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Large Hearts on the Triple Crown Trail
ThoroFan, America's best
racing fan organization has a
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5TH ANNUAL LIVE
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Does the large heart factor play a role in the Kentucky Derby? Is it another useful tool for handicappers or just an interesting notion
that, while entertaining, is a theory better suited to the breeding shed? What do horsemen mean when they say, “that horse has a lot
Let's delve into the components behind these questions.
Unlike humans, a large heart in a Thoroughbred is a good thing. It gives the horse greater stamina and strength. This particularity is
passed amongst Thoroughbred families. The large heart gene is also known as the X-Factor, because the gene is located on the
x-chromosome. Therefore, a colt can only inherit an oversized heart through his dam, and a stallion can only pass on the gene to his
The exact heart size seems to vary from horse to horse. Multiple factors probably contribute toheart size, and the x-factor gene,
being a mutation itself, probably works in conjunction with these; it "supersizes" the heart based on the existing blueprint, so to
The only reliable measurement to determine if a horse has a large heart is to do an ECG of the dam/foal to see if they carry this mutation. The large heart has a
50/50 chance to be passed along to each foal. Therefore, one could have it, while the rest don't. It's a roll of the genetic dice. Other ways to determine (not as
reliable) are to examine the pedigree for large-heart ancestors, winning traits of the family, and some physical aspects.
Pedigree: X-Factor stallions pass the gene to their daughters, thus
becoming good broodmare sires. Secretariat, perhaps one of the most
famous stallions noted for his large heart, was noted as one of the
greatest broodmare sires in modern history. Some mares are "Double
copy" mares, receiving the gene through their sires and dams. For
instance,Secretariat's daughter Weekend Surprise was a double copy
mare, as she received the gene through her sire and damsire,
Buckpasser.Weekend Surprise's dam Lassie Dear and her second dam Gay
Missle (by Sir Gaylord) were also carriers. The large heart gene expressed
itself in Weekend Surprise's sons A.P. Indy and Summer Squall.
Addidtionally, Lassie Dear's daughter Charming Lassie (by Seattle Slew)
passed along the x-factor gene to her son Lemon Drop Kid.
Weekend Surprise's pedigree chart. Note red lines are single copy while blue lines are double copy.
A family's racing ability can also be a clue to whether a horse may have the
X-Factor. For instance, if a mare who carries a copy (or double copy gene)
has ten foals and all of them are winners, she is likely passing along a
large heart gene. If only three or four of her ten foals win, she probably
isn't passing the gene along.
Also, the X-factor gene can be recessive in a mare. Therefore, she may
pass the gene to her babies, but it won't be expressed until her offspring
have progeny of their own. That's one of the reasons we see talent skip a
generation or two. This is how that comes about. A mare with few winners
in her first two generations carrying a recessive heart gene is mated to a
stallion with the large heart gene. Surprise - here's a stakes winner. It isn't
always that simple, but it does explain why one runner out of ten bred
from a mare will be a superstar while the rest languish in claiming races.
Physical Aspects: There is one unique physical characteristic found among horses with large
hearts. That is small, curly ears.Secretariat and Zenyatta are noted for their curly ears. Not
only is Zenyatta a super-mare, but she's also a double copy mare, receiving the gene from her
sire Street Cry (whose entire distaff line is X-factor) and Vertigineux, who receives it from
both her sire Kris S. and dam For the Flag. If Zenyatta passes along the X-Factor gene to her
sons, they should become excellent runners and sires. Zenyatta's yearling colt resembles his
dam and also has her curly ears. Unless Zenyatta's daughters receive the gene from their sire,
it is likely that they won't emulate their dam on the racetrack, but they will pass the X-Factor
along to their sons.
Zenyatta's yearling son
X-Factor & Handicapping: So how does the X-Factor work to the handicapper's advantage?
Nothing is ever mentioned regarding large-hearts and handicapping because it would be
impossible to include the info in the past performances. Not every good racehorse carries a
large heart. Certain Thoroughbred families carry the large heart gene.
Kentucky Derby Contenders: That brings us to the Kentucky Derby. How many winners have carried the large heart? Reviewing the pedigree charts back to
1970, it seems that the the trend comes and goes. Following are the years that, according to their pedigree charts, horses with the X-Factor won the Kentucky
Derby. Note that there is no more than three years between winners who may carry the X-Factor gene.
The gene can be passed from father to son and from mother to daughter, but only generates in the opposite sex. Therefore, a dam may pass along the trait to
her daughters, who in turn would bear sons with the large heart gene.
Keep in mind that heart size alone doesn't guarantee a stakes winner. A horses biomechanics - gait, stride length, conformation, power, health and training all
play a part in their ability. However, if the large-hearted horse has these other factors in their favor, there's a strong chance that they'll be a tough customer in
Review the pedigrees and racing backgrounds of the contender's families. Also, look for a leaf-like “curly” appearance to the ears. A horse doesn't need to be a
statuesque 17 hands high to carry the large heart. Physical size doesn't matter. War Admiral was a small horse, but he carried the double X factor from his sire
1973 – 1975 – yes
1976 - no
1977 – 1981 – yes
1982 – 1983 - no
1984 – yes
1985 – 1986 - no
1987 – yes
1988 – 1989 - no
1990 – yes
1992 - no
1993 – 2000 - yes
2003 - no
2004 - yes
2005 – 2007 - no
2008 – yes
2009 - no
2010 - yes
2011 – 2012 - no
Friday, May 17 8:30 PM EST