What is the X-Factor?
There are seen and unseen attributes in any individual horse. Although beauty and correctness of conformation are easily appreciated, the unseen components may be what sets this horse apart and makes way for greatness. The spirit, character, temperament and will, combined with unseen physical advantages, produce something extraordinary. With particular emphasis on quantifiable traits, the following article addresses the advantage of a larger than normal heart in Equine Athletes. The larger than normal heart is an inheritable trait, and convincing research shows that this genetic opportunity is passed to offspring by way of the X chromosome. The term ‘X-Factor’ is used to describe the inheritability of the large heart gene, because of the association with the X chromosome.
Inheritance of the X-Factor
Genes are the code of life. Each gene is found at a precise location on a chromosome, and is responsible for determining a particular characteristic in an organism. This is done by the formation of specific proteins. Proteins communicate events and status, and form essential structures necessary for life. Proteins have an intimate and inextricable relationship with their immediate environment, and subsequently, the surrounding environment. They participate in a very complex and incessant dialog because the micro and macroscopic structure in the body undergoes constant change. Nothing goes unnoticed at this level, and all things are governed by the information given and received by the genes.
Chromosomes which are the organizing structures for the genes, are united in a special way, forming the double helix structure called DNA. DNA is a pair of chromosomes, one strand from each parent. There are 32 of these pairs that contribute to the genetic makeup of the horse.
Sex-linked chromosomes determine the gender of the individual. This type of chromosome that has female characteristics is called an X-chromosome, and a chromosome that has male characteristics is called a Y-chromosome. Stallions and geldings have one X, and one Y-chromosome, and mares have 2 X-chromosomes. These two chromosomes, are just one of the 32 pair that make up the horse.
How do I find it?
Since the trait is carried on the X chromosome, the heart-line is traced from sire to daughter, and dam to son or daughter. If a sire is known to have a large heart, then he definitely passes it to his daughters, but definitely does not pass it to his sons. If a dam is known to be a double copy mare, she definitely passes the trait to all her offspring, both sons and daughters. A single copy mare has a 50% chance of passing the trait to any given son or daughter.
Without direct knowledge of heart size, it is still possible to make a good guess based on highly correlated information. If a sire appears high on the brood mare sire list, it is highly likely he carries the large heart. Also, mares listed as “Reine-De-Course” are also likely to carry the trait. There are exceptions to both these rules, the most notable being Miss Disco, and her son Bold Ruler. Both were known to have normal hearts. Other circumstantial evidence of the large heart is the ability to win races at more than a mile. The evidence is better as the race is longer and tougher. So graded stakes winners at classic distances are a good bet. There is one more factor that is useful as an indicator for broodmares. The produce record of a single or double copy mare should be considerably above average. Plus her produce should have a tendency to go a distance. Her sons are the best indicator, since they can only inherit the large heart from her. Using the above evidence and known large hearted horses can allow one to have a pretty good idea of the odds that a given horse carries a large heart. You can also have a vet use ultrasound or electrocardiography techniques to directly measure the heart size.
Heart size is just one factor that is important for race horse performance. It is however quantifiable and somewhat predictable. Therefore it is a useful factor for prediction of racing and breeding success. Implications for the Sport Horse Industry are clearly closely related.
The X chromosome is larger than the Y chromosome, and therefore has more space for the genetic markers that describe traits. Heart size, based on current research seems to be carried on the X chromosome alone. Thus, a male horse has a single marker for heart size, while a female has two. A male offspring inherits the Y chromosome from his male parent, and one of two possible X chromosomes from his female parent that determines his heart size. A female horse has 2 X chromosomes and can have one, both, or neither that carry the large heart.
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For Example: |
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Secretariat, a known large heart horse, would not pass an X chromosome to any of his sons but he would pass it on to his daughters. His daughters in turn, pass their X chromosome from him, or their X chromosome from their dams, to either their sons or their daughters... |
In the case of a female, one X chromosome will generally be dominant, so it is possible for the trait to be carried, but not exhibited. The term “double copy” mare is used to describe a mare that carries 2 X chromosomes with markers for the large heart on each one. These are the mares who are most likely to have exceptional produce records. These are the ones we want in our brood mare band.
The conclusion about heart size being determined by the X chromosome alone explains the pattern of broodmare sires. These are sires who are not, in general a huge success as studs directly, but whose daughters end up superior to the brood mare population in general. All the sire’s daughters inherit his X chromosome, and if it carries the large heart, all of them will have at least one, and some of them 2 X chromosomes, that pass on the large heart trait. Thus although he cannot influence his son’s heart size, his daughters rate above average. Their offspring will benefit, and thus the sire gains a reputation as a broodmare sire.
Summary
It is known that certain lines within Thoroughbreds are particularly gifted jumpers. There are stamina lines, and lines which produce more substance. There are lines which produce size, and lines that are known for beautiful movement. Each chromosome carries the genetic material of each parent and that is why a study of pedigree is important. Many traits may be enhanced or diluted depending upon the choices we make.
It is also noteworthy that repeating female families seems to be evident in some of the best racehorses and show jumpers of our time. Female families may also be the key to understanding why certain crosses produce outstanding horses. Talent may be looked for in the sirelines of both sire and dam, and the ultimate ‘power pack’ needed to drive the horse found in superior female families. It makes sense that horses with larger than normal hearts might be more able to perform difficult things with more ease. There are other structural elements that need to go hand in hand with a large heart. Large lung capacity, which includes a deep girth, well sprung ribs, a large windpipe, and large nostrils. If you are going to try to breed a horse with these attributes, it is prudent to breed for fundamentally correct conformation. Don’t put a great engine in a bad chassis. The weakest element will be sorely tested.
So do I assume its better to look at the grand sire behind the dam for a better chance of getting a foal with a large heart? I find your writing very readable as a breeder, could you email me on
ailsae@mac.com
Keith
None of what Marianna Haun wrote made it into peer reviewed scientific papers, yet she passed of what she had ‘discovered’ as science. It was nothing of the sort. In fact, the very method that Haun used, the measurement of QRS intervals by electrocardiogram has been proven in peer reviewed scientific papers to be NOT RELATED to subsequent performance in thoroughbreds.
Equally, while genomic technology has now allowed her to test her theory more fully, nothing is forthcoming. In fact, most of the people involved in cardiovascular measurements, as it relates to elite performance in thoroughbreds have either their own evidence to suggest that it is more likely to be something that the sire passes on (that is a consistent heart morphology that results in elite performance), thus making it impossible for the genes controlling heart morphology to lie on the X-Chromosome, and those that have both cardio and genomic data have also found that heart size as it relates performance does not appear to be related to a gene(s) on the X-Chromosome.
That is not to say that there are not genes on the X-Chromosome that have variants within them that may be an influence on performance, there are, they are just not related to heart size as it relates performance.
Kudos to Ms Haun. It was a great story, weaved back in time to a maternal figure which made it all the more appealing, but it certainly wasn’t an accurate portrayal of how the X-Chromosome or cardiac morphology relate to athletic performance and has done a lot to lead breeders astray.
I understand your points, and they are well taken. Personally, I feel that Ms. Haun is responsible for following her theory to the point of ‘prove or disprove’, and has a responsibility to communicate that to the public, just as she did with the ‘X-factor’. I’d like to understand, more fully, the phenomenon of ‘Broodmare sire’ for example. How and why, if a stallion is passing heart size on to his offspring regardless of the chromosome, why would his daughters produce offspring that perform better than his son’s offspring? Perhaps it relates to the interplay of genes, and the dominance of one allele over the other, both chromosomes playing a part. After years of study, I have explored that question in several other posts in this blog. Conformation is highly inheritable. Why wouldn’t heart size be expressed in that sense, more like height?
X chromosome or not, I think ‘X-factor’ is a great term for a heart the size of a roast turkey. I mean, think about this: an 8.5 pound heart compared to a 19 or 22 pound heart. At more than twice the average size, without any pathology, that big heart has to relate to performance. It has larger vessels (aorta for example) delivering a (possibly) larger volume of oxygenated blood to tissues – removing waste products just as efficiently. Perhaps 1 beat of that larger heart would do the work of 1.5 – 2 beats of the average sized heart. The limiting factor in this case (excluding basic conformation and temperament), would be cellular metabolism, AND the apparatus for air exchange: from nostrils to lungs…
Perhaps Secretariat’s ‘Belmont’, was more related to the fact that his ‘system’ could rid itself of CO2 more efficiently, staving off the effects of fatigue. It has been noted that Sham also had an enormous heart, but perhaps was lacking volume in the intake/exhaust tube, aka, trachea. Don’t know, and it is good to remind readers and breeders, that heart size is only one of many important conformational attributes. Wouldn’t make any sense to have an elite super engine in a chassis with weaknesses…that chassis would suffer…
And while this is all good, and you clearly are a Professional and making your living in the Industry, there are so many people – fans and enthusiasts – that are simply enthralled with the idea of a super heart. I realize that it’s more of an emotional approach, touching on ‘intangible qualities’, but this is where the Thoroughbred Horse serves as an inspiration for the human spirit. That horse, that very special horse that causes tears to overflow their bounds – that horse that stands magnificently, sides heaving and sweat dripping, that horse who puts his/her ‘all’ into performing for the needs of people, that horse who annihilates our expectations and shows us how to be brave – how to succeed – how to triumph – even in death – is what they are seeking to embrace. It’s an eternal romance with the heart of the horse that will never go away…we want to pick ‘em and see ‘em win at the very least.
Many of my readers are fans, a few are movers and shakers in the Industry. Thanks for taking the time to post your thoughts, it is greatly appreciated, and adds valuable perspective.
I am from England and breed racehorses, they are like humans in many ways particularly confirmation and constitution, in the human race I see weak people in mind and body, one could argue that the human race especially in the western world, has ceased to evolve as weak breeds to weak and we treat every ailment with medical processes, the racehorse is evolving in many ways as we select the very best mares and stallions , I doubt a racehorse champion off 100 years ago could live with modern champion.
Arkle was a famous Irish Thoroughbred racehorse. A bay gelding by Archive out of Bright Cherry, I was around at the time he was racing, a machine who could jump 30 enormous fences over 3 miles and carry 12 stone + he was the greatest ever Steeple Chase horse , his heart was removed by the stud when he died and it was described as enormous and I believe it is in a museum in Ireland. You dont need science to tell you what I saw and see every day in humans and horses.
You make excellent references Keith. For those who aren’t familiar with the weight conversion, 12 stone equals a whopping 168 pounds. I think we’ve dubbed some individuals ‘Iron Horses’, having carried less weight over shorter distances, and not having propelled themselves over a single obstacle. One has to wonder about just how much we’ve stripped from ourselves and our animals, as the rigors of survival are continuously lessened. We live less than ‘natural’ lives, toil less, and increase emotional and psychological stress without appropriate outlets.
Horses used to live off the land, and do did we. Now we buy processed food and sit around too much…our horses are contained by walls and fences, and supplied with their nutrition. In some cases, the best nutrition money can buy. But what of it? Why were the horses so much tougher back in the day? It’s worth thinking about…