D. William Davidge
research concerning large heart size and the X chromosome
has spawned many articles and renewed interest in the importance of genes
carried on the X chromosome. It was
recently brought to my attention that in the early 90’s Dan Hunt wrote a
series of articles going to the X chromosome for Owner – Breeder magazine, and
there probably are many more articles and expressed theories floating around. The point that should hit home to current day breeders and
pedigree experts is the fact that all available evidence indicates that it is
certainly more than a mere possibility that the genes that control or contribute
to heart size in the thoroughbred are carried on the X chromosome.
To gain some
understanding as to how the X-factor could be inherited it is important to have
some basic knowledge of genes and chromosomes.
The genes contain DNA that forms the basis of inheritance while the
chromosomes are the vehicles upon which the genes are carried.
In each cell of the horse there are 64 chromosomes, 31 matching pairs and
1 pair that are matching or opposite depending upon the sex of the individual.
During formation of the egg and the sperm each parent contributes 32
chromosomes, or one half of the chromosomes needed.
When the egg is fertilized by the sperm each set of 32 chromosomes come
together to form 31 matching pairs and the sex chromosomes of XY for a male and
XX for a female to make a total of 64 chromosomes.
The sex of the offspring is always determined by the chromosome
contributed by the sperm because the female has two X chromosomes and she can
never contribute a Y chromosome to the egg.
The male has one X chromosome and one Y chromosome.
If the sperm has a Y chromosome the resultant foal will be male, if the
sperm has the X chromosome the foal will be female.
Because the genes
that control or contribute to heart size could be located on the X chromosome it
would be considered to be a sex-linked trait and the laws of heredity dictate
that the mutated gene for large heart size (Xh) cannot be inherited by a son
from the sire because to be a male (son) the sperm must have a Y chromosome.
If the sperm has an X chromosome the foal will be a daughter, hence the
only way that a sire can pass the Xh chromosome is through his daughters.
As a breeder or
owner of race horses one of the most important aspects of the X-factor would be
the predictability of its inheritability. It
is known that Secretariat possessed one of the largest hearts ever observed in
the thoroughbred and coupled with the genetic fact that he would pass this Xh
chromosome to all of his daughters, it is not only predictable but a genetic
fact that all of these daughters would have the ability to pass his Xh
chromosome on to their sons and daughters.
However, because each daughter also inherited an X chromosome from their
dams, the X-factor predictability hinges on whether or not the dams’ X
chromosome was a normal X chromosome or an Xh chromosome.
During the formation of the eggs chance determines which X chromosomes
becomes part of the mix and it is entirely possible that none of the eggs will
contain the Xh chromosome or, in the alternative, all of the eggs could contain
the Xh chromosome. We know that
under these conditions, two factors involved, on average, there is a 50/50
mathematical probability, however reality dictates that a mathematical 50/50
chance does not mean that every other egg would have the Xh chromosome.
The ideal predictability situation occurs when the mare is highly
suspected of having two Xh chromosomes. If
the mutated gene/genes that create the large heart are recessive in the female,
as most sex-linked traits are, any mare that expresses the large heart will be a
double copy mare (both chromosomes are Xh).
The majority of breeders believe that the thoroughbred has evolved and
improved through classic winners, however, with the exception of Seattle Slew
no American triple crown winner has sired a son who became a leading sire and
only one sired a triple crown winner. The
stud careers of dual classic winners and Kentucky Derby winners provides similar
statistics with the noted exception of Northern Dancer. On the other hand, most, if not all, of these breeding sound
classic winners live on in pedigrees through their daughters and most, if not
all, were or are proving to be exceptional broodmare sires.
Keeping in mind that superior racing performance coupled with exceptional
broodmare production are two key elements in isolating X-factor candidates, it
becomes increasingly difficult to dismiss or write-off this statistical
is elusive when dealing with the complexities of pinpointing genetic outcome and
I would only say that pedigree analysis in any form involves mostly speculation
with little or no scientific proof while providing ample statistical proof.
One only has to evaluate the statistics involving full siblings to arrive
at the conclusion that there are no sure things when it comes to breeding
thoroughbreds. Theories such as Dr.
Roman’s views on dosage, the Rasmussen factor and others utilize statistics to
bolster their relative merit but supply little, if no, scientific proof.
at the 2004 leading sires and broodmare sires lists and considering the sons of
Weekend Surprise (A.P. Indy currently #3 on the leading sires list and Honor
Grades (deceased) currently #67 on the leading sires list), Terlingua (Storm Cat
currently #5 on the leading sires list and #29 on the leading broodmare sires
list), Secrettame (Gone West currently #33 on the leading sires list and #63 on
the leading broodmare sires list), Sister Dot (Dehere standing in Japan), Six
Crowns (Chief’s Crown currently #50 on the leading broodmare sires list) and
I’m Pretty (Judge T C currently #81 on the leading sires list) may not provide
scientific proof that Secretariat passed his Xh chromosome on to his daughters
who in turn passed the Xh chromosome on to their sons, however, it most
assuredly provides further statistical data in support of the X-factor at work.
Marianna Haun’s two part article “The X-Factor - Heart of the Matter” and
Lynn K. Joris’ article “Chasing the X-Factor” one question begs an answer,
namely, is the X-Factor gene (“Xh”) dominant or recessive in the female
(mare) or the male (stallion), or both? Joris
states “The answer is that this trait is recessive in the female, but dominant
in the male, as are many sex linked characteristics…”
two most important criteria by which a recessive character can be
considered to be sex-linked are that the number of individuals with such a
character is much higher in the male sex than in the female sex and the trait is
never passed from the male parent to the sons. If Haun is correct when she stated that Winning Colors is a
single copy mare (one normal X chromosome and one Xh chromosome) who expressed
the Xh gene herself, but has only a 50/50 chance of passing the Xh gene to her
offspring, then Winning Colors demonstrates that the Xh gene is not recessive in
the female. In fact, it
demonstrates that the Xh chromosome also is passed to a daughter by her dam and
can be dominate and expressed even if the sire passes a normal X chromosome to
his daughter. Haun stated that
tests indicated Winning Colors does express the large heart and certainly her
record indicates superior performance at a classic distance. At autopsy it was determined that her sire, Caro, did not
express a large heart. This could prove that the Xh gene cannot be recessive in
the female, because single copy mares would never express the large heart in
that it would always be dominated by the normal heart gene.
In other words, if the Xh gene was recessive in the female it would only
be expressed if the female was a double copy mare or homozygous for the trait.
This would be true because the sperm from Caro had the normal X heart
gene, and it should be normal if his heart was normal size, and because he only
contributes one X chromosome to the foal, Caro would always pass a normal X
chromosome on to his daughters and consequently, if the Xh factor is recessive
in the female, even when mated with double copy mares (both chromosomes are Xh),
his daughters would never express the large heart.
The only other explanation is that although at autopsy Caro’s heart
appeared to be normal in size, he did receive an Xh chromosome from his dam, but
his expression of the large heart was slight, and went unnoticed at autopsy.
At conception, Winning Colors also received an Xh chromosome from her dam
making her homozygous for the trait, and, the dam’s Xh chromosome was
dominant, therefore Winning Colors expressed the larger heart of her dam.
The Xh gene
gives the appearance of being dominant in the male because it is always
expressed in the male when passed by the mare, but the real reason for this
expression is when the Xh chromosome is contributed by the mare to her son the
son will always express the large heart because there would be no other heart
size genes available. The X-factor
could be one of the primary reasons Weekend Surprise, a double copy mare, has
produced exceptional sons to the cover of Seattle Slew, Storm Bird and Danzig.
They all would have the Xh chromosome and the mutated gene for large
heart size. A cursory look at
Weekend Surprise’s pedigree reveals that she more than likely inherited her Xh
chromosomes from Secretariat, Princequillo’s Xh from his daughter
Somethingroyal, and Buckpasser’s Xh from his daughter Lassie Dear.
In an attempt
to answer some of the questions posed by Abram S. Hewitt throughout his
memorable Trieste “The Great Breeders and Their Methods” the X-factor could
hopefully shed some light as to the successes and what I believe to be the
primary reasons for those successes.
discusses the achievements of the Alexanders and Woodburn Stud.
Robert Alexander purchased Lexington for the unheard of price of
$15,000.00 in 1856 dollars and I dare say was not disappointed.
Lexington had lost but one race, although he had lost several heats, and
retired as America’s greatest racer of the period. Blessed with all of the attributes that constitute the makeup
of a superior runner, Lexington also could have expressed the large heart.
If he did indeed possess a larger than normal heart the X-factor may have
played a major role in the most accomplished, and with all probability, the most
successful sire ever to stand in America. Lexington
could have received his Xh from his dam, Alice Carneal and therefore all of his
daughters would have inherited the Xh and possessed the ability to express the
large heart, but even more important, to pass it along to their sons and
daughters. Lexington sired 12
champions, six of which were out of daughters of Glencoe and one, Duke of
Magenta, out of a granddaughter of Glencoe.
This should not be considered to be a mere coincidence because Glencoe
could also have inherited the Xh chromosome from his dam, Trampoline.
Glencoe is perhaps best known by X-factor aficionados as the sire of
Pocohontas, the mare generally thought to be the link to Eclipse for the
X-factor. Glencoe was a superior
runner and is said to have won most of his races in little more than a canter.
He was America’s leading sire 8 times although generally noted as a
“filly” sire. His daughter,
Reel, is considered by many to be the greatest American broodmare of the 19th
century and coincidently is the dam of Lecomte sired by Boston (Lexington’s
sire), the only horse to defeat Lexington in more than one heat. Thus the celebrated and much discussed Lexington/Glencoe
“nick” could in reality have been the bringing together of sires with the Xh
chromosome and daughters who all possessed the Xh chromosome.
All of Lexington’s and Glencoe’s daughters would have inherited their
Xh chromosomes and therefore all of these daughters would have had the ability
to pass the Xh on to their sons and daughters.
The majority of pedigree experts write this apparent “nick” off as
chance because of a close proximity of the two stallions involved, their
compatible genetic make-up and/or the many opportunities Lexington was given to
cover daughters of Glencoe.
2 discusses the successes of Daniel Swigert who started out at Woodburn and then
moved on to his own operation. Two
of the three Kentucky Derby winners bred by Swigert were out of suspected Xh
carrying daughters of Lexington as well as Preakness winner Vanguard.
Four time champion Firenze, one of the most accomplished race-mares to
ever run in America was produced by Florida a daughter of Florence by Lexington,
and keeping in mind the fact that a mare could pass her Xh on to her daughters
as well as her sons, Lexington could be the likely source of Firenze’s heart.
The list of notable horses bred by Swigert at the close of chapter 2
lists eleven horses, 7 are out of daughters of Lexington and 1 out of Florida a
daughter of Florence by Lexington.
In a chapter
devoted to the Belmonts, along with discussing the various accomplishments of
Belmont bred runners Hewitt mentions the “female bias” demonstrated by
Gallinule and Star Shoot, both out of daughters of Hermit and the successes of
their daughters in turn. It is
important to note that this “bias” could not pass from father to son, it
would be passed from daughter to son – both Gallinule and Star Shoot could
have received Hermit’s Xh chromosome from his daughters, they in turn passed
the Xh chromosome to all of their daughters.
In later chapters Hewitt again mentions the “female bias” and
discusses Buckpasser and several other noted “filly” sires.
It should not be considered to be merely coincidence that the well known
broodmare sires and sires of superior female runners were superior performers
themselves. Blue Larkspur,
Mahmoud, Princequillo and War Admiral the most likely progenitors of
the four largest hearts all demonstrated superior racing ability and all failed
to produce sons of equal stature that continued the male line with the possible
exception of Princequillo’s son Round Table who had the distinction of being
the leading living sire of stakes winners (82) during the latter stages of his
stud career, and more importantly, when a son did demonstrate superior ability
the dam side could have provided the large heart gene and the cycle would go on
to the next generation. It is also
noteworthy that Round Table, Hill Prince and Prince John, three of
Princequillo’s superior performers all went on to become successful broodmare
sires, not sires of sires which again, might indicate the presence of the Xh
chromosome at work.
Any mention of the
Belmonts inevitably gets to the subject of Man O’ War.
By any stretch of the imagination, Man O’ War was an extraordinary
runner in the annals of racing. Modern
runners such as Secretariat, Seattle Slew, Affirmed, Spectacular Bid, Dr. Fager,
Buckpasser and Damascus to name a few, still are listed under Man O’ War in
most top ten lists of the greatest of all time.
Not only did Man O’ War have all of the genetic material necessary to
create a superb racehorse, he also could have inherited St. Simon’s large
heart from Rock Sand, an English triple crown winner who would have passed the
Xh chromosome on to all of his daughters including Mahubah the dam of Man O’
War. Mr. Hewitt sums it all up by
stating “In fact, virtually all the very highclass stock Fair Play (sire of
Man O’ War) sired carried the blood of Rock Sand. . .”
Again, this should not be written off as mere coincidence, all of Rock
Sand’s daughters would have inherited his Xh chromosome and all of these
daughters would have possessed the ability to express the large heart and pass
it on to their offspring.
Chapter 4 is
devoted to James R. Keene and any mention of Keene brings Domino and Commando
into the conversation. Domino could
have received the Xh chromosome from his dam, Mannie Gray and more than likely
this Xh carried the heart of Reel (Glencoe).
Mannie Gray was inbred 3x4 to Reel, once through her dam’s sire War
Dance (Lexington x Reel) and through her 3rd dam.
Mannie Gray’s Xh could also have come from her sire, Enquirer.
Domino was undefeated as a two year old, six for eight at 3 and four for
eight at 4. He sired 19 named foals
in two crops with 42% stakes winners from foals.
Domino’s best son, Commando could have inherited the Xh chromosome from
his dam, Emma C., who in turn could have received her Xh from her dam Guenn who
most probably carried Lexington’s large heart but there is the possibility
that she carried Glencoe’s Xh from her 3rd dam, Sally Lewis.
Peter Pan, perhaps Commando’s best son as a stallion, could have
carried Hermit’s Xh through his dam, Cinderella.
Peter Pan became a good broodmare sire for H.P. Whitney, the subject of
Chapter 5. Peter Pan was the
broodmare sire of champion 2 year old filly Rosie O’Grady, Preakness winner
Bostonian, champion 3 year old and Kentucky Derby winner Whiskery, Preakness
winner Victorian and champion 2 and 3 year old filly Top Flight.
Chapter 6 concerns
C.V. Whitney and for our purposes we need only discuss Mahmoud, probable
ancestor of one of the four largest super hearts found in today’s pedigrees,
the other three being Princequillo, War Admiral and Blue Larkspur.
Had Mahmoud only sired Almahmoud it would have been enough to cement his
place in the history of the breed because Almahmoud gave us Cosmah and Natalma,
who in turn gave us Halo and Northern Dancer.
For C.V. Whitney
Mahmoud sired champion First Flight and he was the broodmare sire of Silver
Spoon, perhaps Citation’s best runner of either sex and second leading
money earner behind Fabius.
covered in Chapter 8 could provide the basis for a book itself.
Perhaps the single most important American female family was established
by Col. E. R. Bradley when he imported the French mare La Troienne,
she and her descendants continue to influence the breed through five, six and
seven generations including 2004 dual classic winner Smarty Jones.
Col. Bradley is also responsible for Blue Larkspur. His choice of utilizing War Admiral with daughters of La
Troienne, even if forced upon him by Olin Gentry, could be one of the more
important factors that is responsible for the continued success of this female
line. La Troienne produced 14 foals
with 10 winners including 5 stakes winners, champions Bimelech and Black Helen,
plus Bee Ann Mac, Big Hurry and Biologist.
Seven of her daughters produced 15 more stakes winners.
It is entirely possible that La Troienne carried an Xh chromosome and
along with all other genetic factors this Xh chromosome could have contributed
to and continues to contribute to the phenomenal success of this family.
concerns the Hancocks and Claiborne Farm founded by Arthur B. Hancock, Sr., and
continuing today under the guidance of Seth Hancock.
For purposes of the X-factor it is important to note that starting with Celt,
Claiborne had more than its fair share of leading or significant sires but the
lasting impact has been and should continue to be with broodmare sires. Who can say what motivated Hancock, Sr. to stand Princequillo
and Double Jay, or what inspired him to form the first noteworthy syndicate and purchase
Sir Gallahad, III.
What was it about Blenheim,
II that persuaded Hancock, Sr.
to import him for stud duty at Claiborne? One
important aspect that may not have been considered is that all four of these
stallions are suspected of being Xh carrying sires and all four were leading
broodmare sires on many occasions. Today,
as in the past, possible Xh carrying stallions such as Seeking the Gold,
Pulpit, Devil’s Bag, Monarchos, Out of Place, Boundary and Arch
will live on through their daughters.
In more recent
times Buckpasser and Alydar were often depicted as “filly”
sires and a close examination of Alydar’s abbreviated stud career indicates
that of his 69 American runners
that earned over $100,000.00, 30 of these were fillies and 39 were colts.
Alydar sired 9 earners of 1 million or more and this list contains 6
colts and 3 fillies. It must be
understood that the colts certainly had many more opportunities at large purses
than the fillies. Alydar’s runners falling in the category of $500,000.00 -
$999,999.00 is exactly equal with 6 colts and 6 fillies.
I would venture to opine that in reality Alydar was a consistent sire
with fillies or colts. None
of his sons have become top echelon sires but his daughters are worth their
weight in gold and should continue to provide the basis for his name in
pedigrees. Out of
Buckpasser’s 28 $100,000.00 plus earners, only 8 were fillies albeit his
leading money earner was the filly Time and Again, and out of his top 10
earners, 6 were fillies.
“filly sire” is sometimes confused with “broodmare sire” in that most
sires that have produced a predominance of superior performing fillies
eventually end up on the list of leading broodmare sires.
In Buckpasser’s case, although only 8 fillies sired by Buckpasser
earned $100,000.00 or more, he is a multiple former leading broodmare sire and
to have Buckpasser on the bottom side of the pedigree should be considered an
asset. Again, I would not consider
it to be a coincidence that War Admiral, the broodmare sire of Buckpasser,
was a leading broodmare sire and in all likelihood his heart could have been one
of the primary genetic reasons Buckpasser became a superior performer who became
a leading broodmare sire.
I have already
discussed the famous Lexington/Glencoe “nick” and throughout breeding
history there have been several noted “nicks” Bend Or/Macaroni, Isonomy/Hermit,
Phalaris/Chaucer and Fair Play/Rock Sand discussed earlier.
In each case these “nicks” could have involved Xh chromosome carrying
sires as with the more recent Nasrullah-Bold Ruler/Princequillo, Northern
Dancer/Buckpasser, Northern Dancer/Ribot and Northern Dancer/Mr.
Prospector nicks, and of course the most recent A.P. Indy/Mr. Prospector nick.
discussion centers around superior performers on the track versus superior
performance in the breeding shed one point continues to stand out, namely, why
do so many of the male superior performers seldom, if ever, reproduce themselves
through a son. Bold Ruler sired one American classic winner, albeit a great
one, Secretariat. Here was a
stallion that led the sires list 8 times, a modern record, and he was
represented by many sons and grandsons in the stud, but only through Seattle
Slew, a suspected Xh carrying stallion, does his male line continue to flourish.
The accomplishments of stallions such as Northern Dancer and Mr.
Prospector, both suspected of being Xh chromosome carrying stallions, also
confuse the situation because their male lines are the two most successful and
dominant strains active today and both are represented in the stud by many sons,
grandsons and great grandsons. The
primary reason for this phenomenon must be that Northern Dancer and Mr.
Prospector had dominant genes where it counted most, racing ability, as did
Lexington, St. Simon, Stockwell and
any other successful sire of sires,
with or without the Xh chromosome. To
say that all superior runners, all superior sires and all leading broodmare
sires owe their success to the Xh chromosome would be ridiculous, however, to
ignore the research that indicates a larger than normal heart size is a
sex-linked trait carried on the X chromosome would be even more ridiculous.
conducting research for this article and pedigree analysis of past and current
superior/leading broodmare sires one aspect surfaced again and again, the
overwhelming majority of these successful broodmare sires descend from daughters
of successful broodmare sires. While
this fact has certainly been observed and documented in the past, I believe it
further strengthens the argument for the existence of the X-factor and it could
very well be the answer to Abram S. Hewitt’s question posed in print in 1982
“Is there such a thing as “sex bias” in stallions and, if so, under what
circumstances and to what extent is it predictable?”
Mr. Hewitt went on to state “The great English stallion Hermit (1864)
sired both colts and fillies of the highest racing class, but at stud his sons
made little impression (with the possible exceptions of Heaume in France and St.
Blaise in the U.S.), while his daughters were among the very best brood mares of
their time. The same
characteristics held true of Gallinule (1884) and *Star Shoot (1898), both out
of Hermit mares.” These
statements were made in reference to Buckpasser, considering the fact that he
was out of a mare by War Admiral and his second dam being sired by Blue
Larkspur, both leading broodmare sires, was it possible or even probable that
Buckpasser would pass along the same characteristics.
The answer to that part of the question turned out to be a resounding
yes. Of the top 100 broodmare sires
by earnings for 2004, Buckpasser is the broodmare sire of six listed stallions,
#13 Woodman, #19 Miswaki, #22 Private Account, #42 Seeking The Gold, #77 Slew
O’ Gold and #92 El Gran Senor.
Relatively new genetic theories concerning paternal and maternal
influences as to repressed or expressed genes, maternal imprinting and/or
paternal imprinting and the role of mitochondrial DNA must also be considered in
any discussion going to heart size and development.
In addition, the revelation that because mammalian females have many more
genes than their male counterparts due to the fact that females have two X
chromosomes verses one in the male, it could mean that the female contributes to
or influences genetic inheritance to a much greater degree than originally
thought. It brings to mind the
story of John E. Madden who was quoted as stating the stallion was 75% of the
stud and that a mare’s control of form was slight.
However, after observing 12 horses in training he was later able to pick
out the dams of each horse, demonstrating the exact opposite of “slight”
control of form.
The X-factor may
indeed be linked to one specific mutated gene located on the X chromosome,
however, as with most genetic traits, a larger than normal heart more than
likely depends on a number of contributing genes that must be present and
dominant in order for a truly large heart to develop.
Does a larger
than normal heart equate to or contribute to superior performance?
Considering the fact that fatigue in the race horse can be directly
related to depletion of oxygen being distributed throughout the body by the
cardiovascular system, it stands to reason that a larger than normal heart would
be pumping more blood through the system, less fatigue, more staying power,
better performance. Could a larger
than normal heart be in some way responsible for or contribute to stamina?
Here again, logic dictates that it is possible that the stamina influence
attributed to Princequillo and Discovery, to name just two, is a genetically
controlled trait, and in all likelihood, a superior cardiovascular system and a
larger than normal heart are key elements to staying power in addition to
confirmation, lung capacity, physical type, running style, soundness, attitude
and training methods.
It must be
emphasized that a large heart does not by itself guarantee a champion, or for
that matter a winner. With all other factors being equal, conformation,
environment, health, training methods and what have you, the large hearted
individual certainly has an advantage. In
addition, the fact that breeders in general, and more specifically, top echelon
breeders have continued to breed the best to the best and hope for the best, the
large heart gene most likely has been spread throughout the thoroughbred
population to the extent that the majority of leading broodmare sires carry the
large heart gene on the X chromosome. Short
of testing the entire foal crop each year, and confirming by autopsy, it is
impossible to expound on how many foals are produced each year with the
possibility of expressing the large heart gene.
Because of this, the advantage enjoyed by large heart individuals has
by D. W. Davidge © 2005
It should be noted that all statistical information was obtained
utilizing the pedigreequery.com site, therefore, any numbers quoted in
racing and/or money earning information cannot be considered to be up to
date or absolutely accurate. D.W.D.