The use of DNA technology is something most of us are familiar with – mostly thanks to its uses in human forensics. But its applications in the equine world, including DNA profiling for identification and parentage, screening tests for coat colour and some inherited diseases, are growing all the time. Of particular importance to all equine breed registries, regardless of size, is the production of DNA genotypes or profiles for individual horses, and the use of these profiles in verifying parentage of foals before registration.

This technology to produce a genotype for a horse is the same as that used in human forensics and parentage testing. The first step in any form of genetic testing involves isolating DNA from the individual involved. DNA is extracted from the hair follicle cells, and it contains the entire DNA sequence from all of the chromosomes of that horse. From this DNA, 17 species specific microsatellite markers are tested to generate the DNA profile for that individual. Each microsatellite marker has 2 alleles, one from each parent. The two alleles of the foal are compared with those from the dam and from the sire, for each of the 17 markers tested, to ensure that one of each pair has indeed come from the dam and one from the sire. This results in the production of the individual animals DNA profile, which can then be used for identification or for use in parentage verification. Parentage verification using this method is 99.9% accurate.

Equine parentage laboratories worldwide use the same base set of 12 markers, under the recommendation of the International Society of Animal Genetics, so that exchange of information can take place between laboratories and breed societies in different countries. This means that horses don’t have to be retested when they travel internationally, and when imported semen is used it can be accompanied by a DNA profile for use in parentage reporting for any offspring.

Sometimes the results produced are not as expected– there will always be surprises when dealing with horses! Some of the scenarios we see where there are mismatches between foal and/or dam and sire include mares swapping foals in the paddock, and stallions that should not have been with a particular mare using all sorts of devious means (sometimes seemingly impossible) to mate with said mare. Then there are the cases involving human error – mares can be misidentified, transcription errors can occur filling out the paperwork, and hair samples can be put in the wrong envelopes. But with DNA technology and some old-fashioned detective work we can now reach a conclusion on the vast majority of these difficult cases. It must be remembered that we can only produce parentage reports for a foal if we have the DNA profiles for the dam and sire on the database. For this reason, it is extremely important to test all breeding stock (or possible breeding stock) as soon as possible.

Sample requirements:

For horses, the sample of choice for DNA testing is hair – specifically the hair follicle cells attached to hair pulled from the mane or tail.

Animal Genetics price list documents can here found here.

The dwarfism condition, which is characterized by an extreme reduction in size, as well as conformational abnormalities, is a recessive genetic disorder that affects both humans and animals. In miniature horses, the condition has been causatively linked with four different mutations in the Aggrecan (ACAN) gene. Unistel Medical Laboratory currently tests for D1, D2 and D4 mutations. Testing for D3 is under development. Carrier animals appear normal and can perform as well as non-carriers. However, when mated to a carrier female there is a 25 % chance for the offspring to be affected, 50 % of the offspring can be carriers and a 25 % chance for normal offspring. It is therefore important to test your breeding stock to avoid pairings that might result in either a dwarf, or an aborted foetus.

Polysaccharide storage myopathy (PSSM) is a hereditary muscle condition which occurs primarily in horses with Quarter Horse bloodlines such as Quarter Horses, Paint Horses, and Appaloosas. This muscle disease also occurs in other breeds such as Drafts, Draft crossbreeds, and Warmbloods. PSSM is a glycogen storage disorder and is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. The unique feature of PSSM is that in horses with PSSM, sugar from the bloodstream is removed and transported into the muscle at a faster rate, making more glycogen than is seen in horses with normal copies of the gene. The primary clinical sign of this disease is muscle cramping or tying-up. However, clinical signs and severity may vary with different breeds. Signs are most commonly skin twitching, stiffness, firm painful muscles, sweating, weakness, and reluctance to move with light exercise. Horses with PSSM typically have calm dispositions and are in good body condition. In some horse’s symptoms may begin by 2 to 3 years of age while others can remain subclinical. PSSM cannot be cured but it can be managed using a proper diet and exercise routines. Only one copy of the mutated gene is needed to cause PSSM in horses. Unistel Medical Laboratories offers a test to determine whether horses have the mutated copies for the GYS1 gene which could result in Polysaccharide storage myopathy.

Sex chromosome composition in mammals is usually XX for females and XY for males. No androgen is produced in the developing gonad if the Y-chromosome is absent and therefore the embryo develops as a female. Alternatively, androgen is produced if the Y-chromosome is present and therefore the embryo develops as a male. A functional Androgen Receptor is required for male development. Mutations in this receptor can inhibit the development of the Y-chromosome and result in sterile phenotypic females. Affected horses (genotypic males) have external female genitalia and an underdeveloped uterus which results in infertility. The horse also has typical male-like behavior.

Equine cerebellar abiotrophy (CA) is a genetic disorder that results in neurological conditions and is found almost exclusively in Arabian horses. This disorder has a recessive mode of inheritance. This means that carrier animals appear normal and can perform as well as non-carriers. However, when mated to a carrier female there is a 25 % chance for the offspring to be affected, 50 % of the offspring can be carriers and a 25 % chance for normal offspring. At birth, foals that are affected with CA seem normal. Symptoms of CA can take anywhere from six to sixteen weeks to develop. An affected foal’s neurons in the cerebellum begin to die which leads to lack of balance and head tremors. They may also have a wide-based stance, show exaggeration in action of the forelegs and startle easy (which often ends in the horse falling over due to its lack of balance). Foals that are affected with CA are often euthanized or kept as pets as they are not safe to ride. It can therefore be vital for horses to be tested in order not to breed horses that carry the mutation for CA.