The three founding sires of the Thoroughbred breed have essentially been winnowed down to a single dominant branch worldwide, but a recent study of genetic diversity of North American Thoroughbreds revealed “only modest increases” in inbreeding that “isn’t inherently problematic.” Researchers from the University of Kentucky’s Martin-Gatton College of Agriculture, Food and Environment – in collaboration with the University of Nebraska-Lincoln, University of Minnesota, and University of California-Davis – released the results of this study, published in Scientific Reports, in late January. Using whole-genome sequencing (WGS), the research team analyzed the DNA of 185 Thoroughbreds born between 1965 and 2020. “The impetus for this study was to quantify the extent of genetic variation and inbreeding in Thoroughbreds,” Ernest Bailey, a professor at Kentucky’s Maxwell H. Gluck Equine Research Center and the study’s lead author, said in a press release accompanying the research. “By identifying trends, we’re equipping breeders with the data needed to make informed choices that preserve the breed’s health and performance.” For the study, Thoroughbreds were split into two groups: 82 horses born between 1965 and 1986, and 103 born between 2000 and 2020. Researchers identified more than 14 million genetic variations among the horses. Those from the older group showed “slightly more” genetic diversity, while horses in the younger generations showed “modest increases” in inbreeding. The research release stated that the increase “isn’t inherently problematic,” depending on what traits breeders are selecting to amplify or avoid. “Selective breeding focuses on enhancing desirable traits like speed and stamina, but it can also increase the risk of amplifying harmful genetic variants,” said Jessica Petersen, an associate professor at the University of Nebraska-Lincoln’s Department of Animal Science. “Our data gives breeders a clearer road map and tools to navigate these challenges. Inbreeding can help solidify positive traits, like speed or endurance. However, it’s also important to monitor and avoid harmful genetic combinations. With tools like WGS, scientists can proactively identify and manage risks before they impact the breed.” :: Access the most trusted data and information in horse racing! DRF Past Performances and Picks are available now. Sires with miler- and sprinter-type speed are popular in American breeding. Over the past two years, the leading freshman sires were Mitole (2023) and Vekoma (2024), both winners of the Metropolitan Handicap. Mitole went on to win the Breeders’ Cup Sprint and secure a championship. According to The Jockey Club’s Report of Mares Bred, the most popular stallions in America over this same time span were Golden Pal (2023) and Practical Joke (2024), both Grade 1 winners around one turn. One notable takeaway from the recent study was a 10 percent increase in the frequency of a genetic variant linked to the “speed gene,” which influences sprinting ability. This trend reflects the shift in breeding priorities toward shorter races. “DNA doesn’t lie,” said Ted Kalbfleisch, a professor at the University of Kentucky’s Department of Veterinary Science and a study co-author. “This change in the genome aligns with the growing popularity of shorter, faster races and demonstrates how breeding decisions leave measurable imprints on the genetic makeup of the breed.” The Thoroughbred breed can be traced to three sires: the Godolphin Arabian, Darley Arabian, and Byerly Turk. While all three shared prominence centuries ago, the Darley Arabian has since become the dominant line in North America. In the first 15 years of the Kentucky Derby, seven winners came from the Byerly Turk’s sireline, six from the Darley Arabian, and two from the Godolphin Arabian. But the most recent 15 winners of the race – and, in fact, 132 of its 150 overall – can be traced to the Darley Arabian’s descendants. This is not strictly a North American phenomenon: The last 10 respective winners of the English Derby, Japanese Derby, Australian Derby, Chilean Derby, and South African Derby are all from the same line. The 20 starters in the 2024 Kentucky Derby were by 17 different sires, showing a diversity in genetics, although many shared common descendants. The winner of the race, Mystik Dan, was by Goldencents, a son of six-time reigning leading sire Into Mischief, who was represented in the field by Resilience. Another of Into Mischief’s sons, Practical Joke, sired starter Domestic Product. Into Mischief is one of the major living representatives of the great Northern Dancer’s sireline. Northern Dancer was a grandson of Nearco, who was a grandson of Phalaris. Born in 1913 of Great Britain, Phalaris is a representative of the Darley Arabian line and was a common ancestor of all 20 Derby starters via two main sirelines – those of Nearco and Mr. Prospector. Nearco, via several different descendants, appears in the direct male line of 2024 Derby starters Forever Young, Catching Freedom, T O Password, Stronghold, Honor Marie, Endlessly, Epic Ride, Just Steel, Grand Mo the First, Catalytic, and Just a Touch. Meanwhile, Mr. Prospector is the direct male ancestor of Sierra Leone, Dornoch, Track Phantom, West Saratoga, Fierceness, and Society Man. These same trends appear likely to be borne out by the 2025 Kentucky Derby field. Sixteen different stallions are represented on Daily Racing Form’s latest Derby Watch Top 20. Eleven of these stallions trace to Phalaris through Nearco, while another five trace to Phalaris through Mr. Prospector. The dominant line on the Derby Watch is again that of Into Mischief, the sire of five of the 20 contenders. His sons Practical Joke and Authentic are also represented by 3-year-olds in the Top 20. Another influence in this year’s Derby is a great-grandson of Northern Dancer. Medaglia d’Oro, his son Violence, and his grandson Volatile each have horses in the Top 20. :: Want to learn more about handicapping and wagering? Check out DRF's Handicapping 101 and Wagering 101 pages.