Surfing the web I found these:
Hereditary Equine Regional Dermal Asthenia (HERDA) also known as Hyperelastosis Cutis (HC) is a genetic skin disease predominately found in the American Quarter Horse. Researchers at Mississippi State University and Cornell University believe that the origin of this genetic disorder may be the Poco Bueno's sire line. Symptoms of this disorder is a lack of adhesion within the layers of skin due to a genetic defect in the collagen that holds the skin in place. This defect causes the outer layer of skin to split or separate from the deeper layers sometimes tearing off completely. Areas under saddle seem to be most prone to these lesions often leaving permanent scares, preventing the horse from being ridden. The disorder is recessive, which means that a horse must be homozygous positive or have two copies of the defective gene to suffer from the disease. Consequently both the sire and the dam must possess at least one copy of the mutated gene in order for the offspring to be afflicted. Offspring born with one copy of the defective gene and one non-defective copy are considered a carrier and have a 50% chance of passing the defective gene on.
Although there is no cure for the disorder, Animal Genetics has developed a simple DNA test to detect a single nucleotide polymorphism (SNP) mutation with a high degree of association to this disorder.
Hyperkalemic Periodic Paralysis Disease (HYPP) is a muscular disease that affects both horses and humans. In horses, HYPP has been traced back to one horse named Impressive and has the alternative name, Impressive Syndrome, named after this horse. Symptoms of HYPP may include muscle twitching, unpredictable paralysis attacks which can lead to sudden death, and respiratory noises. Severity of attacks varies from unnoticeable to collapse or sudden death. The cause of death is usually respiratory failure and/or cardiac arrest. The HYPP gene is dominant so both homozygous positive (HH) and heterozygous (nH) will cause this muscular disorder. Only homozygous negative (nn) has no HYPP effect. Since HYPP is dominant, the effects of it can also be transposed to other species of horses when intermixing occurs. This makes the recognition and elimination of this disorder very important in preserving the inherited health of all horses.
Glycogen Branching Enzyme Deficiency (GBED) Lacking an enzyme necessary for storing glycogen, the horse's brain, heart muscle, and skeletal muscles cannot function, leading to rapid death. This condition may be diagnosed with a muscle biopsy. Most foals with GBED are aborted or stillborn, and those that survive live only for a few months. Symptoms include general weakness, contracted tendons, seizures, cardiac arrest, and sudden death. There is no known treatment. This genetic disease has been linked to the foundation sire King. (only about 10% of quarter horses are carriers)
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Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"