White Spotting Horse (KIT, Tobiano, Sabino and Roan)

White Spotting (KIT, Tobiano, Sabino and Roan)

White Spotting or Dominant White (W) is a collective name for a series of white patterns in horses. These patterns can vary from a few white markings on the body or limbs to an almost complete lack of pigment over the entire body. Mutations in the KIT gene affect its receptor tyrosine kinase function, and all have an autosomal dominant mode of inheritance, with variable penetrance. KIT plays a broad role in an animal’s development, including helping govern the development and migration of pigment-producing cells, although this is not related to albino. Mutations in KIT can lead to different white spotting phenotypes, some of which will be described below. White patterns can also be produced by mutations in the genes PAX3 (Splashed White, Macchiato) and MITF (Splashed White), which are described elsewhere in the Knowledgebase. This article focuses on white spotting KIT mutations.

There are currently more than 38 identified KIT-variants. Some of them are common but most of these variants have only recently emerged and are only found in specific breeding lines within a breed. CombiBreed tests several W-variants: W4, W10, W18, W19, W20 and W21. Other variants that are considered dominant white/white spotting but sometimes classified differently are: Sabino (SB1), Tobiano (TO) and Roan (Rn). At CombiBreed all three variants are also tested.

Clinical features

Most of the KIT white spotting mutations are homozygous lethal, meaning that embryos with two copies are not viable and do not result in successful pregnancies. This is also true when two different white spotting mutations are combined (compound heterozygosity). W20 and several others can be combined without problem, as can SB1, TO, and some Rn. Genetic testing can be useful to avoid problematic combinations.
Note that this is not related to Lethal White Foal Syndrome (LWS, OLWS, LWO or OLWSF), a fatal condition resulting from the presence of two copies of the LWO allele in the EDNRB gene.

W4

The W4 mutation is found in the Camarillo White Horses (original horse: Sultan (1912), a rare breed known for its pure, completely white colour and pink skin. Mane and tail are also white. To avoid inbreeding and lost pregnancies due to lethal homozygote embryos, outcrossing with Andalusian and Standardbred horses is permitted.

W10

Found in American Quarter Horses (original horse: GQ Santana), W10 has a wide variety of phenotypes, from only white markings on the legs or face till (almost) complete white colouring. Horses with the W10 mutation have a pink skin where the hair is white and eyes have usually a normal colouring. There are no observed homozygotes and the mutation is therefore probably lethal in an early embryotic stage.

W15

The W15 mutation in Arabians originated in the horse Khartoon Khlassic. Most horses with the W15 mutation have a wide blaze, white socks, and patches of white on the belly and torso. At least two W15/W15 foals have been confirmed by genetic testing, but their health and fertility is unknown; one died of unrelated causes as a foal. They are mainly white with some small roany areas of color.

W18

W18 mutation has been found in Swiss Warmblood (CH-Warmblut) horses (original horse: Colorina von Hoff), which results in a speckled white pattern. Homozygotes have not been discovered, and W18 is probably lethal in homozygote form.

W19

The W19 variant has been found in Arabian horses (original horse: Fantasia Vu). Heterozygotes consists of a largely white face, extended white leg markings and irregular markings in the belly area. Homozygotes are viable and detected. Research is still ongoing, but reports describe a (almost) complete white coating and health problems (immunodeficiency, slow growth, infertility) in homozygote animals.

W20

Unlike most variants, the W20 variant is described in many breeds like American Quarter Horses, Warmblood Horses, American Paint horses, Gypsy horses and Welsh ponies. The variant itself has a small effect, and may cause marginal white markings. However, it is mostly known as a white booster for other (dominant) white mutations; in combination it increases the amount of white on the horse. And it can also boost the amount of white in other (non-KIT/non-W) mutations. This variant is autosomal dominant, and homozygotes are viable.

W21

The W21 variant is found in Icelandic horses (original horse: Ellert frá Baldurshaga). Many horses with W21 have wide white blazes and have marbling-type white markings up the legs, under the belly and up the flanks. In some cases it covers most of the body, in others it can be fairly minimal. A blue or partly-blue eye is possible. There are no known homozygotes and it is presumed to be lethal in homozygosity.

SB1 Sabino

SB1 (Sabino) is one of the most common and well-known white spotting patterns. The description Sabino is often used as an umbrella term for white spotting patterns, although they should be described as “sabino-like”. As one of the first discovered KIT variants it has a different name then the typical W variants because at the time it was not known that so many variants existed in horses. The SB1 variant is an incomplete dominant mutation and viable as homozygote. Homozygous animals have significant more white than heterozygous animals. Characteristics are the white patches on the face, legs or belly and the speckled hairs in the midsection. Homozygotes are (almost) completely white. Sabino exists in many breeds including American miniature horses, Shetland ponies, American Paint Horses and Tennessee Walking horses.

TO (Tobiano)

Tobiano (TO) is one of the most well-known white spotting patterns. Like sabino, the term is sometimes used as umbrella term for any white patterns. The Tobiano mutation is located near the KIT gene and responsible for white, vertical patches in the coat of the horse, often crossing the topline. The head is usually coloured with a blaze, and the rest of the body generally has large patches of colour; legs are often at least partly white. Tobiano is autosomal dominant and viable in homozygous form; there is no phenotypic difference. Tobiano is found in many breeds like American Paint horses, American Miniature horses, Icelandic horses, Barock Pintos, Tinker, etc. In some cases tobiano can be “cryptic” or hidden, with only a small snip on the face or partial socks.

Ro (Roan)

Classic roan is characterized by the mixture of normal and white hairs that are evenly distributed over the entire body. Only the extremities of the body (head, lower legs, manes and tail) are untouched and keep their normal colour. Roan is sometimes used as a general term for this type of whitening, but only classical roan is officially a roan. Other patterns (like varnish roan in appaloosa, rabicano or freckling in some W-mutations) can look like roan but there caused by different mutations. Often these patterns don’t have an evenly distribution of white hairs on the entire body, classic roan does.
In literature it is suggested that coat colour Roan is homozygous lethal, but evidence from studies with the Quarter horse breed indicate otherwise. Homozygous Roan horses have been proven to exist, but they are relatively rare.

Inheritance

As their name suggests, most of the KIT variants are autosomal dominant, with one copy of the variant being sufficient to cause white patterning. Some variants are homozygous lethal and do not exist in a homozygote state. It is possible to have a combination of dominant white alleles, although not every combination is viable.

Below a general table for all dominant white variants. The dominant white (W) allele has been generalised to W* so that every variant can be used instead. In sabino, tobiano and roan the W* has to be replaced by their corresponding allele names (TO, SB or Ro). W** is a way to mark a different, second variant then the used W* variant. For more information on each variant see Clinical Features.

Additional information

It’s important to note that in everyday usage, the terms tobiano, sabino, and overo (used to indicate any white pattern, including sabino, not caused by tobiano) are often used interchangeably to describe white colouring on a horse. These names actually refer to patterns caused by specific alleles as described above (Frame overo, responsible for Overo Lethal White in homozygosity, is described elsewhere).

Be careful to test for the right variant when ordering a white pattern test. Because some variants are lethal in homozygous state, this can be essential to produce a healthy foal.

Relevant tests

• P342
• P390
• P391
• P591
• P592
• P659
• P785
• P903