Let’s take a look again at Secretariat’s lineage to get a real life example, since we know him to be a carrier from his 22-pound heart. Secretariat’s dam was a mare named Something Royal, and her sire, Princequillo, is considered one of modern racing’s primary carriers of the x factor gene, but only his daughters will make foals with the enlarged heart. Writer and founder of the theory, Marianna Haun, originally traced Princequillo’s X factor as far back as a mare born in 1837, named Pocahontas. After verifying Pocahontas with scientific researchers, veterinarians, and TB breeders, they were able to take the line even further back, first to famous stud Eclipse born in 1764, and then finally to the very earliest traceable ancestor who carried the large heart gene, Hautboy, one of the founding thoroughbred bloodlines in the mid-seventeenth century.
