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Heart of the Matter .. Part 1

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Research indicates that the female X chromosome is responsible for the large hearts found in outstanding racehorses

Part 1 of 2

... by Marianna Haun
 Author Biography

Click here to go to Part 2

This article first appeared in the Quarter Racing Journal, the monthly racing magazine of the American Quarter Horse Association. To subscribe, call 800-291-7323. Click here to go to the Quarter Racing Journal subscription page on the World Wide Web.

Heart Score | Research | Big Hearts & Big Money
Large QH HeartsX, Y and other definitions
The heart line in Quarter Horses

Ever since the first rider challenged another rider to see who had the faster horse, horsemen have described the fastest runners as having "great heart." The term originally was meant to describe a personality characteristic – something real, but immeasurable, the ability that made a horse dig deep and find the courage not to give up until he crossed the finish line victoriously. Witness SLM Big Daddy and Dashing Perfection’s showdown in the May 10 Remington Gold Cup (G1).

Today, thanks to the truly great heart uncovered at the autopsy of the amazing Thoroughbred Secretariat, it is known that great heart really is a physical description of an extraordinary heart found in certain Thoroughbreds, Quarter Horses and Standardbreds that has passed down the female line on the X chromosome from the legendary Eclipse, foaled in England in 1764.

Following the tradition of burying just the head, heart and hooves of great racehorses, Eclipse was cut open by a London surgeon after his death in 1789. The heart found inside of Eclipse was so much larger than other horses that it was weighed. Eclipse’s great heart weighed 14 pounds, more than twice the normal weight of hearts of horses of that era – approximately six pounds.

Today, the normal weight of a horse’s heart is 8.5 pounds. Even though Secretariat’s heart was not weighed at autopsy, Dr. Thomas Swerczek, head pathologist at the University of Kentucky, estimated it at 22 pounds after finding the second-largest heart in Sham (Secretariat’s Triple Crown rival) and weighing it at 18 pounds.

"I have done thousands of autopsies, and I had noticed differences in heart size in horses before we did Secretariat," Swerczek said. "I had picked up the difference in the male and female hearts and noticed that some were bigger than others.

"But I didn’t pay much attention until Secretariat came along. He was completely out of everybody else’s league. Looking back at what he had done, it was easy to put a connection to it. The heart was what made him able to do what he did. It explained how he was able to do what he did in the Belmont Stakes – a mile and a half race (Secretariat won by 31 lengths in track-record time). You would have to have a large heart to do what he did. It would be impossible for a horse with a small heart to do that."

Recalling the moment Secretariat’s heart was uncovered, Swerczek said, "We just stood there in stunned silence. We couldn’t believe it. The heart was perfect. There were no problems with it. It was just this huge engine."

For more than 40 years, Australian researchers have studied hearts found in racehorses and have demonstrated a strong pattern of performance correlated with large hearts. The heart of Australian’s greatest runner, Phar Lap, weighed 14 pounds. It is encased in glass in a museum in Australia.

But when Australian researchers tried to relate their findings to U.S. veterinarians in the 1960s, they were laughed at because they couldn’t go out and pick out horses with large hearts. From their studies, they felt that large hearts represented a sex-linked characteristic, with the main influence coming from the dams, but they didn’t know where that characteristic came from.

X, Y and other definitions:

Genetically, there are two sex-linked chromosomes in each horse. Males have an X chromosome, which they receive from their dams and pass to their daughters, and a Y chromosome, which they inherit from their sires and pass to their sons.

The Y chromosome is much smaller than the X chromosome and carries less inheritable material. It is basically a sex determinant and when thrown into the mix at the moment of creation makes that individual a male. If the sire contributes an X chromosome, then the resulting foal is a female.

Because the large heart characteristic is on the X chromosome, large-hearted stallions that carry only one X chromosome, which they receive from their dam, can only pass on the large-hearted X chromosome. In the case of their daughters, they carry their sire’s X chromosome and one of the two X chromosomes carried by their dam.

Whichever X chromosome is dominant is the one that is expressed. 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 the large-hearted X chromosome or the small-hearted X chromosome. Depending upon whichever X chromosome is dominant, that mare might express either a large heart or a small heart.

X Factor – Term that geneticists use to describe the female X chromosome, which is attributed with creating the large hearts found in outstanding racehorses.

Heart score – Term coined by Australian researcher Dr. James Steel to communicate his findings of the heart size on the electrocardiogram, which was gained by a correlation of heart weight, stroke volume, cardiac output and aerobic power.

Double copy – Describes the ideal broodmare, which has the large heart on both X chromosomes. If the mare is bred to a large-hearted sire, she always will produce large-hearted foals. Her daughters also will be double copy mares. Double copy mares frequently produce all winners.

One double copy mare is the Thoroughbred Weekend Surprise, a daughter of Secretariat that is out of a double copy dam. Weekend Surprise’s dam, Lassie Dear, produced all winners and so has her daughter, which produced Horse of the Year A.P. Indy and millionaire Summer Squall. Both sires now are producing outstanding daughters, and when mated with large-hearted mares, are producing outstanding sons.

Champion Corona Chick is another double copy mare. She carries the large-hearted X from her sire, Chicks Beduino, which goes to Eclipse through his maternal granddam, Mayshego. On her bottom side, through her dam, Sizzling Lil, Corona Cash has heart lines through Leo and the super heart of Blue Larkspur (TB) through her tail-female line.

Corona Chick’s daughter champion Corona Cash also will be a double copy mare because she is by the large-hearted sire First Down Dash. Corona Cash should prove to be a good producer like her dam.

Corona Cartel, Corona Chick’s son that earned $557,142 before entering stud this year, also should have a large heart. His daughters should be outstanding, and his sons – if he is bred to large-hearted mares – also should find the winner’s circle.

Single copy – Describes a broodmare that carries a large heart on only one of her X chromosomes.

A famous single copy mare that expressed the large heart, but carried the small heart of her sire on her other X chromosome is Kentucky Derby winner Winning Colors. She was measured and found to have the large heart coming from her maternal granddam, Miss Carmie, which carried one of the four largest heart lines from the famous broodmare sire Blue Larkspur. Winning Colors’ sire, Caro, was found at autopsy to have a small heart.

Breeding Winning Colors to a sire with a normal heart size would increase the possibility of her producing a foal with a small heart.

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.

After months of intense pedigree research, I found a genetic connection on the X chromosome from Secretariat’s large heart to Eclipse’s large heart that was found at his death exactly 200 years before. Most large hearts tracked through a mare named Pocahontas, which was foaled in England in 1837, and went to the same daughter of Eclipse, Everlasting, both through her sire, Glencoe, and on her tail-female line.

A few horses track to Eclipse through other Glencoe daughters and through Lexington daughters. The great sire, Domino, carried the large heart of Eclipse through the Glencoe mare Judith and passed it on through his daughters. Lexington, in a recent discovery, was found to descend from an unnamed daughter of Eclipse that was foaled in 1785.

This connection was true for all large hearts found in Thoroughbreds all over the world. Because the breed was founded in England, which for centuries had widespread colonies around the world, Eclipse’s great heart was shipped all over the globe through his daughters. Glencoe, which came to Kentucky after siring Pocahontas in England, spread his large heart through his daughters in America at the same time that Lexington was passing on his heart in America through his daughters.

What makes a "heart score"

Because of the discovery of the importance of the X chromosome in heart size – called the "X Factor" – researchers at the University of Kentucky have been conducting a study to look for the genetic marker for this characteristic.

In the project, more than 400 horses have been measured using an electrocardiogram to measure their heart size and determine a heart score, such as what was used in Australian research. Some 40 years ago, Australian researcher Dr. James Steel coined the term "heart score" to communicate his findings of the heart size on the ECG, which was gained by a correlation of heart weight, stroke volume, cardiac output and aerobic power.

Australian studies rank heart scores in horses according to size. Heart scores of 103 and below are considered small. Heart scores from 104-116 are considered normal. Female hearts from 117 and above are considered large. Male hearts from 120 and above are considered large.

In our study of more than 400 equine hearts, we found heart scores from 100-160. In terms of heart weight, that translates to 6 1/2 pounds to more than 15 pounds. That is quite a range in heart size.

The largest heart in our study, so far, was found in Thoroughbred champion Key to the Mint. He carried the Princequillo heart and had a heart score of 157-160. We also measured an outstanding daughter of Key to the Mint’s that has produced two millionaire daughters, and she had the exact same size of heart as her sire. Most of the Princequillo hearts were between 147-160. The other three super hearts – War Admiral, Blue Larkspur and Mahmoud – were between 140-150. Hearts of 140 and above are considered very large and are generally found in very successful racehorses.

Although we don’t have a heart score on Secretariat, based on his estimated heart weight at the time of autopsy, his heart score would have been around 180. Even in horses carrying the stallion’s heart, we have not found a heart close to that size. It might be that Secretariat, in his heart as well as in his other physical characteristics, was an anomaly. But if his heart hadn’t been so far out of the norm, we never might have discovered what we have about this genetic mutation that produces a high-performance heart.

Big hearts can equal more money

The heart grows until a horse is four years old, and training can add a few points through exercise. While a large heart will not guarantee a winner, studies have shown that it represents approximately 25 percent of what makes a champion and is a consistent characteristic. If a horse has a bad attitude, poor conformation or lousy training, a big heart won’t help. But if everything else is in line, a large heart can mean a super horse.

In addition to a pattern of performance, there also is a positive impact on earnings with larger hearts. In scientific studies of trotters conducted in Denmark by Nielsen and Vibe-Petersen of the Royal Veterinary and Agricultural University of Copenhagen, a positive correlation was shown between earnings and high heart scores. In the study, the 41 stallions with heart scores above 115 earned more than double what the 81 stallions with heart scores below 115 earned.

In Standardbreds, the large heart line to Eclipse traces to a mare named Midnight, which was foaled in 1865. She was out of a Thoroughbred mare that was a daughter of Lexington that was out of a daughter of Glencoe, sire of Pocahontas. This mare and two other mares that descend from her on the tail-female line have had a profound influence on the best American trotters. Their heart line can be found in the pedigrees of the highest earners in the breed, including Peace Corps, which earned more than $4.9 million.

However, one interesting fact found in the University of Kentucky study is that the size of the horse doesn’t seem to have any relation to the size of the heart. The largest hearts aren’t necessarily found in the largest horses.

Research supports the X Factor

After measuring more than 400 horses with an ECG, internationally renowned equine cardiologist Dr. Frederic Fregin found the X Factor to be 100 percent consistent, a rarity in scientific research. We have been able to track the large heart from sire to daughter to granddaughter and grandson and to great-granddaughter and great-grandson.

The heart scores on certain lines have been so consistent that we were able to determine which X chromosome was expressed and to identify the four largest "superhearts" found in today’s pedigrees. As mentioned earlier, these four heart lines come from Princequillo, the largest, and from War Admiral, Blue Larkspur and Mahmoud.

While all large hearts track to Eclipse, some lines, whether from genetic modifiers, natural selection or pattern of breeding, have even larger expressions of the large heart found in Eclipse.

What this means for Quarter Horses

Because Quarter Horses long have been crossed with Thoroughbreds, great Quarter Horse runners can have the same great heart as found in great Thoroughbreds. Today, Eclipse’s mighty heart is passed through a number of sires.

The most successful Quarter Horse sires that pass on the large heart have large-hearted Thoroughbred lines on their bottom sides. Thoroughbred heart lines on the top side won’t do any good because the characteristic isn’t on the Y chromosome, which comes from the sire.

In March, Fregin measured two outstanding representatives of both breeds that reside at the Hall of Champions in the Kentucky Horse Park near Lexington. The Thoroughbred was John Henry, a Horse of the Year that earned more than $6.5 million. He was found to have a heart score of 147. The Quarter Horse was Sgt Pepper Feature, a world champion that earned more than $900,000. His heart score was 143.

Based on the Australian study comparing heart score to heart weight at autopsy, these horses each have hearts that weigh between 14 and 15 pounds. That is extraordinary. Both horses, which have hearts larger than Eclipse, tracked to the same heart line, that of the great Double Jay (TB). ECG tracings indicated other similarities in heart rate patterns, further enhancing the possibility that both geldings carry the same heart.

Double Jay is unique for the large heart line because he goes to Eclipse both through Pocahontas and through Lexington through the female side of Audience, the dam of his broodmare sire, Whisk Broom 2nd. Audience’s dam, Sallie McClelland, is a daughter of the great Hindoo, which was out of the Lexington mare Florence. Sallie McClelland’s dam, Red and Blue, was by Alarm, which was out Maud, a daughter of Pocahontas’s son Stockwell.

When I studied the pedigrees of leading Quarter Horse runners, I found it ironic to see a pattern that has haunted the large-hearted individual for centuries. A Thoroughbred stallion might start his stud career in the rich bluegrass of Kentucky, but when he cannot duplicate himself through his son – an impossibility for the large heart – then the spotlight quickly is turned off and he finds himself sent away, frequently to Texas or elsewhere throughout the West and Southwest. Then that great heart, which Kentucky breeders couldn’t wait for, often finds its way into the best Quarter Horses, lengthening their distances and increasing their stamina and speed with superior cardiovascular systems.

The great heart of Eclipse that was uncovered more than 200 years ago still is racing today in three different breeds. It makes sense that most of those hearts come from Eclipse’s daughter Everlasting.

The heart line in Quarter Horses

Because Quarter Horses long have been crossed with Thoroughbreds, the great heart line found its way into the sprinters years before AQHA was established. Today, Eclipse’s mighty heart is passed through sires such as:

A Classic Dash

Aforethought (TB)

Alamitos Bar

Azure Te (TB)

Beduino (TB)

Bugs Alive In 75


Chicaro Bill

Chicks Beduino

Dash For Cash

Dash Thru Traffic

David Cox

Depth Charge (TB)

Diamond Charge


Double Devil

Easy Jet

First Down Dash

Go Man Go

Hempen (TB)

Holland Ease

Jackstraw (TB)

Jet Smooth

Jonny Apollo

Lake Erie (TB)


Little Request (TB)


Mito Paint (TB)

Moolah Bux (TB)

Moon Deck

Pass ’Em Up (TB)

Piggin String (TB)

Raise Your Glass (TB)

Rare Bar

Rare Form

Rocket Wrangler

Ronas Ryon

Royal Quick Dash

Special Effort

Spotted Bull (TB)

Streakin Six

Takin On The Cash

Texas Dancer (TB)

The Signature

Three Bargains

Three Bars (TB)

Three Chicks

Three Oh’s

Tiny Charger

Top Deck (TB)

Go to Part 2

In Part 2, Marianna Haun explains using the X Factor in making breeding decisions and describes its influence on some of Dash For Cash’s offspring and other noted sires and broodmares. More about the relationship between inherited heart size and racing performance appears in her book, "The X Factor: What it is and how to find it."

Isinglass -- David Manser
Performance Horses -- How much is riding on their X-chromosomes?
First Down Dash ... His Foundation Females Didn't Hurt Him -- by Andrea Laycock Mattson
"LEO ... the All Time Sire" by M. K. Fredlund -- A complete feature on Leo with story, pedigree chart, and progeny records. 
Three Chicks, Tiny Charger, and Tiny Watch -- Photos and excerpts from "Speed and the Quarter Horse" by Nelson Nye  
Domino --  An excerpt from the book "Breeding Racehorses by the Figure System"  by Bruce Lowe, published in 1895, concerning Domino

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