabino patterns are sometimes called "blagdon." To confuse matters further, in Spanish-speaking countries, the term "overo" refers to horses with what are called "sabino" patterns in English; meanwhile, in these countries the term "sabino", which literally translated from Spanish means "speckled" or sometimes "roan", refers to a flea-bitten gray.
Breed standards that recognize the sabino pattern include the Mustang, American Paint, Miniature horse, Morgan, Hackney (and Hackney pony), Tennessee Walking Horse, and the pinto color breed registries. Horse breeds that are generally solid-colored and do not allow most pinto coloring in their breed registries, but who may have representatives with sabino-style patterns (such as the Arabian, Thoroughbred, and Clydesdale) have at times classified sabino horses as roan or even gray.
Genetics Most spotting patterns described as sabino are thought to be created by various mutations, most on the KIT gene, grouped broadly as dominant white (W), which now includes the SB-1 allele. DNA testing is often required to verify the specific genes influencing a pattern. There are over 30 alleles of dominant white identified to date, though only some produce sabino-like spotting. Confusing matters further, some horses may carry more than one allele that produces spotting patterns, thus visual verification of any specific genetic mechanism may be challenging. For example, some blue-eyed horses described as sabino may have a pattern created by the splashed white (SW) gene family.
Variants affecting the KIT gene are also responsible for tobiano and classic roan. KIT is also the gene associated with unpigmented patches of skin and hair on the extremities and midline of humans, mice, and pigs. KIT plays an important role in the migration of early pigment cells (melanocytes) from the neural crest to their ultimate location in the skin. Mutations on KIT appear to limit the migration of melanocytes, leaving the extremities and midline devoid of pigment cells. Other factors, including stochastic events and other genes, affect the amount of unpigmented skin and hair in the fully developed animal.
There are differences in terminology amongst genetics researchers and certain organizations that promote sabino patterns. Because several dominant white alleles produce sabino-style patterns rather than completely white horses, some propose the W gene be called the "white spotting" gene. In some cases, a horse that is homozygous for the SB-1 gene is often called "sabino-white," some researchers prefer the term "maximum sabino" rather than "sabino-white" to describe horses more than 90% white."Why Color Your Horse?" Groups promoting sabino color have a more generous definition; the Sabino Arabian Horse Registry considers a "Maximum" Sabino to be a horse that is over 50% white.
Sabino 1 allele Sabino 1 was identified in 2005 by researchers at the University of Kentucky. The Sabino 1 allele, and the associated spotting pattern, is found in Miniature horses, American Quarter Horses, American Paint Horses, Tennessee Walkers, Missouri Fox Trotters, Mustangs, Shetland Ponies, and Aztecas.Brooks, Samantha (2005). "The SNP was found among American Miniature Horses, American Paint Horses, Azteca, Missouri Fox Trotters, Shetland Ponies, and Spanish Mustangs." SB1 is notably absent from the Arabian horse, Thoroughbred, Standardbred horse, Shire horse and Clydesdale. Sabino 1 is an incomplete dominant trait; homozygous individuals have significantly more white than horses that are heterozygous.
Horses who are heterozygous for Sabino-1 (SB1/sb1) generally have 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.Brooks, Samantha, et al (2005). "[W]hite patches often extend to include the belly and midsection, either as distinct areas of white hair, or as a diffuse scattering of white hairs resembling roan."Brooks, Samantha et al (2005). "Horses characterized as having a Sabino 1 white spotting pattern possessed 3 out of 4 of the following characteristics: 1) two or more white feet or legs, 2) blaze (white patch extending the length of the face), 3) jagged margins around white areas, and 4) spots or roaning in the midsection." Horses homozygous for the Sabino 1 allele (SB1/SB1) are "sabino-white," typically at least 90% white-coated at birth with unpigmented skin under white hair. Horses in both cases have dark eyes.Brooks, Samantha, et al. (2005). "[H]omozygosity for the Sabino 1 gene may result in a phenotype exhibiting extensive white coat color, hereafter referred to as sabino-white in order to distinguish it from White (W)."
The Sabino 1 locus is at the KIT gene. The mutation responsible for Sabino 1 is a single nucleotide polymorphism designated KI16+1037A.Brooks, Samantha et al (2005). "...base substitution for T with A in intron 16, 1037 bases following exon 16." The Sabino 1 mutation results in the skipping of exon 17. The gene in the equine wildtype is recessive (sb1) and the SB1 mutation is dominant, though an sb1/sb1 horse may have white markings due to other factors. Modest Sabino 1 markings can be difficult to tell apart from other white markings; the phenotypes overlap. Sabino 1 and the other KIT alleles in horses are not known to cause blue eyes. Blue eyes are linked to splashed white genes, and green-blue eyes are seen in horses with two dilution genes.
Researchers named the allele "Sabino 1" with the expectation that later alleles would be named "Sabino 2", "Sabino 3", and so on.Brooks, Samantha, et al. (2005), "Consequently, we anticipate future discoveries of other genes encoding different sabino phenotypes (possibly designated SB2, SB3, etc)." However, based on mouse nomenclature, it would have been more consistent to label the SB-1 gene as a W allele as it was part of the dominant white allelic series. The decision to name it "sabino" was to match the existing nomenclature used by horse breeders and to avoid confusion with various controversial known-lethal white genes in horses. However, confusion occurred anyway as more alleles at the same locus were discovered and the subsequent mutations labeled W to match the symbol used in mouse research.
Neither Sabino 1 or any other alleles of W are linked to lethal white syndrome (LWS). LWS occurs when a foal is homozygous for frame overo, which is a mutation on a different gene, EDNRB. "Horse coat color tests" from Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008 Some of the genes in the W allelic series are embryonic lethal when homozygous and only one copy was needed for a horse to be fully white, while others, including SB-1, produced sabino patterns when heterozygous and appear to be viable when homozygous.
"Sabino" but not Sabino 1
The term "sabino" is also used for horses who do not carry SB-1, but have white leg markings above the knees and hocks with jagged margins, wide blazes, and belly spots or roaning. In some cases, the term is defined even more broadly, to include white spots on the lower lip or chin, distal white patches on the legs, or "pointy" leg markings. The "dominant white" alleles W5, W8, W10, W15, and W19 are particularly noted for producing sabino-type markings.
Prior to the mapping of the horse genome and the widespread use DNA testing to verify parentage, assorted rules were created by to distinguish "spotted" from "non-spotted" horses; some breeds selected for bold white patterns, others disqualified horses with "too much white" from registration. The rules were not based on modern genetic studies, as such information was not yet available. In many cases, horses with sabino patterns fell into the middle—what was "excessive" white for one registry might be allowable in another. When early parentage testing using blood typing developed, revealing the variability of white patterns and markings, rules were modified to allow "cropouts" with extra white to be registered when previously excluded, and likewise, various color breed registries began to allow visually solid-colored foals to be registered.
Today, most breed registries allow DNA-verified offspring of registered animals to be registered regardless of color pattern, though some are slow to catch up to modern understanding of genetics. Yet others have refined their breeding goals to include or exclude horses based upon DNA color testing.