Inbreeding and its effects on racing performance in Thoroughbred horses: A review of global evidence with insights from the Hungarian population

The Thoroughbred horse represents one of the most intensively selected domestic animal populations, developed through centuries of breeding focused on maximising racing performance. Originating from a limited number of founder animals and maintained as a closed studbook population since the late eighteenth century, the breed has experienced progressive increases in relatedness and inbreeding. This review examines the development of pedigree structure in Thoroughbred horses and evaluates the potential consequences of inbreeding for racing performance, with particular emphasis on studies assessing performance through race earnings and related indicators. Historical and genetic evidence describing the narrow genetic base of the breed and the subsequent formation of influential sire lines are summarised to provide context for understanding current levels of genetic diversity. The genetic principles underlying inbreeding and inbreeding depression are discussed, including the increase in homozygosity and the potential expression of deleterious recessive alleles affecting fitness and athletic ability. The review then synthesises findings from international studies, while focusing on research conducted in Hungary, that investigate the relationship between pedigree-based inbreeding coefficients and racing performance traits. Empirical results from the literature show varying degrees of association between inbreeding and earnings-based performance measures, with several studies reporting negative relationships consistent with inbreeding depression. Differences among populations, analytical methods, and performance indicators are also taken into account. Overall, the reviewed evidence underscores the importance of monitoring inbreeding levels and maintaining genetic diversity in Thoroughbred breeding programmes to sustain both racing performance and long-term population health.