The x-factor is a strange-sounding name that makes sense with a little explanation. The “x” is for the x-chromosome and factor is, well, a factor.
The quickest explanation of what the x-factor is in equines is a genetic abnormality or mutation that causes an enlarged heart. (Quick note: the x-factor is still a hypothesis, not a proven genetic mutation…yet). Normally, an enlarged heart would be cause for alarm in a beloved pet. For example, domestic cats can have a kind of enlarged heart, called cardiomegaly (if you break down the word, cardio means pertaining to the heart and megaly means abnormal enlargement– so the term literally means a big heart) which can cause breathing problems and shorten their life-span (though if diagnosed early it can be treatable and kitties can live with it and be comfortable, something I saw first-hand as a vet tech). Horses too can have cardiomegaly from heart disease that can lead to problems, but that kind of enlargement is usually seen later in life. The x-factor’s large heart can actually benefit performance horses in their work.
To learn about the emergence of the x-factor, we need to go back many years and generations through the bloodlines of thoroughbred horses. Pedigree information for thoroughbreds goes back hundreds of years, and the lineage of a single horse can be traced back to the 15th century or earlier (the names get less and less ornate the further back you go, and names like “yellow mare” become the norm). Bloodlines mean everything in the horse world, especially for horse breeders. If a certain characteristic or quirk can give a horse an advantage on the track or in the ring, a horse commanding that trait can become a hot commodity for breeding. Though males can produce more offspring, the horses predominantly in control of the x-factor are female, as they have the XX genotype and so can only pass an X chromosome on to their colts of fillies.
Though thought of as a female genetic trait, the first horse noted to have a larger than normal heart was Eclipse, a stallion thoroughbred born in 1764. (He is noted as “temperamental and difficult to train,” which is a lovely description). Eclipse is also attributed to being the foundation horse of the modern thoroughbred breed, and contributing to modern quarter horses as well.
Without delving too far into horse bloodlines, the results (or probably results, remember the X-factor is still technically a hypothesis) of the x-factor gene can be seen in race horse history easily with some big names. Secretariat, Sham, (the horse who was Secretariat’s closest rival), and war Admiral were all descendants of Eclipse and were all found to have larger than normal hearts to go along with their impressive race records. When Sham was necropsied after his death, his heart was found to be 18 lbs, or more than double the size of heart that was expected. Sadly, Sham died of a heart attack (a side effect of the x-factor?). Secretariat has been thought to have had a heart weighing 23 lbs, while Eclipse had a 14lb heart (which is still almost double the average thoroughbred heart weight).
So what good does a large heart do? Why isn’t it a hindrance? Well, it allows more blood to be pumped because of larger heart muscle. This means the muscles of the horse get more oxygen more quickly than a horse with a smaller heart who is the same size otherwise. The x-factor horse will tire less readily and could potentially be better at distance races or even sprints.
Because many stellar race horses have larger hearts, the x-factor has been the presumed indicator of performance. However, like most things genetic, it’s probably not just one gene or one anatomic abnormality that is responsible for success.
I will say I have a horse who may have the x-factor himself. He’s a big boy, and I wouldn’t be surprised if he has a big heart (he already does in spirit). He’s a good performance horse, though now enjoying semi-retirement at 21 years old. He’s fast and doesn’t tire easily. Does he have the x-factor? I’m not sure. He is a direct descendant of Sham (a bit strange as he is a Polish warmblood, but when you consider Sham’s daughter Long Meadows was sent to England and Austria for breeding it makes more sense) and he was a champion jumper in his youth. If he inherited anything from Sham that I’m most impressed with, it’s his personality and willingness to work, which are both stellar qualities in any horse.