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. So, lets begin with the discovery of this factor and then follow up with the controversy.
How can a mare born in 1837 still be important one-hundred-seventy years later? Her pedigree holds the key. This mare, Pocahontas, is the main conduit of the large heart gene- a sex linked gene, which traveled on both her x chromosomes to the champions of today.
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.
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 20lbs., 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 large heart gene is a sex-linked gene, it only travels on the x chromosome. The x chromosome 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.
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.
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 alot of this by going over all the original records, and he found that the Royal Mares, and there were over one-hundred, were bred at Sedbury Stud, 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 x chromosomes. 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.
We have been concentrating on Pocahontas's remarkable value as a deliver of the large heart to the Thoroughbred gene pool. If you take a moment to go back to her pedigree you will find several full and 3/4 siblings. One of these dynamic combinations is the Gohanna Mare in the 4th generation on her sire side and her 3/4 sister Harpalice in her dam's 2nd generation, and they are represented by a son and daughter- perfect. This close up and powerful filly factor insured she would be a valuable broodmare, and these two lines are also her x chromosome lines--therefore the carrier of the large heart. Her dam side has the 3/4 siblings Eleanor and Marmion 3x3 sex balanced. And she has another strong filly factor in the two full sisters Pyrrha and Dux 6x6. She has another stong filly factor in two daughters of Diomed 4x4. There is much more as you can see, but this pedigree is about as good as you can get for a broodmare, and so an excellent example to study (see Understanding Pedigrees).
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.
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 now in 2017 I want to add a few things.
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 veterninary 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.
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. We tend to forget that science is a work in progress and many times genetic traits are not fully explained yet.
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. We must remember all the scientific studies are works in progress, 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 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, that 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.
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 it only occurred once.
Historically we know the early racing studs of England, those from which the Thoroughbred emerged, were made up of Hobby and its descendant, English Running Horses. These remnants from the previous purging by Cromwell and his cohorts, were regathered by D'Arcy into his stud to begin to breed racehorses once again. (Alexander MacKay-Smith, C.M. Prior). Also James R. Hardiman has published his work on the early bloodlines on his Highflyer site , and he found other than one imported Barb mare (Sedbury Mare) and her two daughters bred in England, that all the original mare families were the old English stock.
So we have a nice convergence here of science and history agreeing that the racing factors of the large heart and the speed gene all come down the genetic trail from the Hobby based stock. The same pre-Thoroughbred stock that populated our own colonial racing studs here in America. Speed and sport ability all arose from those talented ancient Celtic Horses.