The coat patterns in spotted horses are genetically related. These are referred to as a leopard spotting complex. Mutations associated with leopard complexes are also linked to abnormalities in the eyes and vision. These abnormalities are known as Congenital Stationary Night Blindness (CSNB).
These patterns are most closely identified with the Appaloosa horse breed, though its presence in breeds from Asia (Tiger Horses) and Western Europe (Knabstrupper or Knabstrup) has indicated that it is a very ancient mutation. European cave paintings have recorded spotted horses, indicating that the phenotype has existed for 20,000 years prior to their domestication approximately 5,000 years ago.
The Appaloosa horse descended from horses brought to America by the Spanish in the 16th century. The name Appaloosa comes from the Palouse River that ran through the Nez Percé (a Native American tribe) territory. The breed is best characterized by its spotted coat pattern commonly referred to as "leopard-complex." The leopard-spotted coat pattern has also been documented in several other horse breeds. These horses include the Pony of the Americas, the Nez Percé Horse, and several gaited horse breeds.
Within the Appaloosa breed, there is a wide range of body types, stemming from the influence of multiple breeds of horses, including Arabian blood lines. Arabian blood lines were introduced during the late 19th century. Although commonly recognized by their colorful coat patterns, Appaloosa horses have three additional identifiable characteristics. These include mottled skin around the nose, lips, and genitals, striped hooves, and white sclera round the eyes.
In 2003, researchers linked the positional candidate gene for leopard complex (LP) to the TRPM1 gene. Further investigations headed by Dr. Rebecca Bellone and the Appaloosa Project (a team of researchers from Canada and the US) identified several genes in TRPM1 that showed complete association. Soon after that, the causal mutation was also discovered. By observing the DNA within this gene, the mutation can be used to identify horses with leopard complex.
PATN1 Gene and its Association with Leopard Pattern
A mutation in the RFWD3 gene is linked to an increased amount of white in leopard pattern horses, known as Pattern 1 (PATN1). When combined with leopard pattern, this mutation behaves in a dominant fashion. This means that a horse needs a single copy of the PATN1 mutation to observe large amounts of white as long as the gene that codes for leopard pattern is present. Horses that are PATN1 positive but negative for leopard pattern will not have spotting. However, they can pass on a copy of PATN1. A foal can receive a PATN1 copy from a solid non-spotted parent and a copy of leopard pattern from the other parent and have a few-spot or a near-few-spot pattern.
Congenital Stationary Night Blindness (CSNB)
It has long been understood that a small percentage of Appaloosas are affected by both Equine Recurrent Uveitis and Congenital Stationary Night Blindness (CSNB), a condition that makes it difficult or even impossible to see in relatively low light.
Research has now shown that CSNB is a recessive disorder that is directly linked to the leopard complex in Appaloosa horses. Recessive disorders are disorders that require two copies of a gene in order to present symptoms. This means that a horse must inherit one copy from both parents. DNA tests for the leopard pattern mutations can also provide clear information as to whether or not a horse is susceptible to CSNB.
Genetic testing of the leopard complex (LP) gene may be beneficial for several reasons. The first reason for testing is to confirm that an animal is a true Appaloosa horse. Another important reason is to identify those animals that are homozygous or LP/LP and could be affected by CSNB.
|Homozygous: Horse has two copies of the LP gene and will pass a copy on to all offspring. Horse has Congenital Stationary Night Blindness (CSNB).
|Heterozygous: Horse has one copy of the LP gene and has a chance to pass it on to every offspring. Horse is a carrier of Congenital Stationary Night Blindness (CSNB).
|Negative: Horse is negative for LP gene and will not be affected by Congenital Stationary Night Blindness (CSNB).
Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus). Bellone RR, Brooks SA, Sandmeyer L, Murphy BA, Forsyth G, Archer S, Bailey E, Grahn B. Genetics. 2008 Aug;179(4):1861-70. doi: 10.1534/genetics.108.088807. Epub 2008 Jul 27. [PMID: 18660533]
Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Hauswirth R, Jude R, Haase B, Bellone RR, Archer S, Holl H, Brooks SA, Tozaki T, Penedo MC, Rieder S, Leeb T. Anim Genet. 2013 Dec;44(6):763-5. doi: 10.1111/age.12057. Epub 2013 May 9. [PMID: 23659293]