Sabino horse

Horses with one copy of the Sabino 1 gene are said to be heterozygous for that gene, and have a distinctive white spotting pattern. The areas of pigmentless white hair are rooted in pigmentless pink skin. Horses with the Sabino 1 pattern typically have irregular, rough-edged white patches on the extremities and the face. These white patches also often include the midsection as "belly spots". Interspersed white hairs around the white markings or on the body, which can resemble roan, are characteristics of Sabino 1, particularly when heterozygous.[5] Typically, Sabino 1 horses have two or more white feet or legs, a blaze, spots or roaning on the belly or flanks, and jagged margins to white markings.[6] Modest Sabino 1 markings are often difficult to tell apart from other white markings; the phenotypes overlap. Blue eyes are not associated with Sabino 1, though horses with Sabino 1 may have blue eyes from an unrelated genetic factor.

Horses with two copies of the Sabino 1 gene - one from each parent - are said to be homozygous for that gene. Homozygous Sabino 1 horses are typically at least 90% pink-skinned and white-coated at birth.[7] Again, the eyes are not usually blue. The term "sabino-white" is used to distinguish homozygous SB1/SB1 horses from so-called "dominant white" horses, which need only a single copy of a "white" gene to have a white coat. Without a DNA test, dominant white horses and sabino-white horses are indistinguishable.

Not all "white" horses are sabino-white or even dominant white. Combinations of other white spotting patterns, such as tobiano with heterozygous frame overo, can produce a horse that is 90% white or more. Cremello horses are superficially similar to sabino-whites, however cremellos have blue eyes, rosy-pink skin and a cream-colored rather than white coat. Gray horses have a white hair coat at maturity but unless they also happen to carry dilution, white, or SB1 genes, they do not have pink skin and are not white at birth.

The Sabino 1 locus is at the KIT gene. Sabino 1 is an incomplete dominant trait; each of the different genotypes corresponds to a separate phenotype. Both Sabino 1 and the many known forms of dominant white spotting in horses involve the same gene, but they are distinctly labeled. "Dominant white spotting" includes many different phenotypes and genes, while "Sabino 1" is reserved by geneticists for the gene that results in an all-white phenotype in the homozygous state.

The alleles, or "versions", of Sabino 1 were designated SB1 and sb1. The gene in the equine wildtype is recessive and the SB1 mutation is dominant.

• sb1/sb1 (also written n/n or +/+) homozygous recessive, wildtype. The horse will not have true Sabino 1 traits, but may still have white markings due to other factors.
• SB1/sb1 (also written SB1/n or SB1/+) heterozygous, sabino. The horse will have Sabino 1 traits such as white markings with jagged edges and body spots or roaning.
• SB1/SB1 homozygous sabino, sabino-white. The horse will have pink skin and white coat on at least 90% of its body from birth.

The laws of Mendelian inheritance, as summarized in the table above, show that crossing two horses with the recessive genotype (sb1/sb1 or +/+) will never produce a foal with the Sabino 1 gene, regardless of the appearance of the coat. By these same rules, the only way to guarantee a sabino-spotted foal (SB1/+) is to breed a sabino-white to a non-sabino.

The mutation responsible for Sabino 1 on the KIT gene is a single nucleotide polymorphism designated KI16+1037A.[8] The Sabino 1 mutation results in the skipping of exon 17.[1] Other areas of the KIT gene are responsible for tobiano, true roan, and a dozen or so dominant white phenotypes. It 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.

Similar phenotypes are known in many other species. There are over 90 unique mutations on the mouse KIT gene, resulting in phenotypes that range from white toes on the hind feet to "black-eyed-white" coats. In mice, the KIT gene is dubbed the W locus.

It is known that sabino-like patterns do exist in purebred Arabians, though studies at the University of California, Davis indicate that the gene (or genes) which produces sabino patterning in Arabians is not SB1, and SB1 has not been found to date in Arabians.[11] The white spotting produced by the sabino trait in Arabians was at one time controversial, and body spotting was viewed as a sign of "impure" breeding, with such horses excluded from the registry. Today, with DNA testing to verify parentage, a large number of Arabians meet the definition of having minimal to moderately expressed sabino characteristics.[12]

Minimal sabino traits exist in Arabians and the term "maximum sabino" was coined to describe spotted Arabians that were close to 50% white.[12] However, there are very few Arabians that visually appear to be true white, and sabino-white has not been verified in any of them. One true white horse originally identified as "bay sabino" was determined to carry an original mutation of a new form of Dominant white, and has passed this trait on to his descendants.[13] The inheritance patterns observed in sabino-like Arabians do not follow the same mode of inheritance as Sabino 1.[14]

A Clydesdale horse with sabino belly spots and leg markings

The Clydesdale and Shire are closely related draft horses originally bred in the United Kingdom. Apart from massive build and copious feathering, both breeds are known for consistent white markings. The most popular and acceptable form of white markings on both breeds includes a bold blaze and four even socks.[15] Breed standards for Clydesdale horses state that "excessive white" is no longer a fault.[16] Conversely, even in modern times, the Shire horse breed standard counts excessive white, body spotting, or roaning as a fault, especially in stallions.[17]

Among Shires and Clydesdales, high white markings follow a pattern similar to that found in high-expression Sabino 1 sabinos. The consistency of the draft-type sabino spotting pattern led researchers to include these horses in the original study that discovered Sabino 1. However, none of the draft-type sabinos possessed the SB1 allele.[1] White markings on the face range from blazes to bald faces to apron faces. White facial markings often extend to the chin or lip, and may wrap around the head with irregular, feathery borders. When white markings on the forelegs extend above the knees, they may trail up the shoulder or up the back of the leg to the elbow with the characteristic irregular, feathered, or roaned borders. White markings above the hocks on the hindlegs are more common, and typically trail up the front of the leg to the stifle joint and flank. When white leg markings extend above the knee or hock, they are often accompanied by body spotting, typically on the belly. These markings are also often accompanied by interspersed white hairs that give the horse a roan-like pattern. Such horses are called "roan" by the Shire and Clydesdale breed registries.[15][16][17]

"Draft-type sabino" differs from Sabino 1 in that, while it appears to be dominantly inherited, it does not result in a sabino-white phenotype when homozygous.[18] As the gene pool for these two breeds is limited, most horses can be expected to possess the gene responsible for their unique white spotting pattern. Yet, if the draft-type sabino gene produced sabino-whites, near white coats would be expected in nearly a quarter of foals. Though near-white Shires and Clydesdales can be found, they are quite uncommon.[19]

The term "sabino" is seldom used among American Quarter Horse breeders, as white patterning was originally considered undesirable. The American Quarter Horse breed was traditionally solid-colored and minimally-marked. Among Quarter Horse breeders, foals with ineligible amounts of white born to eligible parents were referred to as "cropouts," and, until 2004, horses that had areas of white hair rooted in pink skin beyond the gaskin on the hindleg, above the halfway point between the knee and elbow in the foreleg, or beyond the eye could not be registered.[20] Due to the large number of horses that had these patterns, as well as a significant number of double dilute creams also caught up by this rule, the American Paint Horse Association (APHA) formed, allowing these horses to be registered as "Paints". The APHA recognizes sabino as one of the "overo" family of color patterns.

Since the relaxation of the white rule, at least one registered Quarter Horse has tested homozygous for the SB1 allele.[18] Due to this example, as well as the extensive Quarter Horse breeding present in Paint bloodlines, it is clear that the SB1 allele exists in the breed's genepool and that there have been SB1 sabinos that had minimal markings classified as "solid". Quarter Horses may exhibit sabino-like patterning from the same genetic sources as other breeds.

While the American Paint Horse Association (APHA) refers to both sabino and frame patterns as "overo", they are genetically unrelated and visually distinct. The frame spotting pattern is produced by the Ile118Lys mutation on the equine Endothelin receptor type B gene.[21] Frame is characterized by jagged but sharply defined, horizontally oriented white patches on the horse's neck, shoulder, flank and hindquarters. By itself, the frame pattern does not produce white markings that cross the back, or affect the legs or tail. It does, however, often produce bald faces and blue eyes.[21] Horses homozygous for the Ile118Lys mutation on the equine Endothelin receptor type B gene are born all-white or nearly white with some pigmentation along the topline, and die shortly after birth from lethal white syndrome.[22] The expression of the frame pattern varies from minor white markings to the distinctive framed pattern, with some DNA-tested frames appearing solid.[23]

The horizontally oriented white patches are indicative of the frame pattern. The wrap-around blaze, tapered stocking, all-over roaning and roaned edges of the white patches suggest that this is a frame-sabino blend.

Horses carrying both frame and sabino genes can be expected to show characteristics of both patterns. While frame alone is seldom responsible for white leg markings, a frame-sabino blend might have frame body markings and white markings on the legs. Similarly, while frame alone usually produces markings with jagged but sharply defined white patches, the addition of sabino can be expected to have roaned edges and roaning on the body. Blue eyes are not considered a sabino characteristic, but frame-sabino blends may have a lot of facial white and even blue eyes. Frame in conjunction with sabino can produce the "medicine hat" pattern, where only the ears and poll and sometimes the topline may have pigment. (Medicine hat horses sometimes also have a "shield" pattern on the chest).[23] Horses with even less pigment than a medicine hat may be classed as "breeding stock white" by the American Paint Horse Association, may also be horses with both frame and sabino genes.[23]

Since the APHA overo classification system was created prior to modern genetic studies, it is based on physical description and not genetics. Thus, frame-sabino blends of different expressions may be considered "breeding stock solid", "overo", or "breeding stock white". Without genetic testing, horses with frame genetics but lacking obvious frame characteristics may be bred to each other or to identified frames and may produce lethal white foals. The frame pattern and lethal white syndrome are produced by the same mutation, not separate, closely linked conditions.[21] No other white-spotting patterns are indicative of lethal white syndrome.

Terminology as well as identification can be a challenge even for knowledgeable breeders. Standards for horse breeds which do not carry frame or splash spotting patterns, such as the Arabian and Clydesdale, do not consider sabino to be an overo pattern, but sometimes classify sabino horses as roan. To confuse matters further, in Spanish-speaking countries, the term "overo" actually refers to horses with what is called "sabino" in English; meanwhile, in South America, the term "sabino", which literally translated from Spanish means "speckled" or sometimes "roan", refers to a flea-bitten gray.[24]

Splash, or splashed white, is a distinct and uncommon pattern that is also classified by the APHA as "overo" and has not yet been genetically mapped. The splash pattern is characterized by blue eyes and crisp, smooth markings that make the horse appear to have lowered its head and waded through white paint. The legs, tail, underside and head are typically white; the white head is distinct from frame and sabino face markings, which are jagged or wrap around, in that most of the head is white. Splash horses are sometimes deaf.[25] Splashed white is found in paints, Welsh ponies, Icelandics, and Morgans.[26] It is theorized that splash is incomplete dominant and that heterozygous splash horses often have minimal markings that resemble those of sabinos or generic white markings. The most minimal splashes may have only a bottom-heavy, off-center snip and low hind socks.[26] Splash-sabino blends will have characteristics of both patterns, such as jagged patches and roaning, blue eyes and blocky white markings. In horses which minimally express both, accurate identification can be very difficult, and without a DNA test for splash, is primarily conjecture.

This tobiano has an irregular, wrap-around blaze and chin white, the tobiano markings have ragged edges, extend high on the legs, and some roaning is visible. Thus, this horse may carry both tobiano and sabino genetics.

The tobiano pattern is easily recognizable and tobiano horses are categorized separately from other patterns in breeding associations for pinto-spotted horses. Even though they are visually quite distinctive, the simple dominant allele responsible for the tobiano pattern is quite close to known and suspected sabino loci. The tobiano pattern is characterized by smooth, crisp-edged white markings arranged vertically on the body, sometimes as if in bands. The legs are often white, and the tail is often white or partly white. Tobiano is not responsible for facial markings or blue eyes. A tobiano-sabino blend might then have more than the expected amount of facial white markings, and blotchy or roaned edges to the tobiano white markings. Tobiano markings do not usually occur on the stifle and flanks, while sabino leg markings frequently run into these areas; a clearly tobiano-marked horse with white that runs up the legs to the stifle and flanks is likely also a sabino. Due to the additive effect of multiple white spotting genes, tobiano-sabino blends may have more white than is otherwise expected.

Some horses may simultaneously carry genes for both the tobiano spotting pattern and for sabino. This is generally noticed when a tobiano horse has spots with roaning around the edges, or other sabino traits such as belly white, white on the chin, or "high white" markings. Tobiano is a dominant gene, is not related to sabino, but because pintos with different spotting patterns are often crossed on one another, it is easily possible for a horse to carry multiple spotting pattern genes.

Tobiano-sabino blends registered with color breed associations for spotted horses may be categorized as "tovero". Some cases of the "medicine hat" pattern, where only the ears and poll and sometimes the topline (and sometimes the chest has a "shield" pattern) may have pigment, and white or nearly white horses may be tobiano-sabino blends.[23]

This true or "classic" roan has the characteristic darker head and legs.

A true roan is neither sabino nor gray, but instead refers to a pattern of evenly-interspersed white hairs on the body with minimal white hairs on the head and legs and few, if any white markings. The primary characteristics of sabino are white markings on the head and legs, often with roaning at the edges. When both roan and sabino are present in the same horse, it can be difficult to tell whether the roaning is due to sabino or true roan, especially if the white markings entirely cover up the telltale dark head and legs.

In some breed registries, the term "roan" is used to record sabinos, particularly with thoroughbreds and Arabians. Sabinos or sabinos with roaning have also been described as "roan" by Clydesdale, Shire, and Tennessee Walking Horse registries.

The extensive roaning or ticking of this horse's barrel is typical of the rabicano pattern, which is sometimes confused with sabino. High white markings and patches are not controlled by rabicano.

Rabicano is another type of "roaning" or ticking characterized by scattered white hairs centered on the flanks, barrel and white hairs at the base of the tail. Only in the most extreme circumstances is rabicano linked to underlying pink skin. It is unknown if the roaning characteristic of some sabino-type patterns is due to the additional presence of rabicano or a separate mechanism. Rabicanos are also often identified as roans, even among breeds that do not have true roans, such as Thoroughbreds and Arabians.

Gray horses undergo progressive silvering that begins at or shortly following birth. Young gray horses often exhibit a mixture of whitish and colored hairs which can be mistaken for roaning. Grays develop more and more white hairs over the course of several years, most eventually losing all or almost all of their original colored hair. Sabino markings are permanent, and while some changes are not out of the ordinary, drastic color changes are not characteristic of sabino-type patterns. If a horse carries both genes, it will show spotting patterns while young, but they will fade over time as the overall coat lightens to white. Once the horse has fully grayed, the pink skin beneath the original white markings will still exist, but may not be obvious unless the horse has a body-clipped hair coat or is wet.