Last time, in Part One of this series, we looked at a few of the imponderables that influence thoroughbred breeding and the offspring of some very fine racing mares. In Part Two, we’re going to look at some of the inherited traits carried on the X chromosome and how these might influence Rachel’s, Rags’ and Zenyatta’s offspring, just as they have influenced thoroughbreds from as far back as 200 years ago.
But before we get started, a couple of “truths” must be kept in mind. First and foremost is that there is no fail-proof recipe for breeding a thoroughbred champion. There are, of course, any number of breeding beliefs. But to produce a champion, many factors have to be taken into account, from pedigree to race records and from soundness to training.
Secondly, every thoroughbred is a completely unique, unrepeatable individual. So that, while our three exceptional ladies will undoubtedly have fine offspring, they can never reproduce themselves, simply because it is the design of genes, chromosomes and DNA that each of them inherited from their parents that made them who they are. Rachel, Rags and Zenyatta are all originals in that sense — and there will never again be another thoroughbred just like them. Anymore than there could ever be an exact replica of you or of me!
(NOTE: For anyone who needs it, there’s a definition of gene, chromosome and DNA at the end of this article. Just scroll down to the end.)
To begin: there are many reasons that pedigree experts may decide to match a particular stallion with a specific mare. Typically, the choice of a stallion for Rachel, Rags and Zenyatta will involve looking at what each individual brings to the mating through studying sire lines and family (mare’s side), as well as racing performance data over a number of generations and the stallion’s record at stud. This, together with a focus on the characteristics the breeder is looking to breed into a potential offspring, completes the picture. If a mare, or a stallion, has lots of speed, a breeder might look for a stamina influence. Soundness and conformation might be another important consideration. Or a breeder might be looking for offspring with lots of heart room, favouring individuals with this type of conformation, such as Bernardini (2003) or Giant’s Causeway (1997) or Zenyatta or Rags to Riches. A breeder who is looking to breed a dirt horse who is good under most track conditions is looking for a mare/sire combination that favours this kind of adaptability. In this instance, a son of Mr. Prospector (1970) like Smart Strike (1992), or a grandson like Curlin (2004), might be favoured for a mare like Rachel Alexandra, who demonstrated so dramatically that she could win in the mud or in the sunshine.
Regardless of the rationale for pairing a certain stallion and mare, whether or not the qualities being sought come to fruition in the foal is as much a matter of genetics as it is related to the environment in which the foal is raised and trained. The genetic influence begins with the difference between the X and Y chromosomes. The mare carries a pair of X chromosomes (XX) — an X from her sire and another X from her dam. The stallion carries a different pair: a Y from his sire and an X from his dam (XY). This means that, in the case of a filly foal (XX), the stallion contributes the X chromosome he received from his dam. But because a mare has two X chromosomes, she will contribute either the one she received from her sire OR the one she received from her dam, depending on which one is dominant. In the case of a colt foal (XY), the stallion contributes the Y he received from his sire, while the mare provides the X, making it the stallion who determines the sex of a foal. However, neither the sex of a foal nor which of the two X’s the dam expresses to it can be pre-determined.
While the stallion has the role of determining the foal’s sex, it is the X chromosome that carries far more genetic potential (whether it comes from the stallion or the mare). On the X chromosome are factors that influence sound build, temperament, speed and stamina. These are called “X-linked traits.” But the sex-related chromosomes are not the only genetic inheritance a foal receives from its parents. There are 62 other chromosomes, arrayed in 31 pairs, that influence everything else about the foal’s physiology, conformation, coat colour, temperament, racing prowess and so forth. Each pair of chromosomes is composed of inherited genetic traits from both the sire and the dam. In other words, a foal inherits a bunch of important characteristics, including speed and stamina traits, from both its parents.
So although the X chromosome carries more genetic material than the Y, who contributes what as far as the other 31 pairs of chromosomes are concerned has to do with dominant and recessive genes. As they combine into the “code of life” that will become a foal, these dominant and recessive genes — themselves the inheritance of the sire and dam’s ancestors — dance themselves into a pattern that is completely unique to this still unborn thoroughbred. And depending on how the chromosomes that carry the genes arrange themselves, certain traits will dominate over others. So, for example, if Rachel Alexandra expresses a dominant stamina influence via her genes they might well dominate a speed influence from a prospective stallion to whom she is bred. In this case, she will likely produce a foal who needs a longer distance and who may quite honestly be (genetically) disadvantaged in a sprint. This is why, when you open the progeny list for a mare or a stallion, even the successful ones, you usually find everything from soup to nuts! Those pesky genes combine in ways that can advantage — or seriously disadvantage — a champion’s offspring.
In any mating, the hope is for a talented, healthy youngster and breeders base much of that optimism on the performance abilities of its sire and dam. But it’s the genes, chromosomes and DNA that always hold the best hand — and they keep their secrets pretty much hidden until that foal becomes a yearling and goes into training.
There is still another factor that’s loaded with potential, depending on whether or not it occurs in a thoroughbred’s pedigree. It’s quite literally called the “X-factor” because it’s a sex-linked trait. It’s “sex-linked” because it involves the inheritance of a large heart gene that is only carried on the X chromosome — the sire’s single X that is expressed only to his daughters and the dam’s two X’s, one of which she will express to her foal, whether a filly or a colt. And that large heart gene has been found to be a solid indicator of racing performance in standardbreds, thoroughbreds and quarter horses.
Research over the last 20 years in the UK, Australia and the USA has uncovered an inherited pattern for the large heart gene, as far back as the first known instance recorded of a large heart, which was that of the great Eclipse (1764). Now it appears that Eclipse passed on this trait. Over several generations, this large heart gene was passed from sire to daughter and from that daughter to her son, until we arrive at the most influential broodmare in thoroughbred history: Pocahontas (1837). A descendant of Eclipse, Pocahontas was a great producer during her lifetime, her most famous sons being Stockwell(1849), Rataplan (1850), and King Tom (1851) . She also produced five very good daughters, out of a total of 15 foals. Stockwell became the most important of her three sons and appears in the pedigrees of the outstanding individuals Phalaris (1913), Nearco (1935), and Native Dancer (1950). How influential was Pocahontas? Just look at the number of crosses back to her found among these remarkable thoroughbreds: The Tetrarch (7), Man O’ War (9), Nearco (37), Raise A Native (175), Secretariat (249), Northern Dancer (272) and Mr. Prospector (353). Too, in Pocahontas’ pedigree are 13 crosses to the immortal Eclipse — suggesting to large heart X researchers that this trait may well be Eclipse’s permanent gift to the breed.
It was upon the death of the immortal one, Secretariat (1970), that X-factor research was given the impetus it needed to move forward. It was discovered during his autopsy that Secretariat had a huge heart, weighing approximately 22 lbs. (An average thoroughbred heart weighs about 8.5 lbs.) Despite its size, Secretariat’s heart was normal in every other aspect. It was just that the champion turned “running on heart” from a metaphor into a statement of biological fact. In an industry that had focused almost exclusively on the sire, it turns out that the dam is as reliable a transmitter of this gene as the sire. As you might perhaps expect, a mare more reliably passes this gene on to a son. And, as the stallion is only able to transmit the large heart gene on his X chromosome to a daughter, his reliability lies there. In Secretariat’s case, it was Somethingroyal (1952) who expressed her large-heart gene to him, and not Bold Ruler (1954), his sire (who had to donate the Y for Secretariat to become a colt.)
In an almost poetically beautiful turn of events, we now know that Secretariat’s great heart is literally living on, through the progeny of four of his daughters: Weekend Surprise (1980), Terlingua (1976), Secrettame ( 1978) and Betty’s Secret (1977). We know this because the ongoing research into the large heart X has shown that to carry this sex-linked trait to an offspring, a thoroughbred needs to descend directly from at least one of four sire lines: War Admiral (1934), Mahmoud (1933), Blue Larkspur (1926) and Princequillo (1940). Each of these sire lines, in turn, trace directly back to the British mare, Pocahontas.
Today, the large heart principle, or “X-factor” is used as one source of breeding information, in combination with more traditional breeding theories.
As indicated above, the large heart X gene descends, in a zigzag pattern, from a sire to his daughter and from that daughter to her sons. However, not all stallions have this X-linked trait to pass on to their daughters. Only those who trace back to the four heart line sires named above can accomplish the deed. If a mare inherits her X from a sire who descends from at least one of the large heart sire lines, she is called a “single copy” mare. This means that she has a 50/50 chance of passing the large heart on to her son(s) or daughter(s), depending upon which X is dominant. So, even though the large heart gene is an important trait, it isn’t necessarily the dominant X in a mare’s genetic profile. In other words, a mare may be a carrier of the X-factor but never be able to pass it on to her offspring.
If a mare inherits the X-linked large heart from both her sire and her dam, she is called a “double copy” mare. All of Secretariat’s four large-heart daughters are double copy mares. This means that both of their X chromosomes carry the large heart X, making it twice as likely that they will be able to give it to either a son or a daughter.
For example, Weekend Surprise likely expressed the large heart X to her sons, A.P. Indy (1989) and Summer Squall (1987). However, these sires can only pass it on to their daughters. As an example, a filly by Seattle Slew (1974) ex. Weekend Surprise inherits an X from Slew that carries the large heart trait PLUS another large heart X from Weekend Surprise, who, being a double copy mare is more likely to pass the trait on with some consistency to her offspring. In any case, this filly would have 3 out of 3 chances to inherit the large heart gene from one of her parents.
Now, let’s take a look at the pedigrees of our three outstanding ladies, and the gentlemen in their lives, to see whether or not the large heart gene is present in these mating combinations and, if it is, what its chances are of being inherited by a foal from these matings.
Rags To Riches
Rags’ sire, A.P. Indy carries the large heart X which he received from his dam, Weekend Surprise (who had both Princequillo AND War Admiral large heart sire lines). Given his success on the track and at stud, it is reasonable to say that A.P. Indy passed the large heart gene on to Rags, just as he appears to have done with other female offspring. However, Rags’ dam, the excellent Better Than Honour (1996), does not link back (in that zigzag pattern) to a large-hearted sire line. So Rags is very likely a single copy mare, with a 50/50 chance of passing this trait on to a son. (The 50/50 ratio drops in the case of a filly, since both parents contribute an X and one of those X’s will be more dominant than the other.)
Although she is now in Ireland and in foal to Galileo (1998), Rags’ first foal, a filly by Giant’s Causeway, was born in the USA. A chestnut who seems to hold an uncanny resemblance to her dam, the filly’s training has been postponed because of an injury to her sesamoid following a gallop (Thoroughbred Times, 09-06-2011). Interestingly, Giant’s Causeway has two potential sources of the large heart gene in his pedigree, only one of which is active since his sire, Storm Cat (1983), could only give him a Y chromosome. So it is the X he received from his dam, Mariah’s Storm (1991), that he will pass down to a daughter. This means that Rags’ first foal, a filly, had two opportunities to inherit the large heart gene — one from her dam and the other from her sire. If the “X-factor” theory holds true, Rags’ firstborn has an excellent chance of inheriting the large heart gene from Giant’s Causeway.
Rags’ second foal, a colt, is by Henrythenavigator (2005) and the little fellow is reportedly a beautiful individual. This stallion has no connection to any of the four heart line sires in his pedigree. This means that Rags may have expressed the large heart gene on the one X in her pedigree that carries it. Or maybe not. The chances remain about 50/50 because only one of her X’s carries the large heart gene.
Galileo was a great race horse and is proving to be an outstanding sire. He carries the large heart gene from his dam, Urban Sea (1989), who is also the dam of his half-brother, the brilliant Sea The Stars (2006). Urban Sea inherits the gene through her sire, Miswaki (1978), a son of Mr. Prospector who traces back to War Admiral. This bodes well for Rags’ third offspring — if a filly, it will certainly receive the large heart gene on Galileo’s X; if a colt, it has a 50/50 chance of inheriting Rags’ large heart gene.
“Hooves crossed,” wonder- full Rags!
Rachel’s sire, Medaglia d’Oro (1999) does not descend from one of the four heart line stallions in a pattern that allowed him to receive the large heart gene, even though his sire is El Prado (1989), who descends from Northern Dancer and, therefore, the Mahmoud heart line. (El Prado expressed the Y chromosome to Medaglia d’Oro.) Still, no-one could doubt that this stallion isn’t sending great genetic material to his daughters when we think of Rachel’s brilliance, as well as the exploits of Champagne d’Oro (1997), Gabby’s Golden Gal (2006), Tasty Temptation (2006), Payton d’Oro (2006), Plum Pretty (2008) and Vision in Gold (2009).
Rachel’s dam, Lotta Kim (2001), traces back to Pocahontas through her sire, Roar (1993), but not to one of the heart line sires along the way. However, through her broodmare sire, Cure the Blues (1978), Lotta Kim traces back to War Admiral. In other words, Rachel’s dam is likely a single copy mare and, if so, has a 50/50 chance of expressing the large heart gene to her offspring. Curlin, Rachel’s first beau, does not trace back to the large heart sire lines, although through his dam, Sherriffs Deputy (1994), Curlin does trace back to Eclipse.
This adds up to mean that Rachel might be a carrier of the large heart gene, but only if it was passed on to her by her dam. If she is — and it would appear to be a strong possibility, given her amazing racing abilities — Rachel would also be a single copy mare, with a 50/50 chance of expressing this trait to a son or a daughter. As we know that Rachel is expecting a colt, it will be interesting to see whether or not he exhibits this trait.
“Hooves crossed,” beautiful girl!
Although Zenyatta’s sire, Street Cry (1998), traces back to Pocahontas in a zigzag pattern (sire to daughter to daughter’s son), he does not trace back to heart line sires through his dam. And it is Street Cry’s dam that would need to express the large heart gene, first to him and then from him to Zenyatta. However, Zenny’s dam, Vertigineux (1995), descends from the Princequillo heart sire line, making Zenyatta — like both Rags to Riches and Rachel Alexandra — a potential single copy mare.
Bernardini, her first suitor, traces back to the War Admiral and Mahmoud heart sire lines through his dam, Cara Rafaela (1993), making her a double copy X mare. This means that Zenyatta’s foal — if it’s a filly — has three possible chances of inheriting the large heart gene, one from Zenny and the other on the X from Bernardini, which must carry the large heart X . As we have seen, the A. P. Indy sire line boasts a goodly number of outstanding fillies, making it quite interesting to see — X aside — what a filly foal from this pair will be like in terms of racing performance. If the foal is a colt, Zenny becomes the only potential source of the large-heart X, a 50/50 proposition.
However, Vertigineux has also produced other accomplished daughters in Balance (2003) by Thunder Gulch (who traces back to Eclipse through his dam), as well as Where’s Bailey (2002) by Aljabr (who does not trace back to the heart line sires). Her son, Souper Spectacular ( 2007) by Giant’s Causeway is a promising turf performer, having most recently won three in a row over distances from a mile to a mile and a quarter. All of which bodes well for Zenyatta, since it would suggest that Vertigineux tends to express her large heart gene to her offspring fairly consistently, possibly because the gene is a dominant X. (Note: Eblouissante, another Vertigineux daughter, is still in training, so was not mentioned at this time.)
“Hooves crossed,” big girl!
Of course, it must be stressed that these sex-related influences are only one dimension of a much larger “genetic collage.” When one looks carefully at the pedigrees of Rachel, Rags and Zenyatta, as well as the stallions to whom they have been bred, it really is an impressive array of great, great thoroughbred bloodlines. Other influences these bloodlines favour will form part of the final article (part three) in this series.
One thing’s for certain: Rachel, Rags and Zenyatta all gave astounding testimonials to their stamina, courage and heart each and every time they raced. As broodmares, there is absolutely no reason to doubt their potency and the gifts it will bring to their respective offspring!
(P.S. Special thanks to zenyatta.com for inventing the expression “Hooves crossed” that was re-cycled here.)
NEXT TIME: Here’s another genetic trait that these ladies share where dominance has absolutely nothing to do with it. Join us at THE VAULT as we conclude our series with a look at the power of mitochondrial DNA, as well as other influences on the offspring of Rachel, Rags and Zenyatta.
DEFINITION OF TERMS
Gene: a segment of DNA. Genes are the basic biological unit of heredity. What a filly or colt is like, in every way, is its genetic inheritance from its sire and its dam.
DNA: Genes are made up of DNA, a molecule that looks like a double helix or spiral. Each thoroughbred has literally thousands of genes that determine their individual characteristics, or genetic traits.
Chromosomes: Genes, carrying DNA, are arranged along 32 pairs of chromosomes in thoroughbreds. So thoroughbreds all have 64 chromosomes. Each pair of chromosomes represents one chromosome from the sire and one chromosome from the dam. It is these pairings that make every thoroughbred completely unique. In fact, there are over 12 million different chromosomal variations possible in horses!!!!!