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Is the X-factor the answer?

By D. William Davidge © 2005

           Australian 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 evidence.  

Scientific proof 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. 

  Looking 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.  

After reading 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…” 

 The 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[1]

          Chapter 1 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.

 Chapter 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.   

The material 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. 

          Chapter 9 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.  

          The term “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. 

          Whenever the 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.   

          While 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 decreased markedly.        

by D. W. Davidge  © 2005

[1]  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.