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Genetic Disorders – Part I

This is the third in a series of five articles aimed at providing knowledge and resources to horse breeders and buyers as well as discussing the thought processes involved in breeding horses. It is also the first of two articles on genetic disorders. The final article in the series will touch on the selection process and breeding theories.

HWAC acknowledges with appreciation the cooperation and funding by the North American Equine Ranching Information Council (NAERIC) to facilitate the series of articles “HORSE BREEDING REALITIES - REPRODUCTIVE MANAGEMENT” composed by Judy Wardrope, JW Equine.

Articles Series


In the previous article, we delved just below the surface of equine genetics. But, as stated in that Basic Genetics piece, the inheritance of traits is a complex subject. In this article we will dig a little deeper and begin to examine genetic disorders in the horse as well as types of genetic expression.


Understanding the difference between dominant and recessive gives us a start, but as Ernie Bailey, PhD of MH Gluck Equine Research Center at the University of Kentucky and former chair of the Horse Genome Project, observes, “It’s always tricky to categorize things. The subject is mode of inheritance and people have used the following terms: dominant, recessive, multigenic, complex, sex-linked, sex-limited, co-dominance, partial dominance, etc. The terms arise with respect to the array of phenotypes (physical traits) that result from different gene actions.”

Just one example that proves Dr. Bailey’s point is the gene for milk production in mares. While both males and females have genes for milk production, they are only expressed in females, making those specific genes sex-limited.

Dr. Bailey opines, “My own thinking about categories is 1) phenotype is caused by action of a single gene (dominant, recessive, partial dominant, sex-linked, etc.) such that a diagnostic test is developed that confirms diagnosis, or 2) phenotype is complex and caused by a combination of genes, or genes plus environment.”

He adds, “Dominant/recessive are flip sides of a phenotype. Consider hair color in horses: Black phenotype is dominant and chestnut is recessive. The gene is MC1R. The gene isn’t dominant or recessive; its alleles causing the colors are. Animals have two copies of all genes, except some genes on the sex chromosomes. In most cases a single functioning gene is all that is needed for health. Black hair color is the result of a receptor binding melanocortin. If half the receptors on the cell surface cannot bind melanocortin, the cell still has enough functional receptors to produce black pigment. Red hair color is the absence of any receptor binding function; [in other words, it requires] two defective copies of MC1R.

“I think that as far as the breeders are concerned the main question is this: Does a single allele cause a change in phenotype?” One way to find out is through genetic tests, but not all traits have tests attached to them...yet.

Genetic Testing

With the ever increasing number of genetic tests available for horses, breeders have access to information that was previously only available through trial and error. Unfortunately, and with some regularity, the ‘error’ portion resulted in death or suffering in the resultant offspring. In the case of a number of genetic disorders, there is a laboratory test to determine if a horse will be affected by the disorder, is a non-affected carrier of the disorder or is clear of the disorder.

According to Dr. Bailey, “Every month some new test is developed or proposed. Which ones make it to commercial application? Hard to say.” Beyond genetic test results, it is important to know whether any particular disorder is dominant, recessive or has some different mode of expression when it comes to making informed breeding decisions. It is also advisable to know which disorders and/or syndromes affect the type or breed of horses we are contemplating breeding. (We will look more fully at breed-related syndromes and disorders in the next article.)

Mode of Inheritance

Inheritance of genetic disorders depends on the transmission of dominant genes and recessive genes. Genetic defects include any abnormality that is due to a change in the DNA that affects development, such as a new mutation occurring in the embryo's DNA that causes a problem, but that is not inherited from either parent.

Not all horses that possess a mutated gene express clinical signs of a disease or defect. Genes occur in pairs that perform the same function (i.e. determine coat color or some other trait) but the activity of one gene may dominate the activity of the other (recessive) gene. A pair of genes may also be co-dominant, meaning that each contributes equally to the resulting trait.

When a disorder or syndrome is classified as dominant, only one copy of the dominant gene is necessary for expression of the trait. When a horse with a dominant trait for a disorder is bred to a horse that is clear of the gene for that disorder, there is a 50% chance of the resulting offspring developing the disorder or syndrome. In other words, about half of that first horse’s offspring will exhibit the defect, and that half of the affected offspring of that first horse will pass the defect to their offspring even if bred to horses clear of the gene for that disorder. The numbers can multiply quickly, so consideration must be given to that fact when contemplating breeding with horses with dominant genes for a disorder.

If the gene for a particular disorder is recessive, two copies (one from the sire and one from the dam) are required for the offspring to express the disorder. Any offspring with only one copy of the mutated gene will be completely normal; however, he/she will be a genetic carrier of the disorder. But when he/she reproduces, there will be a 50% chance that he/she will pass the mutated gene on to his/her offspring. If the foal of two carriers receives a recessive gene from both parents (each parent has a 50% of passing their recessive gene), the foal will have the disorder. If the disorder is not life-threatening, then the foal could mature and enter the gene pool, but that mature animal would pass on one gene for the disorder 100% of the time because both its genes are associated with expression of the disorder, even though the gene has a recessive role when paired with its dominant counterpart.

If a carrier (one normal gene and one recessive gene) is mated with a non-carrier (two normal genes), each foal has a 50% chance of becoming a new carrier by inheriting a recessive gene from the carrier and a 50% chance of becoming a non-carrier by inheriting two healthy genes. However, when two carriers are mated, the resulting foal has a 50% chance of becoming a new carrier of one recessive gene, a 25% chance of getting both mutations and acquiring the disorder and only a 25% chance of becoming a non-carrier.

But it is not always so simple. As Dr. Bailey points out, “The overo/OLWFS (Overo Lethal White Foal Syndrome) trait is tricky because the mutation causing overo is the same as the one causing OLWFS. It would seem the allele has both aspects. The classification of the alleles would seem dependent upon the genotype. I just read a review where the author objected to referring to the genetics of OLWFS as recessive since the EDNRB gene has a dominant effect causing frame-overo. Interesting. EDNRB exhibits both: recessive for lethal white foals and dominant for overo color pattern.”

Breeding Considerations

It is clear that although testing may provide an answer concerning a horse’s status in regards to a syndrome/disorder, breeders will carry the responsibility regarding the propagation and numbers of affected horses, carriers and non-carriers in the gene pool.

Dr. Bailey says, “If there is a test for a gene causing a disease, then one could eliminate that gene from the population in a single generation. Just require testing and refuse registration to carriers. However, that practice would eliminate a lot of good genes to get rid of one bad one. Breeders have selected horses for generations to increase performance genes in the population. We need to keep these genes in the population. If we outlawed horses that carry deleterious genes, pretty soon we would have a very small population and not one selected for performance. Since some disease genes are recessive and carriers are normal and healthy, the most intelligent approach is to test and set up matings to be sure that affected foals are not produced. Breeders should select for performance, not against disease.”

If given the choice between two horses of equal genetic merit where one is a carrier of a genetic disorder/syndrome or some form of unsoundness and one is not, it seems obvious that using the non-carrier would help reduce the number of carriers in the gene pool over the long term.

David Trus adds, “Disorders should really be discussed in the context of good breeding practices, such as limiting inbreeding, maintaining sufficiently diverse lines within a breed, selection based on multiple traits (e.g. health, growth, performance, temperament, [which] are determined by very different genes) rather than single trait emphasis. So if we acknowledge there are potentially many deleterious genes in a population, getting rid of them no longer makes that much sense – as most people tend to think. Rather, good breeding practices are key. Only a very limited number of disorders will be worthwhile targeting. On the other hand, the appearance of new genetic disorders could be looked on suspiciously as the possible result of poor breeding practices.”

This Paint mare was born deaf, which is genetically linked to her coat colour. However, her deafness did not stop her from competing at the international level. In fact, she was part of Team Canada's silver medal in reining at the 2006 World Equestrian Games. Copyrights to Judy Wardrope/ JW Equine

Breeder Responsibility

It is the breeder’s responsibility to know which disorders their horses are predisposed to propagate, and to take the appropriate steps to ensure the welfare of the foals they will be producing as well as the gene pool at large. Such consideration applies to purebreds, cross-breds and grade horses whether they are registered or not.

In the next article in this series we will look at some breed-specific or type-specific disorders as well as resources for accessing information about them.

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