Research shows inbreeding is increasing rapidly among thoroughbred racehorses

19 Jan 2020


  • Unclear whether tipping point has been reached after largest examination of thoroughbreds to date


  • Pedigree is not a good indication of inbreeding levels


  • Solutions exist for problem which ‘must be addressed at an industry-wide level’



Sunday, January 19th, 2020.New research by scientists at University College Dublin and equine science company Plusvital has established that there has been a significant rise in inbreeding levels in the global thoroughbred population over the last 45 years.

The research, published this week in the peer-reviewed journal Scientific Reports, analysed the genomes of over 10,000 thoroughbreds, the largest set of horses examined to date. The research linked the increase in inbreeding to selection for favourable racing traits and the influence of popular sire lines – 97% of horses in the study traced to Northern Dancer, and 35% and 55% of European and Australian horses had Sadler’s Wells and Danehill, respectively, in their pedigrees. Selection for popular sire lines reduces genetic diversity which can lead to inbreeding depression.

The research team was led by UCD Professor in Equine Genomics, Emmeline Hill, who is also Chief Science Officer withPlusvital.

Professor Hill said: “Inbreeding has always been high in Thoroughbreds, but it is getting higher. It is likely that, unchecked, inbreeding in the Thoroughbred will continue to increase in a market where there is high demand for particular sire lines. The problem with inbreeding is that it can compromise overall population fertility and health. This is a highly significant issue akin to global warming, where inbreeding is accumulating in the population, that must be addressed at an industry-wide level.”

Unlike most managed animal production systems, there is no systematic, industry-mediated genomic selection or population management for the Thoroughbred.

However, Professor Hill stressed that solutions to this issue now exist. “Our new data can assist the industry to monitor inbreeding and DNA-based tools have been developed that individual breeders can use to reduce the problem by choosing genetically diverse stallions for their mares.

“Pedigree is not powerful enough to help any more. Pedigree can be useful in highlighting broad trends in breeding practices, such as the predominance of certain sire lines leading to a high degree of relatedness. However, since Thoroughbreds are now so closely related, there is no longer the resolution in a pedigree to accurately infer relatedness between individuals,” said Professor Hill.

“Multiple studies show that pedigree-based estimates of relatedness are less accurate than DNA-based methods which measure the true genetic relationship between individuals. Suitable outcrosses will be best identified using genetic data.”

Higher inbreeding is not associated with superior racing performance, and Professor Hill reiterated that the research should be seen as a positive for the industry. “The purpose of this research is to provide information that, if harnessed in the appropriate way, could be beneficial to ensure the future sustainability of the breed.”


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