Sabino horse

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.

[1] 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.

[5] 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.

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.

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.

[10][11] 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.

[8] 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.

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.

[15] 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.

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.

Thus, prior to the use of parentage testing as a registration requirement, white spotting in Arabians was controversial and viewed as evidence of crossbreeding or "impure" breeding, with such horses discouraged from registry and penalized in competition.

Once DNA testing began to verify parentage, a significant number of Arabians met the definition of having minimal to moderately expressed sabino characteristics, though the genetic mechanisms involved were not understood.

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 irregular, feathered, or roaned borders.

White markings above the hocks on the hindlegs are common, and typically trail up the front of the leg to the stifle joint and flank.

[25][26][27] The high white markings characteristic of these two breeds follow a pattern similar to that found in Sabino 1, which led researchers to include these horses in the original study that discovered SB1.

[26] Conversely, even in modern times, the Shire horse breed standard counts excessive white, body spotting, or roaning as a fault, especially in stallions.

[27] This pattern, informally labeled “draft-type sabino,” appears to be dominantly inherited, but it does not result in a sabino-white phenotype when homozygous.

Thus, if the draft-type sabino gene produced sabino-whites, near-white coats would be expected in nearly a quarter of foals, but they do not occur.

However, the Quarter Horse arose from multiple sources, including Thoroughbreds and other breeds now known to carry spotting genes.

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.

Splash-sabino blends may have characteristics of both patterns, such as jagged patches, roaning, blue eyes and blocky white markings.

Splash in conjunction with sabino may also produce the "medicine hat" pattern, where only the ears and poll and sometimes the topline or chest have pigment.

[40] The tobiano pattern is characterized by smooth, crisp-edged white markings arranged in a vertical orientation that often cross the back.

In contrast, primary characteristics of sabino include bold white markings on the head and legs, often with roaning at the edges.

It is unknown if the roaning characteristic of some sabino-type patterns is due to the additional presence of rabicano or a separate mechanism.

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.

A horse with classic sabino belly spots, white above its knees and hocks, a white chin and wide white facial markings.
A sabino horse with extensive roaning
A Thoroughbred horse that carries the W5 allele and has sabino patterning
Sabino patterns have long existed in Arabians
A Clydesdale horse with sabino belly spots and leg markings
The horse on the left most likely carries multiple spotting alleles. The horizontallyxoriented white patches on the body of the horse are indicative of the frame pattern. The wrap-around blaze suggest splash white genetics are present. The irregular stockings and roaned edges of the white patches indicate sabino patterning.
Some splashed white horses may closely resemble sabino, but tend to have more white on the head and belly, with sharper-edged white markings
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 sabino or splash genetics as well as tobiano.
This true or "classic" roan has the characteristic darker head and legs.
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.