Leopard complex spotting (LP) is the result of an incompletely dominant mutation in the TRPM1 gene. The TRPM1 gene codes for a protein that is found in pigment and in the eyes. The LP mutation produces a highly variable group of white spotting or depigmentation patterns in the horse. The LP mutation allows for the expression of the various patterns. In addition to LP, other genes control the extent (or amount) of depigmented or white areas on the horse.
A mutation is linked to an increased amount of white in LP horses known as Pattern 1 (PATN1). When combined with LP, 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 LP is present. Horses that are PATN1 positive but negative for LP 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 LP from the other parent. This horse will have a few spots or a near-few-spotted pattern.
Studies have confirmed that the PATN1 mutation is present in many breeds including the Appaloosa, American Miniature Horse, British Spotted Pony, and Knabstrupper, among others. Animal Genetics has found the PATN1 mutation in other horse breeds lacking the LP mutation. This lack of LP causes PATN1 coloration to not appear in those specific breeds, even though those breeds have a copy of the PATN1 mutation.
Holl HM, Brooks SA, Archer S, Brown K, Malvick J, Penedo MC, Bellone RR. Anim Genet. 2016 Feb;47(1):91-101. doi: 10.1111/age.12375. Epub 2015 Nov 16. [PMID: 26568529]
|PATN1/PATN1||Homozygous: Horse has two copies of the PATN-1 gene and will pass a copy on to all offspring.|
|n/PATN1||Heterozygous: Horse carries one copy of the PATN-1 gene and has a chance to pass a copy on to every offspring.|
|n/n||Negative: Horse does not carry the PATN-1 gene.|