This quote in the recent DNA study led by Samantha Brooks in the journal Nature is pretty damning, I thought: The presence of Thoroughbred-specific Y chromosome haplogroups among Arabian racehorses indicates that the large chromosomal blocks of Thoroughbred origin detected in flat racing Arabian horses are likely derived, at least in part, from crosses with Thoroughbred stallions that occurred after the emergence of the “Whalebone” haplotype in the 1800s.”