Sabino horse - AbsoluteAstronomy.com

Sabino is a group of white spotting

Pinto horse

A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...

patterns in horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 (SB1) gene, for which there is a DNA test. However, other horse enthusiasts also refer to patterns that are visually similar to SB1 as "sabino", even if testing indicates the gene is not present. Use of the term to describe non-SB1 "sabino" patterns in breeds that apparently do not carry the gene is hotly debated by both researchers and horse breeders.

Sabino patterning is visually recognized by roaning at the edges of white markings

Horse markings

Markings on horses usually are distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life...

, belly spots, irregular face markings, especially white extending past the eyes or onto the chin, white above the knees or hocks, and "splash" or "lacy" marks anywhere on the body, but particularly on the belly. Some sabinos have patches of roan

Roan (horse)

Roan is a horse coat color pattern characterized by an even mixture of colored and white hairs on the body, while the head and "points"—lower legs, mane and tail—are more solid-colored. The roan pattern is dominantly-inherited, and is found in many horse breeds...

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. Both blue and brown eyes are seen. At one end of the sabino spectrum, the SB1 gene, when homozygous, can produce a horse that is almost completely white

White (horse)

White horses are born white and stay white throughout their life. White horses may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of the dominant white genes, are rare...

with pink or only partially pigmented skin. Some forms of sabino genetics are also thought to be the most common reason for solid-colored horses with "chrome," a term which can refer to horses with bold white markings on the face and high white leg markings. The most generous definition of sabino can include horses with as little white as a chin or lower lip spot.

Even though horses with the Sabino 1 gene can be completely white, no forms of sabino are linked to lethal white syndrome

Lethal white syndrome

Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...

Sabino 1 gene

Sabino 1 was identified in 2005 by researchers at the University of Kentucky

University of Kentucky

The University of Kentucky, also known as UK, is a public co-educational university and is one of the state's two land-grant universities, located in Lexington, Kentucky...

. The Sabino 1 gene, and the associated spotting pattern, is found in Miniature horse

Miniature horse

Miniature horses are found in many nations, particularly in Europe and the Americas. The designation of miniature horse is determined by the height of the animal, which, depending on the particular breed registry involved, is usually less than as measured at the last hairs of the mane, which are...

s, American Quarter Horse

American Quarter Horse

The American Quarter Horse is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other breeds of horses in races of a quarter mile or less; some individuals have been clocked at speeds up to 55 mph...

s, American Paint Horse

American Paint Horse

The American Paint Horse is a breed of horse that combines both the conformational characteristics of a western stock horse with a pinto spotting pattern of white and dark coat colors. Developed from a base of spotted horses with Quarter Horse and Thoroughbred bloodlines, the American Paint Horse...

s, Tennessee Walker

Tennessee Walker

The Tennessee Walker or Tennessee Walking Horse is a breed of riding horse. The breed was originally bred in the Southern United States to carry the owners of plantations around their lands...

s, Missouri Fox Trotter

Missouri Fox Trotter

The Missouri Fox Trotter is an American breed of horse with a unique four beat gait. It was bred in the Ozark Mountain foothills, and used by settlers who valued its smoothness.-Breed characteristics:...

s, Mustangs

Mustang (horse)

A Mustang is a free-roaming horse of the North American west that first descended from horses brought to the Americas by the Spanish. Mustangs are often referred to as wild horses, but there is intense debate over terminology...

, Shetland Ponies

Shetland pony

The Shetland pony is a breed of pony originating in the Shetland Isles. Shetlands range in size from a minimum height of approximately 28 inches to an official maximum height of 42 inches at the withers. Shetland ponies have heavy coats, short legs and are considered quite intelligent...

, and Aztecas

Azteca (horse)

The Azteca is a horse breed from Mexico, with a subtype, called the "American Azteca", found in the United States. They are well-muscled horses that may be of any solid color, and the American Azteca may also carry pinto colors. Aztecas are known to compete in many western riding and some English...

. SB1 is notably absent from the Arabian horse

Arabian horse

The Arabian or Arab horse is a breed of horse that originated on the Arabian Peninsula. With a distinctive head shape and high tail carriage, the Arabian is one of the most easily recognizable horse breeds in the world. It is also one of the oldest breeds, with archaeological evidence of horses...

, Thoroughbred

Thoroughbred

The Thoroughbred is a horse breed best known for its use in horse racing. Although the word thoroughbred is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed...

, Standardbred horse

Standardbred horse

Standardbreds are a breed of horse best known for their ability to race in harness at a trot or pace instead of under saddle at a gallop. Developed in North America, the breed is now recognized worldwide for its harness racing ability...

, Shire horse

Shire horse

The Shire horse is a breed of draught horse or draft horse . The breed comes in many colours, including black, bay and grey. They are a tall breed, with mares standing and over and stallions standing and over. The breed has an enormous capacity for weight pulling, and Shires have held the world...

and Clydesdale

Clydesdale

Clydesdale was formerly one of nineteen local government districts in the Strathclyde region of Scotland.The district was formed by the Local Government Act 1973 from part of the former county of Lanarkshire: namely the burghs of Biggar and Lanark and the First, Second and Third Districts...

. There are many proposed genes responsible for the sabino-like white spotting in these and other breeds. Researchers gave the allele they discovered the name "Sabino 1" with the expectation of finding genes yet to be named "Sabino 2", "Sabino 3", and so on.

Even though horses with the Sabino 1 gene can be completely white, neither Sabino 1 or any other forms of sabino are linked to lethal white syndrome

Lethal white syndrome

(LWS). Foal

Foal

A foal is an equine, particularly a horse, that is one year old or younger. More specific terms are colt for a male foal and filly for a female foal, but these terms are used until the horse is age three or four. When the foal is nursing from its dam , it may also be called a suckling...

s afflicted with LWS are born white or near-white, but have a defective colon and invariably die within 72 hours of birth. A DNA test exists for Lethal White Syndrome to identify carriers.

Sabino 1 markings

Horses with one copy of the Sabino 1 gene are said to be heterozygous for that gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

, 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

Roan (horse)

, are characteristics of Sabino 1, particularly when heterozygous. 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. Modest Sabino 1 markings are often difficult to tell apart from other white markings

Horse 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.

Sabino-white

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. Again, the eyes are not usually blue. The term "sabino-white" is used to distinguish homozygous SB1/SB1 horses from so-called "dominant white

White (horse)

" 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

Tobiano

Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...

with lethal white syndrome frame overo

Overo

Overo refers to several genetically unrelated pinto coloration patterns of white-over-dark body markings in horses, and is a term used by the American Paint Horse Association to classify a set of pinto patterns that are not Tobiano...

, can produce a horse that is 90% white or more. Cremello

Cream gene

The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color that is chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Similarly, horses with a bay...

horses are superficially similar to sabino-whites, however cremellos have blue eyes, rosy-pink skin and a cream-colored rather than white coat. Gray

Gray (horse)

Gray or grey is a coat color of horses characterized by progressive silvering of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike many depigmentation genes, gray does not affect skin or eye color Their adult hair coat is white, dappled, or white intermingled...

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.

Genetics

The Sabino 1 locus

Locus (genetics)

In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

is at the KIT

CD117

Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

gene. Sabino 1 is an incomplete dominant trait; each of the different genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

s corresponds to a separate phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

. Both Sabino 1 and the many known forms of dominant white

White (horse)

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 allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s, 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.

Dam	Sire
	SB1/SB1	SB1/+	+/+
SB1/SB1	100% SB1/SB1	50% SB1/SB1 50% SB1/+	100% SB1/+
SB1/+	50% SB1/SB1 50% SB1/+	25% SB1/SB1 50% SB1/+ 25% +/+	50% SB1/+ 50% +/+
+/+	100% SB1/+	50% SB1/+ 50% +/+	100% +/+

The laws of Mendelian inheritance

Mendelian inheritance

Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...

, 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

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

responsible for Sabino 1 on the KIT

CD117

Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

gene is a single nucleotide polymorphism

Single nucleotide polymorphism

A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...

designated KI16+1037A. The Sabino 1 mutation results in the skipping of exon

Exon

An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...

17. Other areas of the KIT gene are responsible for tobiano

Tobiano

, true roan

Roan (horse)

, and a dozen or so dominant white

White (horse)

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

Neural crest

Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....

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

Stochastic

Stochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...

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.

"Sabino" but not Sabino 1

The term "sabino" can also be used in a descriptive sense for horses with markings often associated with Sabino 1: white leg markings above the knees and hocks with jagged margins, wide blazes, and belly spots or roaning. However, other horse enthusiasts, owners and breeders define the term even more broadly, to include "chrome", "high white", "excessive white" or white spots on the lower lip or chin, distal white patches on the legs, or "pointy" leg markings.

The genetics behind white markings in horses are complex. Some breed registries

Breed registry

A breed registry, also known as a stud book or register, in animal husbandry and the hobby of animal fancy, is an official list of animals within a specific breed whose parents are known. Animals are usually registered by their breeders when they are still young...

select against white, while others select for it; the result is that, prior to the ability to map the horse genome and perform DNA testing to verify parentage, assorted rules were created to delineate "spotted" from "non-spotted." The rules were not based on the behavior of known white spotting genes and many organizations have yet to catch up to modern understanding of genetics. Two horses with identical genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

s for a particular white spotting pattern may phenotypically be considered "solid" or "spotted," depending on the amount of patterning that is visible. Furthermore, the amount of white that a foal ends up with does not solely depend on known white spotting genes. For example, research suggests that chestnuts

Chestnut (coat)

Chestnut is a hair coat color of horses consisting of a reddish-to-brown coat with a mane and tail the same or lighter in color than the coat. Genetically and visually, chestnut is characterized by the absolute absence of true black hairs...

have more white than non-chestnuts, and that non-chestnuts carrying a masked chestnut allele have more white than non-chestnuts without a chestnut allele. White spotting genes also interact with each other, typically in a cumulative fashion.

Research indicates that there are many genes, and potentially different alleles on those genes, that produce so-called "normal" white markings. Further research suggests that when separate genes for white markings are present together, they have an additive effect, producing more white together than either gene would do alone. A similar effect is observed in pinto horse

Pinto horse

s with both the tobiano

Tobiano

and frame overo

Lethal white syndrome

pattern; these "toveros" often have more white than either tobiano or frame overo-patterned horses. Similarly, two apparently solid-colored horses with separate factors for white markings may produce a foal expressing both, with more white than either parent.

Sabino in Arabians

It is known that sabino-like patterns do exist in purebred Arabians

Arabian horse

, though studies at the University of California, Davis

University of California, Davis

The University of California, Davis is a public teaching and research university established in 1905 and located in Davis, California, USA. Spanning over , the campus is the largest within the University of California system and third largest by enrollment...

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. 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

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

testing to verify parentage, a large number of Arabians meet the definition of having minimal to moderately expressed sabino characteristics.

Minimal sabino traits exist in Arabians and the term "maximum sabino" was coined to describe spotted Arabians that were close to 50% white. However, there are very few Arabians that visually appear to be true white

White (horse)

, 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

Mutation

of a new form of Dominant white

Dominant white

Dominant white is a group of genetically related coat color conditions in the horse, best known for producing an all-white coat, but also for producing some forms of white spotting and white markings. Dominant white horses are born with unpigmented pink skin and white hair with dark eyes, although...

, and has passed this trait on to his descendants. The inheritance patterns observed in sabino-like Arabians do not follow the same mode of inheritance as Sabino 1.

Sabino in draft horses

The Clydesdale and Shire

Shire horse

are closely related draft horse

Draft horse

A draft horse , draught horse or dray horse , less often called a work horse or heavy horse, is a large horse bred for hard, heavy tasks such as ploughing and farm labour...

s originally bred in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

. Apart from massive build and copious feathering

Feathering (horse)

Feathering, or feather, is a term used to describe the long hair on the lower legs and fetlocks of some breeds of horse and pony. On some horses, especially draft breeds, the hair can almost cover the hooves...

, both breeds are known for consistent white markings

Horse markings

. The most popular and acceptable form of white markings on both breeds includes a bold blaze and four even socks. Breed standards for Clydesdale horses state that "excessive white" is no longer a fault. Conversely, even in modern times, the Shire horse breed standard counts excessive white, body spotting, or roaning as a fault, especially in stallions.

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. 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

Roan (horse)

-like pattern. Such horses are called "roan" by the Shire and Clydesdale breed registries.

"Draft-type sabino" differs from Sabino 1 in that, while it may be dominantly inherited, it does not result in a sabino-white phenotype when homozygous. As the gene pool

Gene pool

In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...

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.

While the pattern common to Shires and Clydesdales appears to be dominantly-inherited and results in a white spotting phenotype, it is unlikely that the allele responsible will be categorized as "dominant white

White (horse)

." Dominant white is reserved for white spotting alleles thought to result in non-viable embryos in homozygotes. There is no evidence that embryos homozygous for draft-type sabino patterning are nonviable.

Sabino in American Quarter Horses

The term "sabino" is seldom used among American Quarter Horse breeders, as white patterning was originally considered undesirable. The American Quarter Horse

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 "cropout

Cropout

A cropout, crop-out or crop out is a horse with body spots, including pinto or appaloosa spotting, or "high white" horse markings, with a sire and dam who both appeared to have been solid-colored. There are several variations in the definition, depending on the breed registry involved. There are...

s," 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. Due to the large number of horses that had these patterns, as well as a significant number of double dilute creams

Cream gene

also caught up by this rule, the American Paint Horse Association

American Paint Horse Association

The American Paint Horse Association is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas. It was founded in 1965 with the merging of two different color breed registries that had been formed to register pinto-colored horses of Quarter Horse bloodlines...

(APHA) formed, allowing these horses to be registered as "Paints." The APHA recognizes sabino as one of the "overo

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. 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.

Combination patterns and mimics

Sabino may bear some resemblance to other color patterns. Because breeders of pintos

Pinto horse

, particularly American Paint Horse

American Paint Horse

s, often crossbred various color patterns, it is possible for a horse to carry gene

Gene

s for more than one pattern. The presence of multiple white spotting patterns often produces an additive effect and such horses may show characteristics of both patterns. This can, at times, make identification and registration of spotted horses a challenge. Conversely, even if a spotting gene is present, white body markings may be so minimal in some individuals that they are registered as solid-colored. However, they may produce strongly colored offspring, which are sometimes referred to as "cropouts."

Frame and sabino

While the American Paint Horse Association

American Paint Horse Association

(APHA) refers to both sabino and frame

Lethal white syndrome

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

Endothelin receptor type B

Endothelin receptor type B, also known as ETB is a protein that in humans is encoded by the EDNRB gene.- Function :Endothelin receptor type B is a G protein-coupled receptor which activates a phosphatidylinositol-calcium second messenger system. Its ligand, endothelin, consists of a family of three...

gene. 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

Horse markings

and blue eyes. Horses homozygous for the Ile118Lys mutation

Mutation

on the equine Endothelin receptor type B

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. The expression

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

of the frame pattern varies from minor white markings

Horse markings

to the distinctive framed pattern, with some DNA-tested frames appearing solid.

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). 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.

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. 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

Roan (horse)

. To confuse matters further, in Spanish

Spanish language

Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...

-speaking countries, the term "overo" actually refers to horses with what is called "sabino" in English

Modern English

Modern English is the form of the English language spoken since the Great Vowel Shift in England, completed in roughly 1550.Despite some differences in vocabulary, texts from the early 17th century, such as the works of William Shakespeare and the King James Bible, are considered to be in Modern...

; meanwhile, in South America, the term "sabino", which literally translated from Spanish means "speckled" or sometimes "roan", refers to a flea-bitten gray

Gray (horse)

Splash and sabino

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. Splashed white is found in paints, Welsh ponies

Welsh Pony

The Welsh Pony and Cob are closely related horse breeds including both pony and cob types, which originated in Wales in the United Kingdom...

, Icelandics

Icelandic horse

The Icelandic horse is a breed of horse developed in Iceland. Although the horses are small, at times pony-sized, most registries for the Icelandic refer to it as a horse. Icelandic horses are long-lived and hardy. In their native country they have few diseases; Icelandic law prevents horses from...

, and Morgans

Morgan horse

The Morgan is one of the earliest horse breeds developed in the United States. Tracing back to the stallion Figure, later named Justin Morgan after his best-known owner, the breed excels in many disciplines, and is known for its versatility....

. 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. 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.

Tobiano and sabino

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

Locus (genetics)

. 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

Color breed

A color breed is a term that refers to horses that are registered based primarily on their coat color, regardless of the horse's actual breed or breed type....

associations for spotted horses may be categorized as "tovero

Tovero

The Tovero coloration is a mix of tobiano and overo colorations in Pinto horses and American Paint Horses. The genetics of pinto coloration are not always fully understood, and some horses have a combination of patterns that does not fit cleanly in either category.Some characteristics of a 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.

Roan, rabicano, and sabino

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.

Rabicano

Rabicano, also called white ticking, is a horse coat color characterized by limited roaning in a specific pattern: interspersed white hairs most dense and originating from the flank and the tailhead...

is another type of "roaning" or ticking characterized by scattered white hairs centered around 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 and sabino

Gray horses

Gray (horse)

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.

"Maximum White" Sabino

There are differences in terminology amongst genetics researchers and certain organizations that promote sabino color, particularly in Arabians. Some researchers, such as Dr. Phillip Sponenberg, use the term "maximum sabino" rather than "sabino-white" to describe horses more than 90% white. 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.

Breeds that recognize sabino patterning

Breed standards that recognize the sabino pattern include the Mustang

Mustang (horse)

, American Paint, Miniature horse

Miniature horse

, Morgan, Hackney

Hackney (horse)

The Hackney Horse is a recognized breed of horse that was developed in Great Britain. In recent decades, the breeding of the Hackney has been directed toward producing horses that are ideal for carriage driving. They are an elegant high stepping breed of carriage horse that is popular for showing...

(and Hackney pony

Hackney pony

The Hackney pony is a breed of pony closely related to the Hackney horse. Originally bred to pull carriages, they are used today primarily as show ponies. The breed does not have its own stud book, but shares one with the Hackney Horse in all countries that have an official Hackney Stud Book...

), Tennessee Walking Horse, and the pinto color breed

Color breed

A color breed is a term that refers to horses that are registered based primarily on their coat color, regardless of the horse's actual breed or breed type....

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 the sabino gene pattern expressed by high white, belly spots, lacy or roaning patches and white extending past the eyes include the Clydesdale, Arabian, Thoroughbred and Shire.

The most controversial expression of the sabino gene complex was in the American Quarter Horse, which for years did not register horses with "cropout" color or blue eyes, i.e. typical sabino patterns, nor cremello or perlino horses. This exclusion of cropout foals, even from two solid-colored parents, led in part to the formation of the APHA registry. However, since the advent of DNA testing to confirm parentage, the AQHA has repealed this controversial "white rule", allowing light-colored horses and those with body spots to be registered.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Sabino 1 gene

Sabino 1 markings

Sabino-white

Genetics

"Sabino" but not Sabino 1

Sabino in Arabians

Sabino in draft horses

Sabino in American Quarter Horses

Combination patterns and mimics

Frame and sabino

Splash and sabino

Tobiano and sabino

Roan, rabicano, and sabino

Gray and sabino

"Maximum White" Sabino

Breeds that recognize sabino patterning

See also

External links