The x-factor theory

The x-factor theory is one of the most disputed theories in breeding circles. Some swear by it while others swear it off. This article briefly outlines the theory as proposed by Marianna Haun (very briefly - Marianna Haun wrote several books and this summary is just a bare basics overview), several criticisms and looks at what science in fact can tell us. The x-factor theory has not been proven or disproven. This article does not argue for or against, but merely summarizes some key facts, presents the common criticisms and also briefly outlines other development in genetics that are relevant.

To understand the theory one needs to first know about the legendary thoroughbred Eclipse, foaled in England in 1764. At his death in 1789, a medical examination of Eclipse found that his heart was much larger than other horses. At that time the average heart weighed approximately six pounds but Eclipse’s heart weighed 14 pounds. When Secretariat passed away his heart was not weighed but it was observed that it was immense. Dr Thomas Swerczek, the head pathologist at the University of Kentucky, estimated it at 22 pounds, while the normal weight of a horse’s heart today is 8.5 pounds. Basic physiology would tell you that larger heart and larger lungs on average gives a better runner, be it a horse, a human or some other animal. There is no question that  having such a massive heart is advantageous for a thoroughbred (but while it is a good thing it is not absolutely necessary: there are great racehorses with average size hearts).

The obvious question was: where did the large heart come from genetically? (It was taken for granted that it was genetic.) According to Marianna Haun, in her article "Heart of the Matter" she writes that "Following the publication of my first article about Secretariat’s heart in the spring of 1993, I began to search for the source of the great heart. University of Kentucky geneticist Dr. Gus Cothran said that if large hearts were a sex-linked characteristic like the Australians believed, then the characteristic would track to a single source and would be a genetic mutation found on the X chromosome, which passes through the female line."

Looking for a possible pedigree link, she eventually found one and then postulated the theory that there was a gene responsible for the size of the heart, a large-heart gene, and that this was sex-linked, i.e. that it was on the x-chromosome (which explains the name). This would explain how the large heart would pass, genetically, from Eclipse to Secretariat.

Since genetics is unknown to some, let us briefly explain what is meant by x-chromosome.

All foals receive one sex chromosome from each parent (just like humans), this is called the XY sex-determination system in science. Males (stallions) always have one X and one Y chromosomes while females (broodmares) have two X chromosomes. Each foal receives one chromosome from the father and one from the mother. Since the mother is XX, i.e. she has two X chromosomes, we know that she passes on an X chromosome to her foals. The father has both an X and a Y chromosome and can pass on either one to the foal. If the father passes a Y chromosome to the foal the it becomes XY (a male), and if the father passes a X chromosome to the foal the foal becomes XX (a female).

We see that genetic characteristics and traits inherited on the X chromosome can be inherited from father to daughter, from mother to daughter and from mother to son but NEVER from a father to a son (because a father will ALWAYS pass a Y chromosome to a son). The Y chromosome is smaller than the X chromosome and carries less genetic material. When a stallion produces a daughter, it is his X chromosome that is passed on to the foal. The stallion only has one X chromosome (his other chromosome is a Y chromosome) and this X chromosome he must the one he received from his mother. (This is because the stallion received his Y from his father and his X chromosome from his mother) Genes are stored in gene pairs. Some genes are dominant while others are recessive. If a human has two genes for blue eyes, the eyes will be blue. If a human has one gene for blue eyes and one gene for brown eyes, the eyes will be brown because the brown gene is dominant. But even though the eyes are brown, the person still possess a gene for blue eyes too, and this is why people with brown eyes can have children with blue eyes. In fact, if both parents have one gene for brown eyes and one for blue eyes then both will have brown eyes - but there is a 1 in 4, i.e. 25 %, chance that their child will have blue eyes.

Similarly, whichever X chromosome is dominant is the one that is expressed. Furthermore, for a female the has two X chromosomes, does she have one X chromosome with a large heart gene or does she have two? If a mare is a single copy (meaning she carries the large heart on only one of her X chromosome), she could pass on either her large-hearted X chromosome or her small-hearted X chromosome. Depending upon whichever X chromosome is dominant, that mare might express either a large heart or a small heart

The term "Single copy" mare describes a broodmare that carries a large heart gene on only one of her X chromosomes.

The term "Double copy" mare describes a broodmare that carries a large heart gene on both of her X chromosomes. In the x-factor theory this is the ideal broodmare. If she is bred to a large-hearted sire, she always will produce large-hearted foals and her daughters also will be double copy mares. The theory postulates that double copy mares frequently more successful horses than any other mares.

With this theory comes also the concept of a "x-factor positions" or "x positions" as they are sometimes called.

Consider the pedigree for a broodmare above. For simplicity no names are shown. If the current horse has a large heart gene, where could this come from? Because a father will never pass on any X chromosomes to their sons, a horse in the white pedigree positions can never be the source of any large heart genes in the current. (If the horse was a male, the top half of the pedigree would be all white and he must have inherited the X chromosome with the large heart gene from his dam.) Furthermore, if you know that a certain ancestor carried a large heart gene then by using the concept of x-factor positions you can look for stallions with that ancestor in x-factor positions.

Sometimes somebody who claims to believe in x-factors will say something like "just double up on anybody in an x-factor position" but that is not compatible with Marianna Haun's theory. That theory does not postulate that as long as you double up on anybody in an x-factor position, this will in any way improve your chances. Marianna Haun is very clear on using large-hearted sires. You do not want to simply double up on "anybody" in x-factor positions, you want to double up on those that carry the large heart gene in those positions.

Under the x-factor theory average and poor sires can still be great broodmare sires. The great Secretariat, clearly a large heart horse, produced ok as a sire but his sire career did not in any way match his fantastic race career. However, he became known as a very good broodmare sire. Under the theory, any sire can pass on his large heart gene to his daughters but not his sons. The sons are dependant on their dams for the large heart gene and the importance of the sire is hence diminishes. Since the daughters can get a large heart gene from their fathers then large heart stallions like Secretariat can still achieve greatness as broodmare sires. To exaggerate: the failure of a large heart stallion at stud is then because his did not get many large heart broodmares.

The x-factor theory is not meant to explain everything. As Marianna Haun have said "Heart size will not help a racehorse be a champion if he is unsound, has bad conformation, is not interested in running, is poorly trained or any number of factors that can impact the success or failure of a racehorse."

So is the x-factor theory valid? That is a source of endless debate. Below are outlined a few of the common criticisms of the theory.

Even though there was an Australian study in 1977 that found a correlation between heart scores and performance, later studies did not support that hypothesis. Most genetic studies on horses are also using data from the Horse Genome Project at the University of Kentucky in their research. If a genetic link exists then using that data often (but not always) leads to a scientific link being found quickly. Several scientists have began to investigate the x-factor theory but has since abandoned their studies. Although not conclusive, this indicates to skeptics that no clear x-factor link was found using the Horse Genome Project data.

Some have also pointed out that heart size can change in association with pathology and that large heart size is sometimes a sign of heart problems, hence a large heart is not always a good thing and not always inherited by genes.

Several extensive equine genetic studies have located genes related to heart size on autosomal (autosomal are non-X and non-Y) chromosomes, but have not located any such genes on the X chromosome alone, which some critics have interpreted as disproving the whole x-factor theory altogether.

The explanation of how poor sires can be great broodmare sires, especially Secretariat, has also met with much criticism. The criticism is that while Secretariat has become a great broodmare sire that is not because of him but rather because he was bred to many of the best broodmares from all the best maternal families. Secretariat was a much better broodmare sire not because he is an x-factor carrier but because his daughters come from some of the best and finest maternal lines around and this is the primary reason. Secretariat is just benefitting from being mated to some of the finest broodmares at the time and it is their maternal families that shine.

Finally, some have criticized the theory for its oversimplification. How can racing performance primarily be a factor of one gene pair when the horse consists of a lot of different genes?

Generally speaking, geneticists researching racing performance are of the opinion that performance is a result of a large number of genes. Various research projects are starting to piece together a very small part of the puzzle. When Marianna Haun postulated her theory the research into equine genetics was non-existent. Since then there has been several interesting findings, three of which are mentioned below.

Some surprising findings on large hearts and heart function in 2002 was discovered by a University of Toronto team as part of a research project on cancer and heart disease ( The team, lead by Josef Penninger, found that heart size and heart function is controlled by the PTEN and PI3K alpha and gamma proteins at work in the body's immune system. If PTEN is not present, then the PI3K protein works to make the heart bigger and when the PI3K protein is missing the heart was only half-size. The research was done on mice but Dr Penninger confirmed in a e-mail to Sophia Pedigrees that the same principles applies to horses and should work there as well. This seemingly contradicts the x-factor theory by making heart size a function of the presence of proteins and their interaction. At the time of writing this article there has, however, been very little further research into this subject and none on the equine side.

Another scientific breakthrough comes from Dr Emmeline Hill of the University College, Dublin, an Irish geneticist who published a paper in the journal PLoS ONE called "A sequence polymorphism in MSTN predicts sprinting ability and racing stamina in Thoroughbred horses." Basically her studies has identified a myostatin gene which is responsible for muscle mass development and can predict with a very large accuracy the muscle types of thoroughbred horses and if they are better at shorter or longer distances (with a combination of "C" and "T" genes). No similar research has yet to be conducted on harness horses. Her company Equinome, has a list of all online research papers found at

The Swedish company CapiletGenetics, led by geneticist Lisa Andersson, has done similar research into the gaits of horses (with a combination of "C" and "A" genes) and has identified a single gene which predicts the horse's ability to maintain a certain gait at higher speeds.