Pocahontas and the Large Heart

X Chromosome and Sex Linked Genes

How can a mare born in 1837 still be important 170 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 3 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 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 and 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 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 alot of this by going over all the original records, and he found that the Royal Mares, and there were over 100, 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.

[2012 Update! Science has caught up with our equine scholars: In 2010 Dr. Mims Bowers and her researchers revealed that their DNA evaluation of the Thoroughbred has shown that the 'royal mares' were not arabian mares as was long believed by many, but were British and Irish 'native' mares. In 2012 Dr. Emmiline Hill and her team isolated the 'speed' gene in the Thoroughbred and discovered that it originated with those same sprinter Hobby-bred mares (native) of the early racehorse studs.]

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 4 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 4 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.

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 7 lines of Pocahontas it contains.

Back to the Mare

This article first published in 2005 on Sport Horse America, is re-written and enlarged for 2007 and now revised in 2012.

2012 update- I have been a little disappointed by a few blogs I have chanced upon that are discounting the work Marianna Haun and her team did. 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, the work they did was ground breaking, it 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, whether there is one gene or many or several other factors involved.

2013 Update: For those of you who want to learn more on sex-linked characteristics and tesio methods my book is now available: North American Sport Horse Breeder.