HERIDITY

A horse inherits one copy of each gene from each parent. When looking at the testing results for PSSM type 1 it will read either n/n, n/P1 or P1/P1 (by some labs also noted as n/PSSM1 and PSSM1/PSSM1).
n/n means the horse did not inherit the mutated GYS1 gene from either of the parents. It does not have PSSM type 1.
n/P1 means the horse inherited the GYS1 gene from one of its parents. It has PSSM type 1.
P1/P1 means the horse inherited the GYS1 gene from both parents. It has PSSM type 1.

PSSM type 1 is a dominant disease, which means that a horse is affected, even when it only has one copy of the gene. How symptomatic a horse will be depends on the individual and the way it is managed. Note: some labs use the term “carrier” as a description clarifying the test result n/P1. Though this term is used more often when a result shows the inheritance of one copy, this term is misleading in the case of PSSM type 1. There is no such thing as a “carrier”, as a horse with an n/P1 test result is affected.
When a horse tests P1/P1 it means it has  inherited a copy of the gene from each parent. This automatically means that both parents carry at least one copy of the GYS1 gene. A horse that tests P1/P1 will always pass on a GYS1 gene to its offspring, meaning all offspring off this horse will have PSSM type 1.

Heredity works a bit differently in horses with PSSM2/MIM. These variants are referred to as “semi-dominant”. It still means that if the horse inherits one copy of the faulty gene, it will be affected by the associated disease. For example, a horse that tests n/P2, the horse has a type of Myofibrillar Myopathy associated with that gene. However, the effect the gene has on the expression of the disease (symptoms) increases when the horse carries either two copies of the same gene (for example P2/P2) or is positive on multiple genes on the panel (for example n/P2, n/P3, n/Px). In other words: the more MIM genes the horse tests positive for, the more symptomatic it will likely be. This also applies to a combination of PSSM2/MIM, PSSM type 1 and IMM/MYHM. For the western breeds (American Quarter Horse, American Paint Horse and American Appaloosa), testing positive for Malignant Hyperthermia (MH) on top of copies of above mentioned muscle diseases is also known to increase the severity of the horse’s symptoms.