Genetic Testing

Genetic Testing Information

The PHAA offers the following tests through Racing Australia and their contract Laboratory Massey University.

All testing requests for DNA must come through PHAA, no other parent validation results taken after 2012 will be accepted from other providers unless this horse is from overseas.

OLW / OLWS - Overo Lethal White (Syndrome) is a dominant gene and is responsible for the 'Overo' or 'Frame Overo' white pattern in Paint Horses. Not all horses who carry the gene will display the colour characteristics and may appear as 'solid'. In the same vein not all horse who display the phenotype pattern will carry the gene when tested. Additionally when in combination with other coat patterns such as Tobiano or Splashed white, the overo pattern may be disguised. It its homozygous state (O/O) OLW produces a fatal condition known as Overo Lethal White Syndrome. Foals born as O/O will die within 36 hours of birth. It is for this reason that the PHAA recommend that all horses with overo breeding be tested for OLW prior to being bred themsleves. The PHAA discourages the breeding of overo to overo to avoid the 25% chance of producing a homozygous Lethal White foal.

HERDA - Hereditary Equine Regional Dermal Asthenia (HERDA) is a recessive genetic skin disease predominantly found in the American Quarter Horse. Within the breed, the disease is prevalent in particular lines of cutting horses (Poco Bueno). HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions. Heterozygous horses (1 copy - n/hrd) are carriers and not affected by the disease. In order to be affected by the disease, an individual must carry two copies of the gene (homozygous hrd/hrd) inherited from both sire and dam. HERDA is characterized by hyperextensible skin, scarring, and severe lesions along the back of affected horses. Affected foals rarely show symptoms at birth. The condition typically occurs by the age of two, most notably when the horse is first being broke to saddle. There is no cure, and the majority of diagnosed horses are euthanized because they are unable to be ridden and are inappropriate for future breeding.

PSSM1 - PolySaccharide Storage Myopathy (PSSM) Type 1 is an inherited muscle disease that affects many and divers breeds of horses. The clinical characteristics of PSSM vary from muscle pain, cramping and cell damage with exercise, to progressive muscle atrophy. Research led 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. PSSM1 is a dominant gene with the horse only requiring one copy (n/P1) to show symptoms of the disease. There is a wide variation in the display of symptoms of positive horses, from none (insignificant) or minor to quite severe, in rare cases leading to death. Research has shown that the presence of MH (Malignant Hypothermia) may increase the severity of symptoms. It is also believed that other genetic factors affect the severity of symptoms.

MH - Malignant Hyperthermia (MH) is a life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia, specifically the volatile anesthetic agents and the neuromuscular blocking agent, succinylcholine. In susceptible individuals, these drugs can overwhelm the body's capacity to supply oxygen, remove carbon dioxide, and regulate body temperature, eventually leading to circulatory collapse and death if not treated quickly. Less than 1% of Quarter Horses are affected, and those that are seem to trace to two specific bloodlines. The genetic disorder suggested to be an autosomal dominant trait.

The disease can be triggered by factors that may include exercise, stress, breeding, illness and concurrent myopathies. An additional genetic mutation in RYR1 gene (MH) influences PSSM and can increase the severity of the symptoms of PSSM in Quarter Horses and related breeds.

GBED - Glycogen Branching Enzyme Deficiency (GBED) is found in Quarter Horse or related bloodlines and causes late-term abortion or death of foals by a few months of age. It is an autosomal (non-sex cell) recessive disease. This means that horses can be carriers and not show signs of the disease, but have affected offspring. Foals with disease are homozygous gb/gb and receive an abnormal allele from both the dam and the sire. Foals born 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.