The X-Factor (Large Heart) Theory

If I existed in the universe of social media (which I do NOT), I would *LIKE* the pedigree of Epsom Derby winner Ruler of the World. He is by Galileo out of Love Me True, by Kingmambo.

Any nag by Galileo out of a Kingmambo mare obviously possesses a good pedigree. What interests me about his pedigree is that his third dam is Lassie Dear, by Buckpasser.

Lassie Dear probably requires no introduction. Suffice it to say that she was the dam of Weekend Surprise and others of note.

Lassie Dear is a reine-de-course of course. She also appears on lists of double-copy Thoroughbred mares (mares who allegedly pass on the “large heart,” or the X-Factor).

If you do not know what I am talking about, read Chapter 13 of Racehorse Breeding Theories. I have referred to this book frequently in the past of course. It has many excellent chapters. It also has some chapters that are real stinkers. Chapter 13 is one of the latter. But for all its faults, this chapter does do an adequate job of explaining the X-Factor Theory and whats its proponents believe.

That theory can be summarized as follows: “The large-heart-gene, which became known as the X-Factor, was later determined to be passed along on the female X chromosome” (page 251).

The chapter purports to examine the pedigree paths through which this gene is passed. For example, page 258, “The War Admiral heartline is among four of the largest heartlines that have been identified in today’s pedigrees. The other three are the heartlines of the Thoroughbred stallions Mahmoud, Blue Larkspur, and Princequillo.” Secretariat and his rival Sham allegedly received their large-heart genes from their common broodmare sire, Princequillo.

So I decided to have some fun with this notion. Specifically, I decided to examine War Admiral, Princequillo, Mahmoud, and Blue Larkspur as sires of dams in the female line of sales foals of 2003-2007 and see what their results actually were.

The results in terms of prices are summarized below.

Sire                             Foals              Gross             Average          Maverage      Price Index

Princequillo                1,392         $91,396,804     $65,659            181.62                1.11

War Admiral                 536         $39,772,450      $74,202           198.05                1.21

Mahmoud                      489         $27,842,577      $56,938            173.05                1.06

Blue Larkspur               126          $3,523,485       $27,964            124.87                0.77

Totals                           2,543        $162,535,316    $63,915            180.62                1.11

Just a reminder, these are foals in the female line ONLY. Princequillo, for example, showed up 1,046 times as the sire of the fourth dam, 332 times as the sire of the third dam, and 14 times as the sire of the second dam for a total of 1,392 foals. War Admiral showed up 493 times as the sire of the fourth dam and 43 times as the sire of the third dam for a total of 536 foals.  Mahmoud showed up 470 times as the sire of the fourth dam and 19 times as the sire of the third dam for a total of 489 foals. Blue Larkspur showed up 124 times as the sire of the fourth dam and twice as the sire of the third dam for a total of 126 foals.

The overall average for all 70,714 sales foals of 2003-2007 was $54,140. Princequillo, War Admiral, and Mahmoud were all above that figure. Blue Larkspur was below it. The overall maverage for all sales foals of 2003-2007 was 163.11. The first three were all above that figure. Blue Larkspur was below it.

Princequillo was a foal of 1940, War Admiral 1934, Mahmoud 1933, and Blue Larkspur 1926. So the youngest (Princequillo) showed up most often, and the oldest (Blue Larkspur) showed up least often and was the only one of the four with below-average prices. If you add ’em all up, that comes to a total of 2,543 foals that sold for an average of $63,915 (above the overall average of $54,140), a maverage of 180.62 (above the overall maverage of 163.11), and a Price Index of 1.11 (1.00 being average).

Some pretty good stakes winners were included among these 2,543 foals. Princequillo sired the third dam of Sun King (3,240 Performance Points) and the fourth dam of of Good Ba Ba (3,685) and Kinsale King (2,404). War Admiral sired the fourth dams of Shadowbdancing (2,399) and Master Command (2,337). Mahmoud sired the fourth dam of Haynesfield (2,719). Blue Larkspur was shut out in terms of stakes winners with 2,000+ Performance Points.

The overall results are summarized below. APPPSW stands for average Performance Points per stakes winner, a measure of the quality of the stakes winners involved. The overall figures for all sales foals of 2003-2007 are 3.41% stakes winners from foals and 616 Performance Points per stakes winner.

Sire                           Foals          Stakes Winners          %          APPPSW          PPI (result)

Princequillo              1,392                   40                     2.87            673                    0.92

War Admiral               536                   18                     3.36            719                     1.15

Mahmoud                    489                   15                     3.07            588                     0.86

Blue Larkspur             126                    3                      2.38            223                     0.25

Totals                        2,543                  76                      2.99            649                     0.92

Without belaboring these results too much, you can see that only War Admiral had positive results (above 1.00). The other three all had negative results (below 1.00).

The chart below puts it into even better perspective by comparing prices with results.

Sire                            Price Index                  PPI (result)

Princequillo                     1.11                              0.92

War Admiral                   1.21                              1.15

Mahmoud                        1.06                             0.86

Blue Larkspur                 0.77                             0.25

Totals                               1.11                              0.92

War Admiral did have positive results, but not as good as his prices. Those 536 foals sold for prices 21% above average and achieved results only 15% above average. The 1,392 Princequillo foals sold for prices 11% ABOVE average and achieved results 8% BELOW average. The 489 Mahmoud foals sold for prices 6% ABOVE average and achieved results 14% BELOW average. The 126 Blue Larkspur foals sold for prices 23% below average and achieved results 75% below average.

The total 2,543 foals for all four sires sold for prices 11% ABOVE average and achieved results 8% BELOW average.

If Princequillo, War Admiral, Mahmoud, and Blue Larkspur did indeed pass along a gene for large heart, you would expect to see pretty good results for them as sires of dams in the female line (the large-heart gene is passed along the female line, according to the theory). In fact, they had pretty miserable results (except maybe for War Admiral, who was close to his price expectations).

So the evidence to support this theory with reference to these four sires is pretty thin. Empirical evidence, that is. Imagination is an entirely different matter.

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6 Responses to The X-Factor (Large Heart) Theory

  1. Ned Williams says:

    Thanks for the analysis. I found this chapter in Racehorse Breeding Theories quite interesting, but was a bit skeptical when the large heart quotient was linked to certain physical characteristics such as head or ear shape. My question for you is: Is it possible to compare your findings above, with foals from the male line as compared to the dam side. The X factor should make the dam side significantly better than the male line side within these four sires extended families. If I am reading this correctly you have compared these four sires dam side progeny with the breed (within certain well defined parameters) as a whole. Can the dam side be compared with the sire side? I may be off track and it looks like you have already made a convincing argument. Just wondering if this is even possible over generations or makes sense? Please forgive me if I am way off track.


    • ddink55 says:


      I may not understand your question completely, but I think you want to know how these four sires fared outside the female line. Most sires have their best results down in the female line, especially sires such as these four which were not particularly good as sires of males (the one possible exception being Princequillo as the sire of Round Table).

      You might check out an earlier post on Princequillo fourth generation. That post was based on sales foals of 1999-2002. Princequillo qualified among the top 20 for that group. The other three did not (too old). At P8 in the fourth generation (sire of the third dam), Princequillo had a Price Index of 1.06 and a PPI (result) of 1.61. That compares very favorably with his overall numbers for all eight positions in the fourth generation of 0.96 (price) and 0.83 (result). I am pretty confident that the other three sires would follow the same pattern if I went back far enough in time to where they did show up often enough to make an analysis of their stats somewhat meaningful.

      Incidentally, I should make it clear that I am not discounting the importance of heart size and/or efficiency to racehorses. I am merely skeptical that this trait can be tied so specifically to certain ancestors (as the X Factor Theory does and severely oversimplifies the situation in doing so).

      Also, I am not done with this theory yet. I have at least one more post about it in the works. Hope you will find it interesting.

      Passingly yours,


  2. JPhillips says:

    Why are people even discussing these bizarre breeding ‘theories’ any more? Genetics will very shortly debunk them (but also explain the few valid observations) through irrefutable scientific evidence. Nicks and pedigrees will be out the window too, so the average buyer/owner will no longer need to worry about understanding misleading catalogue pages and well-marketed pedigrees. Transparency which will hopefully encourage new owners/interest/money into the industry. Using these theories and looking at crosses contributes to success about as much as waving at magpies does. But then some people think smoke and mirrors is a good thing, for them.

    • ddink55 says:

      Your question was rhetorical, but I would say that people have continued to discuss bizarre breeding “theories” because the evidence to debunk them has been sorely lacking. Not that such evidence is difficult to find, just that not many people have made the effort to do so.

  3. JDB says:

    A fair analysis of the X Factor hypothesis would have to include all potential X passing pedigree positions in which the four sires named above may occur, not just 4 of them. In a male’s 5 gen. pedigree the number of such positions = 7, in females it’s 11.

    It’s a moot point, however, since there is no lack of highly credible genetic evidence debunking this hypothesis which holds that heart size is controlled on the X chromosome. The domestic horse genome was sequenced six years ago, SNP chips developed, and since then the genome of no breed has been scrutinized as closely as the TB. As first proposed ~20 years ago the X factor hypothesis was somewhat simplistic though not altogether implausible and certainly not “bizarre”. But as it turns out the factors controlling heart size aren’t on the X and the hypothesis is false.

    Genomic evidence has already begun to redefine how we study and analyze pedigrees and will no doubt continue to do so. Any ‘transparency’ it might offer must be regarded in full context, i.e. the genome is only one factor in the equation that produces a successful racehorse.

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