Should breeders pay attention to sire-lines in pursuit of producing athletes?

This article was first published in October 2019

The recent proposal by the American Jockey Club to limit book sizes to 140 stimulated various articles bemoaning the "loss of diversity" in the thoroughbred. Nancy Sexton touched upon the subject in these pages when she asked whether Dream Ahead, whose daughter Glass Slippers had recently won the Prix de l’Abbaye (and whose son Donjuan Triumphant later won the Champions Sprint), might counter this by stimulating a revival of the 'In Reality/Man o' War sire-line'.

Nancy contrasted the present situation of dominance by the Northern Dancer and Mr Prospector sire-lines with the situation in 1990 when "breeders had access to fine horses descending from Hyperion (via Forli, Aureole and Tudor Minstrel), Blandford (via Busted and his sons), Djebel (via Ahonoora and Blakeney), Sharpen Up and In Reality". She commented that: “Many of those lines have since disappeared.”

The Man o’ War sire-line traces back to one of the three stallions often referred to as the foundation sires of the thoroughbred, the Godolphin Arabian via his grandson Matchem. Another branch of the line continues in the US to Tiznow.

However, an even greater threat to the continuation of the three foundation sire-lines is the parlous position of the Byerley Turk/Herod line, once the most dominant of all. The recent death of Dunaden and the transfer to jumps breeding of Notnowcato reduced its already slim chances of survival, previously damaged by the gelding of Cirrus Des Aigles, Deacon Blues and Borderlescott.

Unless its few remaining representatives, such as Pearl Secret (2019 fee: £4,000), can produce unexpected results, the conventionally defined sire-line from Herod through Djebel will soon be as dead as a Norwegian Blue parrot.

There is an irony here, though, since recent genetic research into the inheritance of the Y sex chromosome (the chromosome passed on by a male to all his sons but none of his daughters) has provided an unexpected twist in the story.

This is that the Y chromosome found in numerous products of the Galopin/St Simon sire-line, the dominant one in the late 19th century, traces to Herod, not Eclipse as would be expected from recorded pedigrees.

It seems that a rumour current when Galopin won the Derby in 1875 was not, in fact, the Fake News suggested by his breeder. This was that Galopin was actually sired by Delight, a male-line descendent of Herod, rather than Vedette from the Eclipse line.

It is likely that the two stood at the same stud for a brief period in 1871, the year Galopin was conceived. Mares at this stud were 'paddock-bred'; and undesirable advances by Delight to his aunt Flying Duchess (Delight was a grandson of Flying Duchess’s sire, The Flying Dutchman) may have caused the Herod line to remain alive and well via Galopin and his son, the great St Simon.

Or at least it would have if only the sire-line of St Simon were itself alive and well. It is alive, in the form of the few residual male-line descendants of Princequillo and Ribot standing in various parts of the world; but it is far from well, with the male produce of Round Table, Prince John and Alleged proving unable to prolong the line much beyond the next generation. So even with the help of a Galopin/St Simon connection, the Herod sire-line faces extinction.

So should we deplore the 'loss of genetic variability' caused by the probable loss of one of the three sire-lines originating in the 18th century? Should we launch a Save Herod campaign – somewhat ironic given the original Herod’s approach to heredity – backed by petitions to clone Alleged for breeding purposes (with judicious gene editing to calm the temperament), so as to preserve the influence of the once-mighty Herod within the thoroughbred breed?

Well, there may be people that are sufficiently concerned that they are already planning Herod Extinction Rebellion protests; but perhaps some reflection on how inheritance actually works might reassure them. Who knows, it might even suggest that considerations of sire-line are not entirely relevant to discussion of genetic diversity in the thoroughbred?

The reasons for this are simple. Firstly, all that members of the same sire-line are certain to have in common is their Y chromosome. However, this is just one of 64 chromosomes in the equine. It is also by far the smallest , carrying little information other than to tell the animal that it is a he rather than a she, with no specific instructions relating to speed, stamina, soundness, temperament or other factors that determine the racing ability of a thoroughbred. These are contained on chromosomes independent of the sire-line.

Now take a look at the pedigree chart of a thoroughbred, say Frankel. There you will find the sire-line indicated along the top of the pedigree, from Galileo to Sadler’s Wells to Northern Dancer right the way to Eclipse 20 generations ago. In those same 20 generations the female line can be traced along the bottom of the chart, from Kind to Rainbow Lake to Rockfest back to Promise.

There are 1,048,576 positions in the 20th generation of a pedigree (I counted them just to be sure). The male line accounts for just one of these. If a sire-line disappears, its influence on future generations does not disappear – it reduces by one name out of 1,048,576 in the 20th generation.

Typically, Herod is at least as distant as this, so the extinction of his male-line will have virtually no effect on the diversity of the breed. Neither would the disappearance of the Man o' War, Blandford, Djebel or Hyperion sire-lines given that even their names are only found in generations where several hundred others are present.

I have made numerous studies of the influence of various great producers on the evolution of the thoroughbred. Over 100 years ago Lt-Colonel Vuillier formulated the Dosage Methodology, which provides an excellent means of assessing this but – despite Vuillier’s claims to the contrary – no guidance whatsoever on how to breed good racehorses.

The methodology involves counting occurrences of particular ancestors in the pedigree of leading racehorses and then weighting them by how far back in the pedigree they are found, to produce a 'Dosage' (or Influence) score.

Applying this methodology to study the pedigrees of the best horses from various eras since 1850 gives a clear indication of the relative importance of different stallions in breeding history. It also supports Vuillier’s claim that after many generations a stallion’s influence becomes ‘fixed’ within the closed breed, being unlikely to change in the future.

From such studies I venture the following predictions; and I hereby challenge anyone to prove me wrong by updating this work in 100 years time:

• With or without the continuation of his male-line, Herod is and will remain the most important influence on the breed, far more than Eclipse.
• With or without his male-line’s survival, St Simon is and will remain the most influential sire born in the 19th century.
• Northern Dancer will prove the major genetic influence from the 20th century, perhaps even exceeding St Simon in his influence on the breed.
• Phalaris will become the common source of the sole surviving sire-line, with branches via Nearco (Northern Dancer, Bold Ruler, Sunday Silence) and Native Dancer (Mr Prospector).
• Nonetheless he will rank well below Northern Dancer, Nearco, Sadler’s Wells and Galileo in his overall contribution to the breed.

So what does all this mean as regards the relevance of sire-lines to the study of thoroughbred breeding? Well, they are certainly among the easiest things to remember about a pedigree and this might encourage an interest in studying breeding.

They are also relevant if you are fascinated by the struggle for stud supremacy between great horses, in the way sire-lines wax and wane, often based on unpredictable elements such as Mr Prospector proving a far better sire than the racehorse that dominated his crop, Secretariat.

However, if you are trying to work out how to breed a good horse, a study of the stallion’s tail is likely to give you just as good a guide as his tail-male ancestry; and, if you are wondering whether to send a mare to Dream Ahead in order to protect the genetic diversity of the breed, you would probably do better to use the very worst stallion you can find, for his genes will probably diverge far more from those of any decent mare than do those of Dream Ahead. The produce is unlikely to run as fast, of course, but think about all that genetic diversity!

So in terms of the importance of sire-lines on practical racehorse-breeding, my advice would be to bear in mind the words of Freddie Mercury in his paean of praise to the great Galileo: "Nothing really matters, anyone can see. Nothing really matters..."

If you enjoyed reading this, you might also like...

Incredulity to acceptance: ten years on from the discovery of the speed gene

Are the most attractive jumps foals really the best racing prospects?

A reconnaissance mission to learn the secrets of France's best jumps breeder