I thought it would be helpful to post known genetic disorders and post any information that might be helpful to forum members.
I borrowed the below information from a website (link pasted below)
Glycogen Branching Enzyme Deficiency (GBED) – Recessive. Found in some lines of Quarter Horses and related breeds. Always fatal in utero or within 4 months of birth. Results from lack of the enzyme allowing branching of glycogen (sugar units) in muscles and resulting lack of nutrients required for survival. Prevent by not crossing two carrier horses. A genetic test for GBED is in development by the University of California at Davis.
Hereditary Equine Regional Dermal Asthenia (HERDA), Formerly called Hyperelastosis Cutis (HC) – Recessive. Found in some offspring of the Quarter Horses stallion “Poco Bueno.” Average life span is two-four years. Skin layers separate due to collagen defect, and horses cannot be ridden. Prevent by not breeding two carrier horses.
Hyperkalemic Periodic Paralysis (HYPP) – Dominant. Genetic mutation of sodium channel in muscle membranes that results in zero to severe muscle symptoms and possible death from heart failure. All affected horses trace back to the Quarter Horse Stallion “Impressive.” AQHA registered horses have a designation of N/N for normal, H/N for single gene and H/H for both mutated genes on the registration papers. Prevent HYPP by testing all Impressive-bred horses and not breeding horses with H/N or H/H designation. Management of affected horses is possible with medication from a veterinarian and/or carefully balanced low-potassium rations.
Recurrent Exertional Rhabdomyolysis (RER) – Dominant. Genetic form of tying-up or muscle damage and stiffness usually found in young, excitable Thoroughbred racing fillies. Management is possible by reducing starch and adding fat and following carefully managed exercise program. Diagnosis is possible with muscle biopsies.
Overo Lethal White Syndrome (OLWS) –- Recessive. Disorder of Paint and Pinto horses resulting in pure white color and incomplete formation of the digestive tract. Always fatal in utero or within hours of birth. Prevent OWLS by not crossing two horses with Overo (one form of pinto spotting) coloring or breeding. A test for the Overo gene is available from the American Paint Horse Association.
Polysaccharide Storage Myopathy (PSM) – Dominant. Genetic form of tying-up with muscle damage and inability to move, usually found in Quarter Horses, Draft Horses and crossbreds. Horses make and store abnormal muscle glycogen and cannot tolerate dietary starches and sugars. Horses with PSSM can be maintained with low-starch and low-sugar rations and precise exercise protocols. Diagnosis is possible with muscle biopsies.
Severe Combined Immunodeficiency (SCI) – Recessive. Lack of normal immune function in Arabian Horses. Usually fatal early in life. Prevent by not breeding two carriers. No genetic test at this time.
Information taken from the below website: