Pocahontas and the Large Heart

X Chromosome and Sex Linked Genes

Before we get started on this interesting subject let me address the x-chromosome issue. We all assumed sex-linked characteristics must be present on the x-chromosome...this trait if nothing else shows that assumption may be wrong. The large heart factor appears to travel on mare lines, like Pocahontas, down to their descendants...some say it is not on the X-chromosome, but the mystery still remains for the evidence is it travels down in a sex-linked manner. So, lets begin with the discovery of this factor and then follow up with the controversy.

When a equine writer and a group of scientists and universities teamed up to explore the interesting fact that some Thoroughbreds have a measurably larger heart and that it seemed to travel in a sex-linked manner the report has enthralled us ever since.

In 1997 a free lance writer who specialized in racing Thoroughbred articles, rocked the equine world with the publication of her book: "The X Factor". Marianna Haun's love of the magnificent Secretariat had led her to do extensive study of his pedigree, and she took note of a few things. It appeared to her that Secretariat's huge heart actually came down the genetic trail from a mare named Pocahontas.

Secretariat- 7 generation pedigree

In her research she collaborated with veterinarians, genetic researchers and breeders. The researchers were aware of the huge size of Secretariat's heart because his autopsy had revealed it to be nearly three times a normal Thoroughbred heart size. It was a whopping 22 lbs! And Marianna thought she might know how he acquired it because of her pedigree study. The result of this breakthrough was one of the most fascinating discoveries in recent times for students of equine genetics.

Pocahontas- 7 generation pedigree

It has been known for over a hundred years that some Thoroughbreds have larger hearts than the average. When good race horses died it was a common practice to bury their head and heart, and in so doing, people took note when they came across a huge heart. It is also known now that generally these big muscle pumps (large size heart) are not pathological- rather they are normal functioning but are just about twice as big: 12 to 20 lbs., as compared to the normal Thoroughbred heart which ranges between 7.5 and 8 lbs. As an illustration, it is sort of like one car with a standard 4-cylinder engine compared to the same size car with a turbo V8. It is a bigger engine in the chest.

The researchers noted that the large heart is sex-linked, it only travels on the mare lines. Because of this they assumed that it must reside on the x chromosome, which is the female contribution, mares have two x chromosomes, one each from their sire's dam and her own dam, and they pass one of their x's to each of their children. Stallions get their x chromosome from their dams. Stallion daughters get one of their two x chromosomes from their sire and one from their dam. Colts cannot get the x chromosome from their sire, they get the y instead- which is what makes them male. 

[Note: this work was done before the equine genome was mapped.]

The geneticists believe the large heart trait may be a mutation, and because of this belief they told Marianna that it should be traceable back to a single ancestor. The studies which were then carried out revealed that Marianna was right about Pocahontas, and they were able to follow the large heart trait in the Thoroughbred, back to the stallion Eclipse, most usually through his descendent: Pocahontas. Pocahontas is linebreed, 7x7, to Everlasting, a daughter of Eclipse, once from her dam and once from her sire's dam.

Everlasting Pedigree

Further study has traced the large heart gene all the way back to Hautboy, one of the very earliest Thoroughbreds, but they have lost the genetic trail after that. Hautboy's got his large heart gene from his dam, who is only named "Royal Mare". Early bloodline research is difficult, and there has been confusion about what the Royal Mares were. Alexander MacKay-Smith cleared up a lot of this by going over all the original records, and he found that the Royal Mares, and there were over one-hundred, were either bred at Sedbury Stud or gathered in there from the scattered remnants of the Hobby and Running Horses from previous racehorse studs, usually by Lord D'Arcy and his relatives. Close review of the old records show that they were not imported as is so often stated, but that they had been bred from the English Running Horse and its forerunner, the Irish Hobby, with some Oriental blood added. He published his findings in his ground breaking Speed and the Thoroughbred, a fascinating and excellent read.

Eclipse, he gave the Large Heart to all his daughters

Eclipse's heart, when he died in 1789, was weighed and found to be 14 lbs, over twice the normal Thoroughbred size of that era= 6lbs (Haun). Eclipse got his large heart from Spilletta, his dam, who carried it on both her damlines. Spilletta got one of her large heart genes from Regulus (her sire), it came up his dam line from Hautboy. Spilletta's dam line also goes straight to Hautboy. Having the large heart gene on both her x chromosomes meant she gave it to all her progeny."

Pocahontas and her foal Stockwell

Pocahontas, died at the advanced age of 34, and produced babies for 20 years. Her stallion sons: Stockwell, King Tom and Rataplan are found in most Thoroughbred pedigrees in multiples. This single mare has probably had more influence on the Thoroughbred breed than any other single horse since the Spanker Mare. 

The team that worked on this research took heart measurements, and were able to connect specific sizes to certain bloodlines. From this they have determined thus far that there are four main conduits of the large heart gene in the modern Thoroughbred- all premier broodmare sires who gave their heart gene to their daughters only. And the researchers determined that approximately 28% of the Thoroughbred population carries this trait. (The large heart is also found in the Standardbred and the Quarter Horses- America's other racehorse breeds).

Sport horses and any other breed that has Thoroughbred may also carry this trait, and if your horse has unusual stamina, longevity and vitality, this is a strong indication that it may carry the large heart.

The four major modern carriers: Princequillo- found to transmit the very largest heart of them all, this is the heart Secretariat was running on. He received it from his dam: Somethingroyal, a daughter of Princequillo. War Admiral- the second largest variation of this gene, this comes to War Admiral from his dam: Brush Up, a daughter of Sweep. The great champion Seattle Slew ran on this heart and this same heart was found in the champion Whirlaway who received it from another daughter of Sweep. Blue Larkspur- almost as big as the War Admiral heart, the great Roberto conquered the Europeans with this heart. Mahmoud- The English Derby winner, gave his large heart to Northern Dancer.

Since the original studies, other research has identified additional avenues to the large heart besides Pocahontas, who still remains the principle source. One of these is the stallion Sterling who goes back to Everlasting also. The broodmare of the century: La Troienne (Teddy daughter- which makes Teddy a carrier also) has multiple lines to Eclipse and Everlasting, some through Pocahontas and some through Sterling.

La Troienne 7 generation pedigree

Heart size is not a guarantee of a champion, but it sure helps. Haun 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."

The same applies to our sport horses, if we are fortunate enough to have the large heart gene in our horses, it is not a guarantee of their success in sport- but it is a nice edge. .

To this day Pocahontas has a profound effect on pedigrees. For instance the Fair Play-Rock Sand nick owes much of its genetic power to the seven lines of Pocahontas it contains.

Back to the Mare


This article was first published in 2005 on one of my earlier websites: Sport Horse America. Since then it has been re-written in 2007 for this site, revised in 2012, and in 2017 added a few things. But now 2018 because of the tremendous controversy about this "gene" or "trait" I have edited the text and think we should talk about this a bit.

First, Marianna Haun has passed on 2/12/16 after a long illness. Her body of work stands today as her monument, and we in the equine community are much richer for all she contributed to our knowledge of sporting traits.

Many years after her books came out some were saying that she was wrong, that there is no 'large heart gene' and that her work was too simplistic. Marianna Haun and her team, which was comprised of some of the best veterinary and genetic research people available then - and even now, did ground breaking work, and that work opened the door for more research, and since then many dedicated scientists and researchers have built on the base they provided. This is to be expected, it is always for others that come after the ground breakers to enlarge the path, and clear the debris and branch out. For those of you who have chanced on these blogs and become discouraged, realize the work they did was excellent, there is a large heart that travels down the genetic trail through specific ancestors in a sex-linked manner, whether there is one gene or many or several other factors involved.

Here is a well written piece by pedigree expert Anne Peters that suggests all should just throw Marianna's work into the trash can. I can understand her frustration, but cannot agree with her dismissal of this work. If you go back the article you will see I quote Marianna saying this trait will not guarantee a successful racehorse and that many other factors come into play. Yet Anne must have missed that point, for she criticized saying that was the message. And that Marianna, a writer not a breeder, did not do a field test of any significance in her  breeding program on this trait is an unfair point to dismiss her work in my opinion. I also like to test my work, but am limited in this by many factors, as I am sure Marianna was.  The fact is hundreds of horses were indeed tested and measured for this trait, done in a responsible and approved way at universities, which really does establish the trait is proven. It just upsets me a little to have her work which was done in tandem with many reputable scientists, veterinarians and universities, be kicked to the curb with such put downs.

So now the cry is it is a fraud, or a myth. How the heck can we learn anything with such closed and fearful minds? It is there, they measured it over and over, it travels in a sex-linked manner, and certain bloodlines are proven carriers of it. Back in 1977 Dr. Stewart and Dr. Steel published a paper that outlined that the heart size is determined by the female, not the male and that it travels in a sex-linked manner. That science has not discovered how this occurs is par for the course. Science is a work in progress, so its not on the x-chromosome you say, well explain how it is there then. 

Chantal Spleiss at MagicMatchGenetics has suggested a solution to me that I want to share with you about the science dilemma that the issue of the large heart gene presents. As mentioned some scientists have said there is NO large heart gene on the X-chromosome, but then we have the evidence that Marianna's team found: that it is indeed a sex-linked characteristic and it does travel like a X-chromosome line, and it can be measured and quantified in the horses that carry it. 

Chantal explained to me there is a mysterious (at least to me) genetic process where another factor called epigenetics comes into play and can over ride the typical gene functions, so that this factor could effect the normal heart size dictated on the X-chromosome by enlarging it. And there may very well be some other genetic path as well, that has not been discovered yet. None of that changes the fact that the large heart is alive and well in 28% of the Thoroughbred population. We must remember all  the scientific studies are progressing, and here we are again with an example that there is more to learn. We know his factor exists...it can be measured...it travels in a sex-linked fashion...it is passed by identified bloodlines...how these facts are done genetically is still only partially known.

Marianna wrote her books in the late 1990s, and at that time the equine genome had not been mapped completely. She was operating under the traditions we all were at that time, and had been taught like the rest of us, that the early pre-TB studs were populated with Arabian mares, called Royal Mares. And it followed then when she traced the large heart gene back through the ages and ended up with Hautboy and his Royal Mare dam, and so she assumed his dam was an Arabian mare, and then of course that the large heart gene came from that desert breed. 

Later DNA studies by Dr. Bower and her team, published in 2010 surprised us with the proof that those early mares were not Arabians but instead the humble 'native English mares'. Then in 2012 Dr. Hill and her team traced the 'speed gene' back through mtDNA to one of those mares as well. So this trait resides with the native Celtic Horse of the British Isles.

All these groups, Marianna's team, that of Drs. Hill and Bower, all look at these wonderful sport traits as mutations because of the primitive horse theory they are working from (see DNA Studies), and then further assume because their theory demands it be a mutation, that it only occurred once.

So here we are 40 years after Drs. Steel and Stewart noticed the heart traits come from the female and 20 years after Marianna and her team worked on the large heart factor, and still there is some mystery. 

Root Large Heart Mares

Speed Gene

Stamina Gene

Gait Keeper Gene