Understanding the 5-panel and Other Equine Genetics
What is 5-Panel testing?  This is a question I have had to answer many times over the last few months.  The 5-panel is test for five different genetic diseases found in quarter horses and other stock horse breeds.  The diseases tested for in the 5-panel are PSSM1, HERDA, GBED, HYPP, and MH.  Starting in 2014 any AQHA stallion breeding 25 or more mares must be 5-panel tested  through AQHA/UC Davis before their 2015 foals can be registered.  Starting in 2015 ALL AQHA stallions must be 5-panel tested before their 2016 foals can be registered.   

What do PSSM , HERDA, GBED, HYPP, and MH stand for?

PSSM - Polysaccharide Storage Myopathy -The University of Minnesota describes Polysaccharide Storage Myopathy or PSSM (PSSM1) an inherited muscle disease that affects many and diverse breeds of horses. The clinical characteristics of PSSM vary between breeds, from muscle pain, cramping and cell damage with exercise, to progressive muscle atrophy. Our previous work lead to the discovery of a dominant genetic mutation in the GYS1 gene that is responsible for PSSM in many horses (McCue et al 2008). This mutation results in the accumulation of abnormal complex sugars within skeletal muscle of horses. This form of PSSM is termed Type 1 PSSM. More information about the breeds affected with Type 1 PSSM can be found here: Equine Neuromuscular Lab. Subsequent work in our laboratory has also demonstrated that a mutation in a second gene important in muscle function RYR1, can make the clinical signs of PSSM worse in certain individuals (McCue et al 2009).  There are no "carriers" of PSSM.  PSSM is a dominant disorder.  Which means a horse only has to carry one copy of the disease to be affected.  If the horse tests PSSM positive, he has the disease and should not be bred.

PSSM N/N - Horse tested negative for PSSM and does not carry the PSSM gene mutations. The horse will not pass on the defective mutations to its offspring.

PSSM n/P1 - Both the normal and PSSM alleles were detected. Horse tested heterozygous for PSSM. The horse is affected with the PSSM genetic disorder and there is a 50% chance that this horse will pass a PSSM allele to its offspring.

PSSM P1/P1 - Positive for the dominant PSSM gene mutations, indicates the animal carries two inherited copies. Homozygous PSSM horses are genetically bound to pass the gene to 100% of their progeny when bred meaning all foals will be have at least one copy of the dominant PSSM gene mutation.


HERDA - Hereditary Equine Regional Dermal Asthenia.  Also known as Hyperelastosis Cutis, HC is a genetic skin disease predominantly 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. The symptom 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 the saddle seem to be most prone to these lesions, often leaving permanent scars and 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, but are in no way affected by carrying the gene.  They are just like any other horse.  HERDA only comes into play when you go to breed your horse.  If the horse is a carrier, it must only be bred to tested N/N stallions/mares

HERDA N/N - Horse tested negative for the gene mutation that causes HERDA and will not pass on the defective gene to its offspring

HERDA Hrd/N - Both the normal and mutant copies of the gene detected. Horse is a carrier for the HERDA mutation and can pass on a copy of the defective gene to its offspring 50% 0f the time.

HERDA Hrd/Hrd - The horse carries two copies of the HERDA mutation and is homozygous for HERDA. The horse is affected with the HERDA genetic disorder.


GBED - Glycogen Branching Enzyme Deficiency (GBED) is a fatal condition caused by the bodies' inability to properly store sugar. In a normal horse, the body stores sugar as energy by converting glucose to glycogen. This inherited disorder prevents the body from producing the enzyme needed to branch the glycogen structure, preventing the horse from being able to adequately store the sugars. This means that the horse will not be able to store enough energy to fuel important organs, such as the muscles and brain.
Foals born which are affected by GBED suffer from a range of symptoms associated with this lack of fuel, such as low energy, weakness and difficulty rising. Other symptoms include low body temperature, contracted muscles, seizures, and sudden death. Unfortunately, GBED is always fatal; most affected foals will die before the age of 8 weeks. GBED often causes the foetus to be aborted in utero. Research suggests that as many as 3% of aborted Quarter Horse foals were homozygous for the GBED mutation.
Studies show that the mutation responsible for GBED is carried by as many as 10% of Quarter Horse, Paint Horse breeds and related breeds. GBED is an autosomal recessive trait, meaning a foal can only be affected if the foal inherits the disease from both parents. Horses that are carriers of the GBED have one copy of the mutation but do not have any symptoms associated with the disorder. This makes DNA testing important to screen for carriers and prevent this fatal condition.   GBED is a recessive disorder and it takes two copies of the gene for the horse to be affected.  Horses carrying only one copy are not affected by the gene and are just like any other horse.   GBED only comes into play when you go to breed your horse.  If the horse is a carrier, it must only be bred to tested N/N stallions/mares.

GBED N/N - Horse tested negative for the gene mutation that causes GBED and will not pass on the defective gene to its offspring.

GBED N/Gb - Both the normal and mutant copies of the gene detected. Horse is a carrier for the GBED mutation, and can pass on a copy of the defective gene to its offspring 50% 0f the time.

GBED Gb/Gb - The horse carries two copies of the GBED mutation and is homozygous for GBED. The horse is affected with the GBED genetic disorder.


HYPP - Equine Hyperkalemic Periodic Paralysis Disease (HYPP) is a muscular disease caused by an inherited genetic mutation. 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.
HYPP is a dominant disorder meaning both homozygous positive (HH) and heterozygous (nH) horses will be affected. Only homozygous negative (nn) horses are not affected by HYPP.   If a horse tests HYPP positive, he has the disease and should not be bred.

HYPP N/N - Horse tested negative for the gene mutation that causes HYPP and will not pass on the defective gene to its offspring.

HYPP N/H - Both the normal and HYPP alleles were detected. Horse tested heterozygous for HYPP. The horse is affected with the HYPP genetic disorder and there is a 50% chance this horse will pass a HYPP allele to its offspring.

HYPP H/H - Positive for dominant HYPP gene, indicates the animal carries two inherited copies. Homozygous HYPP horses are genetically bound to pass the gene to 100% of their progeny when bred and all foals will be HYPP horses.  AQHA  no longer allows the registration of HYPP H/H horses.

MH - Malignant Hyperthermia or MH is a genetic muscle disorder that affects Quarter Horses and related breeds. Horses with the MH mutation may not show any physical signs of the disorder until triggered by exposure to anesthesia or extreme exercise or stress. Symptoms can include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. Symptoms develop rapidly, and if not treated quickly, this condition can be fatal.
MH is inherited as an autosomal dominant trait, so the disorder can be passed on even if only one parent has the defective gene. The mutation can be present along with PSSM and if a horse also has PSSM, the symptoms associated with MH can be more severe. Therefore, testing for both PSSM and MH is recommended for Quarter Horse breeds.
Although this condition is rare, testing for MH is recommended in case a horse must undergo anesthesia. Horses that are known to have the MH mutation can be given medication prior to administering anesthesia to help reduce the severity of the symptoms.  MH is another dominant disorder and horses testing positve for it should not be bred. 

MH N/N - Horse tested negative for MH and does not carry the MH gene mutation. The horse will not pass on the defective gene to its offspring.

MH N/MH - Both the normal and MH alleles were detected. Horse tested heterozygous for MH. The horse is affected with the MH disorder and there is a 50% chance this horse will pass a MH allele to its offspring.

MH MH/MH - Positive for dominant MH mutation, indicates the animal carries two inherited copies. Homozygous MH horses are genetically bound to pass the gene to 100% of their progeny when bred and all foals will be MH horses