On Valentine’s Day, Let’s talk about the X-Factor… THE LARGE HEART of the successful racehorse.

I don’t know that much about the TB breed.  For me, since I was little, the muscular and smaller horses (Morgans, Mustangs) caught my eye.

As I’ve grown, my penchant didn’t change.  I own smaller, muscular horses.  But I have joined the fan clubs (literally) of TBs such as Zenyatta, Secretariat, Seabiscuit and Rachel Alexander.  Why?  Well, I think it is because I admire their racing ‘heart’.

Now, I always thought I admired the ‘heart’ that one could describe as the will or the drive inside the horse.  But, now I’m coming to understand that the actual physical heart muscle of some of these TBs are the reason they were so successful – (plus their will and drive, I’m sure…).

What do I mean?

Well, for you TB fans, the X-Factor is old news.  But, for the rest of us, let me bring to you the X-FACTOR.




Many of you, including me, heard that the legendary Secretariat had a huge heart, literally.  His heart, although not actually measured during his autopsy for some reason, is now thought to have weighed approximately 22 lbs.

The normal horse heart weighs 8.5 lbs.

Since the heart is the muscle that pumps oxygen around the body, having an extra large one would be quite an advantage when running long distances.

So, was he an anomaly?

Yes and no.

Yes, he had an exceptionally large heart.  But, was/is he the only one?  Probably not.

In fact, writer Marianna Haun thinks she has discovered the gene for the ‘great equine heart’.  She named it the X-Factor.

Large heart



Marianna Haun is a member of the National Turf Writers Association and has covered the racing industry since 1992,  first as a staff writer for the Thoroughbred Times for two years, and now as a free-lance writer for the Associated Press.

But, Marianna has that ‘x-factor’ of another kind… the inquisitive kind.  The huge hearts of Secretariat and his arch rival, Sham (18 lbs), led her on a personal quest…  Could these large hearts contribute to racing wins?  Is it no coincidence that these two horses, and Phar Lap (14 lbs), had huge careers that paralleled their heart muscles?  Marianna was determined to find the gene.

And, she did.

Amazing.  She didn’t do it in a lab, she did it on paper.  Hours and hours and hours of poring through pedigrees to find the lineage.  She found the trail to the great heart without the smoking gun, so to speak.

The large heart traced back to a sire named, Eclipse and his daughter, Everlasting.  They lived 200+ years ago in England.  You can read the details here.





You’d think the stud would be important in passing on the great heart.  And, he is… but not as important as the dam.

You see, the large heart genetic market lives on the X chromosome.

Males have only one X chromosome.  Females have 2.

So, if you have a male who has the large heart marker on his one X chromosome, he cannot pass it onto his son since the son gets his Y chromosome.  The sire can only pass this great heart marker onto his daughter.

But, if you have a female who has the large heart marker on BOTH of her X chromosomes, you have 100% rate of passing it on to either her filly or colt.

So, the dam is very important in this trait.   Breeders ‘in the know’ try to match their double marked mare with a single marked stud.

Here is an excerpt written by Marianna Haun:

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.

The X-Factor



What is interesting and eye-opening about this is that for years I had heard that Secretariat wasn’t that great of a stud.  But, really what they meant is that he couldn’t produce himself in a colt.  If ‘himself’ meant his large heart, well, he couldn’t reproduce that…

It is impossible for a sire to pass on the great heart gene since he cannot pass on his heart gene (on the X chromosome) to a male offspring.  His male offspring can only receive his Y chromosome.

But, he could help create great-hearted daughters through his X chromosome.  Which he did.

After the great-heart marker was discovered, sires like Secretariat became known as ‘great broodmare sires’.  They could pass on their great-heart gene to their fillies.  And, if crossed with a double marked, large hearted mare, the progeny would be guaranteed a large heart.

Another excerpt from Marianna Haun:

Because the large heart characteristic passes via the female line and the racing industry for centuries has judged a sire by his sons. Large-hearted stallions that accomplished amazing feats on the track often have been panned because they were unable to duplicate themselves in their sons. Extraordinary Thoroughbreds such as Secretariat, Omaha, Citation and Whirlaway are examples.
Man o’ War was an exception because he was lucky enough to be bred to a mare that had a larger heart than he did. That mare, Brushup (TB), was by Sweep, which also gave his high-performance heart to Dustwhirl, dam of Whirlaway.
Brushup produced Man o’ War’s greatest son, Triple Crown winner War Admiral, which has become one of the four largest heart lines in today’s pedigrees (joining the Thoroughbreds Princequillo, Blue Larkspur and Mahmoud). War Admiral’s heart is in Seattle Slew, broodmare sire of champion Cigar, and also fuels 1997 Kentucky Derby winner Silver Charm.

broodmare sire



OK, so Marianna Haun has written several articles.  Here is a link to one that will link to others.  And, she wrote two books, “The X Factor:  What it is and how to find it:  The relationship between inherited heart size and racing performance” – and – “Understanding the Power of the X Factor”.

I thought those would be very interesting reads so I tried to buy them.

Uh oh.  No can do…

I wonder why they are out of print?  I have no idea except that the few that are floating around are very, very, very expensive.  So, perhaps Marianna is using these copies to pay her way through life – and so it should be.  I mean, she did all this research, she may as well get paid.  Why should she give it away for $5.99 on Amazon which could eventually be discounted to $.99 a few years down the road, eh?

So, if you want one of these books, here is what I’ve found:


Uh huh.




Another interesting tidbit for you QH breeders…

Because Quarter Horses have been crossed with TBs for a while now to create appendix or running quarterhorses, the great-heart now runs through their blood as well.

So, I think it is a great idea to rummage around horsey garage sales and see if you can pick up either of Marianna’s books from an unsuspecting owner.  I’m sure they are around… Wouldn’t that be a find?!

For me, I WOULD LOVE to read her book.  Totally.  I’m so into genetics, I would find it fascinating.  But, I expect I’ll just have to resort to her few articles.  And, to be honest, those articles are fairly heady so I think Marianna gave away much of her studies in those reviews.  I’m sure she wants the breed she loves to prosper.  So, I’m sure one can find out the information if they dig into her writings.

And, she’s on Facebook…. Maybe drop her a line, eh?


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HORSE AND MAN is a blog in growth... if you like this, please pass it around!

5 comments have been posted...

  1. Harold Matthews

    Eclipse was a stallion, he carried the large heart gene on his X gene which he got from his mother,and passed it on to his daughters on his X gene Since the sons get the Y gene from the sire it is not passed by the sire on to the sons, only the daughters. The daughters on the other hand can pass the large heart gene on to either sons or daughters as they get the X gene which carries the large heart gene from the mare Eclipse had a 15 pound heart. There are other sires with larger hearts which trace back on different lines The Darley Arabian. Byrley Turk, Goldophin Arabian had standard size hearts The large heart gene originated in mares before these studs

  2. Dee

    some large libraries have that book available…. might be worth a check if you really want to read it. I found it at a University library in Illinois…and there are several other libraries, including in California that have it.

  3. dawndi Post author

    Carol! You are correct! Thank you for the catch! I’m mortified by my hasty negligence here. All fixed, thanks to you!

  4. carol in utah

    I thought Eclipse was a stallion…foaled 1764…eclipse award named after him…x-factor traced to mare named Pocahantas in Britain…will have to re-read Ms Hahn articles again…confused

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