Appaloosa Patterns Horse (PATN1, CNSB, Leopard Spotting)

Appaloosa Patterns Horse (PATN1, CNSB, Leopard Spotting)

The leopard-like spotting pattern distinctive of the Appaloosa breed – a white coat flecked with small, dark spots – is referred to as Leopard Complex Spotting (LP) or Appaloosa Pattern. Several breeds are known to have the Appaloosa coat colour: this are amongst others the Appaloosa, Falabella, Miniature Horses, Noriker, and Knabstrupper. The trait is caused by an incompletely penetrant dominant mutation to the gene for the transient receptor potential cation channel, subfamily M, member 1 (TRPM1). Appaloosa Pattern-1 (PATN1) on the other hand is a modifier of the leopard complex spotting (LP) and controls the amount of white in the coat, increasing the amount of white present in horses that also have the LP mutation. It is located on the RFWD3 gene. So LP and PATN1 are two different mutations, influencing each other.

Clinical features

Leopard Complex Spotting (LP) includes a highly variable group of white spotting – or depigmentation – patterns in horses. Appaloosa horses have three additional identifiable characteristics: mottled skin around the muzzle, anus and genitalia, striped hooves and white sclera round the eyes.

Aside from affecting the development and distribution of pigment-producing cells, TRPM1 (LP) also fulfils a function in vision. Horses that are homozygous for the LP mutation are also affected by complete Congenital Stationary Night Blindness (CSNB). Horses affected by CSNB are born unable to see in dim light. This may cause nervousness, confusion and difficulty in training. Affected horses may also display strabismus and nystagmus (abnormal and/or involuntary movements of the eyes). CSNB does not change or worsen over time. Appaloosa coloured horses that carry at least one copy of the LP allele have a higher risk at developing ERU (Equine Recurrent Uveitis, moon blindness) and blindness. Homozygous animals (carrying two copies) are more often and severely affected by this disease then heterozygous animals (carrying one copy). Research is still ongoing, but it is suggested that the presence of PATN1 also increases the risk for ERU and blindness.

Inheritance

Appaloosa patterning is a complex trait, meaning that multiple genes influence the actual coat colour (phenotype). The combination of alleles from these genes determines the phenotype. At the moment, only two genes are known that influence the appaloosa patterning: LP and PATN1. LP serves as the basis for appaloosa patterning, without LP no pattern is visible. PATN1 is a modifier of LP, it determines to what extent the pattern is visible by increasing the amount of white in horses that carry the LP allele. LP inherits is an incomplete, autosomal dominant trait. Horses with one copy have a different phenotype then horses with two copies. PATN1 is an autosomal dominant trait. Horses with one or two copies of the PATN1 already have more expressed whitening in their coat then non-carriers.

Interpretation of the combined results

*See additional information.

Additional information

Appaloosa patterning is a complex trait, with various modifying genes that determine the exact patterning. Not all of these genes and effects are known. Also, the difference between all phenotypic patterns in non-carrier, heterozygote and homozygote horses can be hard to determine. Patterns occur in all kinds of gradients and therefore can overlap each other. A near leopard can look like a full leopard and the other way around. Therefore, the phenotypes mentioned above are based on the general trend observed on the two known mutations. Exceptions, probably caused by undiscovered genes, can produce anomalous results.

Relevant tests

• P305
• P311