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Animal genetics2023; 54(3); 408-411; doi: 10.1111/age.13309

Inbreeding depression and durability in the North American Thoroughbred horse.

Abstract: The proportion of the genome containing runs of homozygosity (ROH) affects production traits in livestock populations. In European and Australasian Thoroughbreds inbreeding, quantified using ROH (F ), is associated with the probability of ever racing. Here, we measured F using 333 K SNP genotypes from 768 Thoroughbred horses born in North America to evaluate the effect of inbreeding on racing traits in that region. Among North American horses, F was not associated (p = 0.518) with the probability of ever racing but was significantly associated with the number of race starts (p = 0.002). Among raced horses, those with a 10% higher F than the mean inbreeding coefficient were predicted to have 3.5 fewer race starts compared to horses with a mean inbreeding coefficient. Considering the trend of increasing inbreeding and a decline in the average number of race starts per runner in North America, mitigating inbreeding in the population could positively influence racing durability.
© 2023 The Authors. Animal Genetics published by John Wiley & Sons Ltd on behalf of Stichting International Foundation for Animal Genetics.
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Publication Date: 2023-02-26 PubMed ID: 36843349DOI: 10.1111/age.13309Google Scholar: Lookup
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Summary

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This research provides an analysis of the impact of inbreeding on the performance and durability traits of North American Thoroughbred horses, showing that higher levels of inbreeding, as identified through runs of homozygosity (ROH) in the genome, are associated with fewer race starts.

General Overview

This study seeks to better understand the impact of inbreeding on production traits, such as racing performance and stamina, in North American Thoroughbred horses. This is accomplished through the measurement of runs of homozygosity (ROH), or long stretches of DNA that are identical on both chromosomes, which is a sign of inbreeding.

  • The researchers analysed genetic data (333 K SNP genotypes) from 768 horses born in North America.
  • ROH was used to quantify the level of inbreeding (denoted as F in the study).
  • The main goal was to evaluate the effect of inbreeding on racing traits, including the probability of the horse ever racing and the number of races it starts.

Key Findings

The researchers found that the level of inbreeding in Thoroughbred horses had no significant association with the likelihood of the horse ever racing. However, there was a significant correlation between increased inbreeding and a reduction in the number of race starts.

  • In the North American horses studied, there was no statistically significant association (p = 0.518) between the inbreeding coefficient F and the probability of a horse ever racing.
  • The inbreeding coefficient F was significantly associated (p = 0.002) with the number of race starts, meaning higher inbreeding can predict fewer race starts.
  • Horses with a 10% higher inbreeding coefficient than the mean were predicted to start in 3.5 fewer races compared to horses with an average inbreeding coefficient.

Implications and Conclusion

The study’s findings suggest that mitigating inbreeding in Thoroughbred horses could lead to improvement in their racing performance, specifically in terms of their racing durability. This hints at the need for careful breeding strategies in the production of racing Thoroughbreds in North America, especially considering the observed trend of increasing inbreeding and a decline in the average number of race starts per racing horse.

  • The observed trend in North American Thoroughbreds shows both an increase in inbreeding and a decrease in the average number of race starts per horse, suggesting a potential negative impact of inbreeding on racing durability.
  • Thus, countering inbreeding trends in the population could positively influence the durability of Thoroughbred race horses, leading to an increased average number of race starts.

Cite This Article

APA
Hill EW, McGivney BA, MacHugh DE. (2023). Inbreeding depression and durability in the North American Thoroughbred horse. Anim Genet, 54(3), 408-411. https://doi.org/10.1111/age.13309

Publication

Animal genetics
ISSN: 1365-2052
NlmUniqueID: 8605704
Country: England
Language: English
Volume: 54
Issue: 3
Pages: 408-411

Researcher Affiliations

Hill, Emmeline W
  • Plusvital Ltd., The Highline, Dun Laoghaire Industrial Estate, Dublin, Ireland.
  • UCD School of Agriculture and Food Science, University College Dublin, Dublin, Ireland.
McGivney, Beatrice A
  • Plusvital Ltd., The Highline, Dun Laoghaire Industrial Estate, Dublin, Ireland.
MacHugh, David E
  • UCD School of Agriculture and Food Science, University College Dublin, Dublin, Ireland.
  • UCD Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin, Ireland.

MeSH Terms

  • Horses
  • Animals
  • Inbreeding Depression
  • Genotype
  • Homozygote
  • Inbreeding
  • North America
  • Polymorphism, Single Nucleotide

Grant Funding

  • 11/PI/1166 / Science Foundation Ireland

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Citations

This article has been cited 6 times.
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    doi: 10.1186/s12915-025-02393-7pubmed: 41034993google scholar: lookup
  2. Karlau A, Azcona F, Molina A, Trigo P, Sánchez-Serrano JP, Demyda-Peyrás S. Exploring the Genetic Link Between Coat Colour and Morphological Traits: The Case of Peruano de Paso Horse. Animals (Basel) 2025 Sep 17;15(18).
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  3. Yokomori T, Tozaki T, Segawa T, Itou T. Genomic regions and candidate genes associated with forehead whorl positioning in Thoroughbred horses. J Equine Sci 2025;36(1):11-18.
    doi: 10.1294/jes.36.11pubmed: 40115733google scholar: lookup
  4. Azcona F, Molina A, Demyda-Peyrás S. Genomic-Inbreeding Landscape and Selection Signatures in the Polo Argentino Horse Breed. Int J Mol Sci 2024 Dec 24;26(1).
    doi: 10.3390/ijms26010026pubmed: 39795883google scholar: lookup
  5. Bailey E, Finno CJ, Cullen JN, Kalbfleisch T, Petersen JL. Analyses of whole-genome sequences from 185 North American Thoroughbred horses, spanning 5 generations. Sci Rep 2024 Oct 2;14(1):22930.
    doi: 10.1038/s41598-024-73645-9pubmed: 39358442google scholar: lookup
  6. Gmel AI, Mikko S, Ricard A, Velie BD, Gerber V, Hamilton NA, Neuditschko M. Using high-density SNP data to unravel the origin of the Franches-Montagnes horse breed. Genet Sel Evol 2024 Jul 10;56(1):53.
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