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A 2021 journal article titled "Genetics of Thoroughbred Racehorse Performance" by Annual Reviews. 

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A 2021 journal article titled "Genetics of Thoroughbred Racehorse Performance" by Annual Reviews. 

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Ernest Bailey|Jessica L. Petersen|Theodore S. Kalbfleisch

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Genetics of Thoroughbred Racehorse Performance | Annual Reviews

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Thoroughbred horses have been selected for racing performance for more than 400 years. Despite continued selection, race times have not improved significantly during the past 60 years, raising the question of whether genetic variation for racing performance still exists. Studies using phenotypes such as race time, money earned, and handicapping, however, demonstrate that there is extensive variation within these traits and that they are heritable. Even so, these are poor measures of racing success since Thoroughbreds race at different ages and distances and on different types of tracks, and some may not race at all. With the advent of genomic tools, DNA variants are being identified that contribute to racing success. Aside from strong associations for myostatin variants with best racing distance, weak to modest associations with racing phenotypes are reported for other genomic regions. These data suggest that diverse genetic strategies have contributed to producing a successful racehorse, and genetic variation contributing to athleticism remains important.

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"Candidate genes for physical performance in the horse", an online article written in 2011 by Science Direct.

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"Candidate genes for physical performance in the horse", an online article written in 2011 by Science Direct.

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A 2020 article from the American Quarter Horse Association that gives information about breeding and how someone should go about it. The article is titled "Breeding Responsibility".

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A 2020 article from the American Quarter Horse Association that gives information about breeding and how someone should go about it. The article is titled "Breeding Responsibility".

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New research examines the prevalence of genetic diseases in American Quarter Horses by discipline.

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"Inside Your Equine Athlete – Understanding the Genetics of Performance" an article written by The Chronicle of the Horse in 2023.

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"Inside Your Equine Athlete – Understanding the Genetics of Performance" an article written by The Chronicle of the Horse in 2023.

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Inside Your Equine Athlete - Understanding the Genetics of Performance - The Chronicle of the Horse

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Let’s be real – DNA is a thing. Every human, animal, and plant has it. To some, it may be a complex, mysterious thing, but most folks at least understand that DNA, or your genetic code, is the blueprint for what you are and what you can do. Every minute there are thousands of people and billions of dollars dedicated to decoding the human genome and understanding what it means for each human. “Big deal,” thinks the horse person, “who cares…what about my horse?!” Let’s face it, if you are reading this, you could probably take or leave most humans, it is your horse that matters. So then how does your horse’s genetic story play into his or her life? How does it affect yours? Read on, dear horse person, and let’s explore.

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An online article written by University of Kentucky Ag Equine Program in 2022 titled "Inbreeding and Genomics".

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An online article written by University of Kentucky Ag Equine Program in 2022 titled "Inbreeding and Genomics".

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Inbreeding and Genomics | Equine Programs

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Inbreeding has played a key role in the improvement of livestock breeds, resulting in more uniform populations with highly specialized performance traits. Selection for desirable traits entails identifying individuals with superior performance and often mating them to relatives (inbreeding) who possess the same superior traits. The goal of this practice is to increase the frequency of the desired characteristics and thus of the beneficial genes in the offspring. At the same time, negative consequences of inbreeding are well known. In small populations such as captive bred species, the loss of diversity associated with inbreeding is a major concern, and significant losses of diversity may lead to extinction. The increased expression of recessive deleterious genotypes can also lead to embryonic loss or other defects, some of which can be fatal. Furthermore, inbreeding can lead to a phenomenon called inbreeding depression. Inbreeding depression is commonly manifest in poor performance of traits that are complex (due to contributions of many different genes), such as fertility and athleticism. Mindful of the dangers inherent with inbreeding, breeders traditionally balance the benefits and dangers of inbreeding by monitoring their breeding stock, culling poor performers and avoiding matings of closely related individuals. Recently, genetic tools have become available that provide an alternative approach to unambiguously quantify and manage inbreeding relative to the traditional use of pedigrees. Today, a genomic survey of a horse’s DNA may cost $70 to $180. A comprehensive whole genome sequence, including analyses, may cost $1,000 to $2,500. So far, over 1,000 horses have had their entire DNA sequenced in connection with research projects. Those genome sequences have been used to identify the genetic bases of diseases, coat colors and even some performance traits. Nevertheless, the overall performance of horses is complex, involving over 20,000 genes and probably millions of other functional elements. Studying genes one at a time is unlikely to be effective to significantly improve performance. Genomic tools, however, make it possible to identify associations between the genome and traits that contribute to success or which may cause problems. One of the areas in which genomics excels is in determining levels of inbreeding. An animal’s inbreeding coefficient is the likelihood that both parents transmitted the same piece of DNA to their offspring that they each inherited from a common ancestor. Traditionally, we measured inbreeding by identifying all common ancestors – those that appear in the paternal and maternal sides of an individual’s pedigree. After common ancestors are identified, the relationship between the parents of the individual in question can be calculated. Using this method, on average, pedigree-based inbreeding coefficients for Thoroughbred horses are reported to be between 12.5%-13.5%, however individual horses may have values that range from less than 5% to over 20%. When genomic measures have been made in other species, geneticists discovered that inbreeding levels calculated from pedigrees are poorly correlated (50%-80%) with genomic measures of inbreeding. This is not surprising since pedigrees inaccurately assume a random and equal transmission of genes each generation. Which variant of each gene is inherited, however, is not predictable. For example, full-siblings share, on average, 50% of their genes; however, at any particular part of the genome they may share 0, 50 or 100%. Further, genes are not randomly distributed in a breed since selection practices are applied in mating horses. If we are good breeders, the genetic constitution of our current generation is not a random representation of the ancestors, but rather, a selection of the genes contributing to their success. There are other ways to apply genomics to horse breeding. As noted above, both the genome and the traits we value are complex. Our genomic tools are powerful, and we can begin to seek genetic patterns correlated with measures valued by horse owners. The limitation for such studies is the quality and availability of data for traits related to fertility, conformation, durability and athleticism. Collecting these data and using genomics to identify genes associated with these complex traits would be a more sensible way to improve performance rather than simply seeking to limit inbreeding. Ernest Bailey, PhD, professor, and Ted Kalbfleisch, PhD, associate professor, both in the Department of Veterinary Science at the Gluck Equine Research Center, and Jessica Peterson, PHD, University of Nebraska-Lincoln, provided this information. Source: January 2020 Equine Disease Quarterly. The Equine Disease Quarterly is published in January, April, July, and October each year by the Department of Veterinary Science. It is funded by Underwriters at Lloyd’s, London. Material published in the Equine Disease Quarterly is not subject to copyright. Permission is therefore granted to reproduce articles, although acknowledgement of the source and author is requested.

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An article written by Animals in 2021 titled "Impaired Reproductive Function in Equines: From Genetics to Genomics".

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An article written by Animals in 2021 titled "Impaired Reproductive Function in Equines: From Genetics to Genomics".

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Impaired Reproductive Function in Equines: From Genetics to Genomics

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Fertility is one of the key factors in the economic and productive success of the equine industry. Despite this, studies on the genetic causes affecting reproductive performance are scarce, especially in mares, where the genetic architecture of the reproductive traits is extremely complex. Today, with the increasing availability of new genomic methodologies for this species, we are presented with an interesting opportunity to understand the genetic basis of equine reproductive disorders. These include, among others, novel techniques for detecting chromosomal abnormalities, whose association with infertility in horses was established over 50 years ago; new sequencing technologies permitting an accurate detection of point mutations influencing fertility, as well as the study of inbreeding and molecular homozygosity, which has been widely suggested as one of the main causes of low reproductive performance in horses. Finally, over the last few years, reproductive performance has also been associated with copy number variants and candidate genes detected by genome-wide association studies on fertility traits. However, such studies are still scarce, probably because they depend on the existence of large and accurate phenotypic datasets of reproductive and/or fertility traits, which are still difficult to obtain in equines.