Horses in History: The X factor

Horse Nation’s in-house historian Lorraine Jackson explores the science behind a genetic mutation that results in a larger-than-average heart–a huge advantage for racehorses.

From Lorraine:

After another emotional and triumphant finish for I’ll Have Another at the Preakness Stakes on Saturday, it was all racehorse history for me over the weekend. But this column definitely took a scientific turn in nature, as I take a look into a genetic theory called Thoroughbred racing’s “X factor.”

While watching Secretariat’s triple crown races for what must be the umpteenth time, I started reading more in depth about this mysterious X factor, which Secretariat himself was purported to be  benefiting from. In essence, the theory hypothesizes that some thoroughbred broodmares have a recessive gene which, when rarely matched to a carrying stallion, would produce foals with extraordinarily sized hearts (sometimes 2-3 times the size of a regular TB).  This genetic mutation would actually be of enormous benefit for a racehorse, the same way that having a bigger engine in your truck allows you to pull a bigger rig, or a racecar to accelerate more quickly. For a horse like Secretariat, that meant he could run splits in the 1.5-mile Belmont progressively faster without tiring, in contrast most horses getting progressively slower as they tire at the end of a gruelingly long race.

But the gene was a tricky one to track. As the name suggests, the X factor only mutates on the X chromosome, which of course all horses (and humans, and ducks and all other animals) have from their mother. Males have an X and a Y chromosome, while females have two X chromosomes. When a foal is born, it receives one of its dam’s two X chromosomes, and depending whether it gets its sire’s X or Y chromosome will determine if it is male or female. The result is that there will be few male carriers, and even less males that actually have the mutation itself. This means that a stallion cannot produce hundreds and hundreds of foals with this mutation, only a broodmare with the mutation can produce sons or daughters that will be affected. Ergo, only a few champions throughout history have sported the “reverse grinch syndrome,” a heart many sizes too big!

Clear as mud?  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.

Hautboy would have gotten the mutation from his dam, known to the ages only as “Royal Mare,” which historian Alexander MacKay-Smith says refers to the hundreds of royal mares that were bred by Lord James D’Arcy around 1660. D’Arcy was appointed by Charles II to oversee the Royal Stables and expand a breed of racing horses for England, and the Royal Mares and Hautboy were among these foundational bloodlines. Unfortunately, these royal mares were the end of the road for TB historians looking for the X factor, when the mares had no names.

Hautboy foal circa 1705

Royal mares

In contemporary racing history (early 20th century to present) there are four TB racing stallions considered to be major carriers who would have produced X factor horses: Princequillo, who was already mentioned in the lineage of Secretariat; War Admiral, from his dam Brush Up out of Sweep, (Sweep would later produce large hearts in Seattle Slew and Whirlaway); Blue Larkspur, who made a few large hearted European champions; and Mahmoud, whose X would be carried down to racing and breeding great, Northern Dancer.

As fascinating as it is to get sucked into the power of the X factor gene, every researcher on the project was emphatic that the mutation alone does not a champion make.  There are likely countless Thoroughbreds in the world this very minute sporting a Reverse Grinch Heart that are living common lives–A true champion requires soundness, a quality upbringing, exceptional training, an optimal conformation, and more than anything, a true will to run.  You can’t help but look on in awe as Secretariat barrels down the stretch of the Belmont so far ahead he could not hear pounding hooves behind him. All that propelled him forward was a gentle hand-ride by Ron Turcotte, the screams of tens of thousands of fans, and his own personal will to run with all his might. Perhaps there are still many more unspoken mysteries about a horse with a giant heart, and we have yet to learn them.

Secretariat winning the Belmont

Go riding.


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