The X Factor
You Gotta Have Heart
A new book suggests that heart size correlates to equine performance and is inherited through the broodmare
The California Thoroughbred - May, 1997
Ever wonder why some stallions seem to be better broodmare sires than sires of sons? The genetic makeup of the horse that dictates the many characteristics of a successful athlete should, by conventional logic, be equally transmitted to both genders of offspring, right?
That logic is headed out the window with the publication last month of an intriguing book, The X Factor, by Marianna Haun, published by The Russell Meerdink Company, Ltd., Neenah, Wisc.
As spokesperson for the group of geneticists and cardiologists, Haun proposes, with some very solid reasoning, that the size of a horse’s heart correlates well with its racing performance and that the inheritance of heart size is linked to the X chromosome. This linkage suggests that the genes related to heart size are passed from stallions to their daughters and then to the next generation.
We were fortunate to meet with some members of the X Factor team as they came through California, searching for more evidence of performance and inheritance of heart size. Traveling with the enthusiastic Haun were the internationally renowned equine cardiologist, Dr. Fred Fregin and his wife, Laura. Dr. Fregin is also Director of the Marion du Pont Scott Equine Medical Center in Leesburg, Virginia. The opportunity to discuss these concepts for an hour or so led to some excitement on our part, and, after reading the book, led to the commitment to share these ideas with readers of the California Thoroughbred.
The idea of heart size of horses as it relates to performance is not new. The first serious attempts to correlate the two traits began in the early 1950’s in Australia. Dr. James Steel, a professor of veterinary medicine at the University of Sydney worked out the "heart score" using the electrocardiogram (ECG). In a major report of his findings on 2,500 horses, Steel published convincing evidence of the relationship of heart score to racing performance.
In the years subsequent to those studies, many investigators around the world followed up on these observations, confirming the heart score as one important characteristic for successful racing. And as actual heart measurements became available by post-mortem examinations of horses with a recorded heart score, an additional factor emerged. There was an excellent correlation between the heart score and the actual size of the heart. Thus, a higher heart score means a larger heart and an increased stroke volume of blood put into circulation. When a heart score of 100 is compared to 120, the cardiac output doubles. The potential for improved performance is obvious.
The next logical questions that breeders would ask at this point are: "Is heart size a heritable trait." "How is it inherited?" That’s where this discussion reverts to a detective story, with Marianna Haun playing the role of lead investigator, or at least, coordinator of information.
The Australians first noted the possibility of sex linkage of the inheritance of large hearts in Thoroughbreds. If that is correct, the large heart characteristic is transmitted on the X chromosome. Stallions have but one X chromosome, which passes to their daughters. Mares have two X chromosomes, of which one is passed to their sons and one to their daughters. A trait which is based in the genes of an X chromosome cannot be passed to a stallion’s sons, but to his maternal grandsons. Males inherit these traits from their dams.
So, what evidence do we have that this is really how it is, and how can we confirm it?
The support for the idea could come from looking at heart scores in families that contain superior runners, and confirmation could come from mapping the specific genes involved. The former information on families is the subject of a major portion of the book, told in the sequences of deductive reasoning that make for a good detective yarn. Haun begins with her initial awareness of the heart size of the great Secretariat, which was described after his death in 1989. In conversations with Dr. Thomas Swerczek who conducted the autopsy, it was obvious that his enormous but normal heart (estimated at 22 pounds) was a major factor is his superb racing career. Haun suggested that the revelation of the horse’s heart size was "a record making event akin to his 31-length record breaking triumph in the Belmont..."
Four years later, Dr. Swerczek supervised the autopsy of Sham, Secretariat’s old rival, who’s heart weighted 18 pounds, also notably large. Confronted by the evidence of two great runners with two great hearts, the X-Factor team began looking for links between the two. The fact that both horses were out of daughters of Princequillo started the wheels turning. The words of the Australians, suggesting a sex linkage, fueled the idea, and the race was on.
Haun details the progress of the investigation. A description of the huge heart of Eclipse, foaled in 1764, suggested a computer search for a genetic trail linking Eclipse with Secretariat. Hundreds of computer hours later, the genetic link turned up; a trail that led to one mare, Pocahontas, foaled in England in 1837. The details of the family lines are clearly presented, as the trail of inheritance of every large-hearted horse follows the X chromosome back to this great mare. This is the X Factor.
Of considerable interest is the association of four major sire lines which contribute greatly to the concept of sex-linked transmission of this large-heart trait. In the Princequillo line, in addition to Secretariat and Sham, we find other greats like Mill Reef and Key to the Mint. The line of War Admiral is represented by Seattle Slew, Hoist the Flag and Buckpasser, among others. In the Blue Larkspur line we find Mr. Prospector and Halo and many others. Finally the descendants of Mahmoud include Northern Dancer, Gallant Man and Rahy. Some of these horses appear in more than one of these four lines. Just look at this abbreviated list and consider the kinds of broodmare sires that are represented.
The study of the X factor is now following two lines of exploration. Dr. Gus Cothran of the University of Kentucky, heads the genetic side of the investigation. The focus here is to define the genetic marker of inheritance using DNA analysis.
"If detailed pedigree analysis confirms the transmission of this trait on the X chromosome, as it suggested by work to date, then we will have a unique opportunity to find a gene associated with a postive effect on the heart," Cothran said. "Until now there have been genes identified in mammals that affect the heart, but all of them do so in a negative manner," he added.
The other line of study is the determination of heart score on individuals that fit the family criteria for inclusion in the set of data. That duty is in the able hands of Fregin, who is well into his second hundred ECG examinations of breeding and racing Thoroughbreds for this project. The heart scores generated by Dr. Fregin are derived using the same approach and equipment as was used by the Australian workers. Thus, comparisons can be made between studies. Fregin's reactions to the results up to now are optimistic.
"My initial conservatism has been hard to maintain as new family linkages are found," he said. "Although relevance of this study waits to be fully documented by the scientific community, the opportunity to 'color outside the lines' has been great fun," he added.
So why was this distinguished team wandering around southern California in early March? I thought you’d never ask.
It seems that Marianna grew up in Five Points, Calif., immediately adjacent Harris Farms, Inc. She became familiar with the Thoroughbred operations of the farm early on. That background became more important when she started pondering the X Factor. Moscow Ballet, a stallion standing at Harris Farms and who’s second dam is by Princequillo, is producing the numbers of top performing daughters. When Fregin measured the heart score for Moscow Ballet at 147 (anything over 120 is well above average) and for his top daughter, Soviet Problem, at 150, it became clear that a laboratory for exploring the X Factor was right here in California. Numbers of sons and daughters of Moscow Ballet and some of their offspring are in the state, on the farm and at the tracks, so here is a great chance to study multiple generations of transmission of the large heart characteristic.
After confirming large heart scores for several stakes-winning daughters, like Dominant Dancer, Teresa Mc and Ballerina Gal is was obvious that this population was a gold mine for the study. So on March 8, when the X Factor team dropped in to Southern California, it was to fill in some heart scores (and collect blood samples for the DNA work) from various descendants of the stallion currently in training at Hollywood Park and Santa Anita. It was very obvious from the conversation that the opportunity was seen as an important step to completing the puzzle.
Where is this work headed, and what are the implications for Thoroughbred breeders? If the greatest expectations come to pass, an important contribution to the field of equine genetics will be made. That contribution could lead to methods which could enhance the chances for breeding better athletes. If the gene or genes are identified and a marker can be used to find them, a valuable screening technique could be available.
That consequence is probably a long way down the road considering the amount of work left to do on the DNA analysis. Also, we must consider the possibility that the many other variables that affect performance, such as environment, nutrition, bone development and mental attitude, to name a few, could overshadow this single genetic contribution.
But in the meantime, serious students of the breed and serious breeders of the horses should read this book. It presents the story of the X Factor in an entertaining and informative style. The pedigree information is valuable to it’s own sake. The scientific aspects of the work are presented in everyday terminology.
And like most detective stories, I couldn’t put it down!
-A.C. Asbury, DVM