Anne Peters



Anne Peters
P.O. Box 3926
Midway, KY 40347
(859) 846 - 9794


Pedigree Theories and Selection Techniques

The "X Factor" or Large Heart Gene - Thumbs Down

Despite having been disproved by science for years, the X Factor, or Large Heart Theory continues to be touted by its fans as a truth in horse breeding. People, please listen! I have always stressed that when considering a pedigree theory, if it doesn't make genetic sense, then throw it out. Here's one to throw out.

How did this idea get started? After the death of Secretariat in October, 1989, a necropsy was performed on the great champion by the University of Kentucky veterinarian, Dr. Thomas Swerczek. One of Swerczek's findings was that, by his estimation, based on visual examination only and from his long record doing necropsies, Secretariat's heart was roughly twice the size of the normal Thoroughbred heart, in fact twice the size of the heart of Secretariat's great sire, Bold Ruler.

From this observation, writer Marianna Haun developed a theory which she called "The X Factor," suggesting that Secretariat's great heart size was inherited from his dam, Somethingroyal, through one of her X chromosomes (see Chart 1), particularly the one acquired from her sire Princequillo, one of the great broodmare sires of modern times. The idea was that when Secretariat passed on that same X chromosome to his daughters, he also passed on his large heart gene, but when he passed on his Y chromosome (to his sons), he did not pass on such greatness. This would hopefully explain why he became better known as a top broodmare sire (females are all XX, getting one X chromosomes from each parent) than as a sire of top colts (who are YX, getting their Y chromosome from their sire and their X chromosome from their dam).

Of course, this concept doesn't exactly jive with reality, since Secretariat was a very successful, if under-appreciated, sire with 57 stakes winners (8.72% from foals), not the least of which were the top male performers Risen Star (Champion at 3, Preakness and Belmont Stakes), Medaille d'Or (Champion at 2 in Canada), General Assembly (Travers Stakes), and Kingston Rule (Melbourne Cup), besides his fillies which included Horse of the Year Lady's Secret and the brilliant sprinter Terlingua. One could argue that those good staying colts, Risen Star, General Assembly and Kingston Rule, got a large heart gene from their dams and not from Secretariat (if I chose to believe the theory to begin with), but it seems obvious that a horse who wins the Belmont Stakes by 31 lengths in new record time for a mile and a half, as Secretariat did, has the genetic wherewithall to sire good staying colts, X Factor or not.

Continuing her speculation, Haun extrapolated on the concept by zig-zagging backwards through Secretariat's pedigree, eventually landing on the great 19th Century broodmare Pocahontas (foaled in 1837 and sired by *Glencoe) many generations back as the source of Secretariat's large heart-producing X chromosome. Through similar speculation, Haun determined that nearly every top broodmare sire is a carrier of the "X Factor," and can all trace their pedigrees back to Pocahontas along this "heart-line." Convincing, yes? Well, actually no. Since most Thoroughbreds, including Secretariat, go back to Pocahontas many times over, finding a "heart-line" path to her wasn't very difficult (there are several possible paths in Secretariat's pedigree), and we know all of these "large heart" carriers weren't as great as Secretariat. Labeling these historic stallions as X Factor carriers was easier said than done, since most of them were unavailable for analysis to confirm their heart size, being long dead.

Working with a scientific researcher, Haun began the task of actually measuring heart size in living individuals through heart scans. Using a mare with a proven measured large heart, she bred a few foals with large hearts, although unfortunately none of them ever raced, so she could not prove her hypothesis that the large heart gene is what made Secretariat great.

This is an intriguing and colorful theory that a lot of people want to believe, and many will spend too much time mapping the heart-lines in a pedigree, to show which ancestors might carry one or even two copies of the large heart gene on their X chromosome. Their goal, of course, is to breed a horse with a large heart and win classic races, but unfortunately, there are a few problems with this theory and methodology.

First and foremost, there is no proof that a large heart translates into greatness, stamina, or even above average success on the race track. While most good equine athletes have strong hearts, there are too many other factors at work in producing a great runner and a large heart is useless without them working in concert with each other. On the other side of this argument, there are several really outstanding runners and sires that allegedly have average-sized hearts (Mr. Prospector and Caro come to mind, as well was Secretariat's sire Bold Ruler), and this presumed deficiency didn't hold these horses back in either career, on the track or as sires of classic runners.

The relationship of stamina to a large heart slips away when Haun describes the great mare Miesque having inherited the mighty heart of her sire Nureyev, when in fact both father and daughter were outstanding milers rather than being able to run further successfully, using the stamina that the large heart is said to supply.  

A second point to consider is that without actually having a horse's parents and other ancestors confirmed as owners of large hearts through actual measurement, mapping the X Factor through a pedigree is entirely a guess based on non-fact, no matter how neatly or how many ways one can find their way back to Pocahontas.

Third, and most importantly, while it wouldn't be surprising to find that natural heart size has a genetic component, and so could be inherited in a reasonably predictable manner, the mapping of the equine genome has proven that heart size is not passed on the X chromosome. If Haun hadn't been so adamant that the large heart factor was on the X chromosome specifically, she wouldn't have dug such a deep hole for her theory.

As I've said, if a breeding strategy doesn't make genetic sense, then it's not a valid breeding strategy. If the Large Heart Gene does not exist in horses on the X chromosome as the theory states, that's about as solid a genetic debunking as you can get.

Copyright by Anne Peters 2016.

Chart 1: A horse's sex (like humans and other mammals) is determined by the sex chromosomes with alleles X and Y, getting one from each parent. A female is what happens when the pairing is XX and a male happens when it's XY. A female can pass on either one of its X alleles to its offspring, but a male can pass on either a Y (resulting in a son) or an X (resulting in a daughter), so he is actually the sex determiner.
                                                              Nasrullah    (XY)
                          Bold Ruler (XY)           Miss Disco (XX)
Secretariat (XY)                                      Princequillo (XY)
                          Somethingroyal (XX)    Imperatrice  (XX)
(We don't know which X chromosome Secretariat got from Somethingroyal, the one she got from Princequillo, or the one she got from Imperatrice. The Large Heart theory presumes it was the one from Princequillo, but it could just as easily be the one from Imperiatrice.)