Breeding Theories – What’s In and What’s Out

Since man and horse became a team, we have attempted to influence nature in a multitude of ways under the guise of ‘breeding methods’.  Whether it be to facilitate conception, breed a particular gender, or come up with a ‘best of breed’, man has been relentless in his attempts to hit upon the magic formula.

Many theories have been disproved by technological advances or the mere passage of time.  Although the Thoroughbred is only a couple of centuries old and is certainly the breed into which the most time, money and manpower is invested today, in bygone eras, horses warranted this status by being an indispensable mode of transport as well as a principal leisure vehicle.

Many horse breeding methods recorded since man put quill to papyrus seem blatantly preposterous today, but with the benefit of a huge boost to the knowledge base in the past couple of decades with equine genome mapping and sophisticated testing techniques, there are also more recent theories and methods which can now also be either discounted or confirmed.


Blood was regarded with great reverence in centuries gone by and probably accounts for the term ‘bloodlines’ to describe a horse’s ancestry.  A principle which went by the grand title of the ‘Telegonic Theory’ was that a pregnant mare’s blood was infused with the blood of the covering stallion through the developing foetus.   Even after foaling, traces of the blood, and hence the characteristics of the stallion, were believed to remain in her veins and could be passed on to the next foal, even though the mare may be covered by a different stallion.   Her blood was seen to be the vehicle for the accumulation of characteristics of all stallions to whom she fell pregnant.  Therefore, the more foals she had, the more prepotent her blood.

The theory of telegony, explained rather vaguely as being ‘through channels as yet unknown to science’, was disproved by a Professor Ewart, who experimented with a zebra stallion and a miscellaneous group of mares.  Not surprisingly, in no case did subsequent matings to horses produce offspring with zebra characteristics.

It wasn’t until 1900 that the work of Austrian botanist, Gregor Mendel solved most of the mystery surrounding inheritance.  He came up with the discovery that the physical characteristics of living things were inherited via genes rather than the blood and introduced mathematical probability into breeding and propagation.

Lowe Family Numbers

Australian Bruce Lowe developed his own theory in the latter 1800’s and set about tracing and numbering families back to 43 foundation mares in the first General Stud Book according to their success at producing winners of the three English Classic races; the Derby, Oaks and St Leger. He numbered the families 1 through to 43 (the last 9 families producing no Classic winners), the application of the system being to breed to the lower numbered families 1, 2 etc to give a greater chance of breeding a Classic winner.

Partly because Bruce Lowe did not regard the families of winners of America’s top races as worthy of inclusion, and partly through the changing fortunes and fertility of various lines, his work has been disregarded as an accurate guide for this purpose.

The Bruce Lowe family number system has however left a legacy for line breeders today. The number you used to see in brackets beside a lot’s name in sales catalogues and next to the names of ancestors in tabulated pedigrees are Lowe’s family numbers. While not all horses with the same family number are now ‘related’ to any useful degree, it narrows down the number of families and lines which need to be searched to assess the level of linebreeding present in the pedigree.

Family numbers have since been sub-categorised, with a letter following the number.  For instance, Family 1-x traces to Blue Hen mare La Troienne, while family 1-w traces to Blue Hen mare Marchetta.

Lowe also used the Telegonic Theory to account for the difference in ability of full relations earlier last century.  He asserted that if a mating produced a good result it should not be repeated, as the subsequent foals would then have too much of the sire’s blood, and thus upset the optimum balance.

Relatively recent theories tend to steer away from external factors influencing inheritance, but include those involving the age of the parents.

Age of Parents

A contentious issue is the opinion that as a mare ages, her offspring decline in quality.  This is true of mares whose uterine function has deteriorated (and thus the nutrition of the foetus) and may also be true for more complex reasons at the cellular level, however if it applied to all mares, horses such as Bois Roussel (dam aged 22), Klairon (dam 21), Damascus (dam 20), Galopin (dam 19), Nearco, Native Dancer, (dams 18) and St. Simon (dam 17) to name a few, would not have contributed to the early development of the breed.

Australian Thoroughbreds Bernborough, Comic Court, Surround, Redcraze, Baguette and Tontonan, and more recently Commands, Octagonal, Apache Cat, Denman, Manhattan Rain and Northern Meteor are among those who are out of dams 14 years or over. Considering there are fewer older mares at stud, and that they have fewer foals, their strike rate is a lot better than some may think.

There is also definite evidence that an individual stallion’s sperm quality can decline with age. Older stallions however, generally serve fewer mares due to declining fertility, which of course also affects their stakes winners and winners annual crop stats.

Line Breeding and Inbreeding

The great Italian breeder Federico Tesio developed theories based on reinforcement of dominant ancestors through inbreeding (duplication within four generations) and line breeding.

Tesio was an outstandingly successful breeder, especially considering his very small number of mares and limited budget.  In modern terms of success rate, you could say that Tesio is to horse breeders as Danehill is to sires.

In the days when Italy was a much bigger racing force in Europe than today, Tesio was an active breeder for 45 years, producing 24 Italian Derby winners and several Champions – all from a band of mares producing an average of 12 foals a year.

Tesio’s champions include the unbeaten legends Nearco and Ribot. Ribot was the better racehorse, his record of 16 wins including two runnings of the Prix de l’Arc de Triomphe.  He was the best racehorse in the world at the time and really only Secretariat, Nijinsky, Sea Bird and Brigadier Gerard could stand alongside him as candidates for best racehorse of the 20th century.

Champion sire Danehill (Natalma 3m x 3f), the great Black Caviar (Vain 3f x 4f) and top class Aussie racehorses-now-sires Brazen Beau (Bletchingly 4m x 3f), Hallowed Crown (His Majesty 4f x 4f) and Stratum Star (Kaoru Star 4m x 4f) are examples of inbreeding.

Countless champion racehorses are closely line bred including superstar Winx and champion sire Snitzel, these theories are still extremely popular. Nowadays it is unusual to find a pedigree that does not carry some degree of inbreeding or linebreeding (good or bad) within five generations.

Any inbreeding should be conducted with caution however, as duplicating an ancestor up close not only increases the chances of inheriting desirable traits, but also undesirable traits. It is generally accepted that inbreeding tends to be more successful when the duplicated horse is represented by a daughter and a son (sex balanced).

Experience and thorough ongoing research is required to identify which ancestors respond best to both inbreeding and line breeding patterns and luckily, there are many resources available for analysts.

Rasmussen Factor

A more specific version of Tesio’s methods, this theory developed by US Thoroughbred columnist Leon Rasmussen requires the duplication of a female ancestor in a mating through different progeny, within five generations. Again, Danehill fits into this mould.


Sire over damsire patterns (nicks) are easy to prove and with certain sires, there is definitely an affinity with the daughters of certain other sires.  Some of these nicks used to be explained by proximity – a farm standing two stallions putting the daughters of one to be covered by the other sire.  This is not so relevant on today’s commercial farms, where the majority of mares are from outside sources, so any obvious nicks are genuine.  Fastnet Rock for instance, has produced 24% stakes winners to runners from Galileo mares, whereas his overall stakes winners to runners ratio is 8%.

Once a nick or potential nick is identified, many breeders take the hint and send mares by that damsire to the stallion, further perpetuating the cross.

Cluster Patterns

Cluster patterns refer to a group of common ancestors repeated in a mating. Clusters can involve line breeding, inbreeding and even the Rasmussen Factor. In my experience, this method is a great way of encompassing and duplicating groups of ancestors for maximum effect.

Large Heart Gene (X-Factor)

The idea that there was a specific and traceable gene for the inheritance of a large heart (and theoretically superior for producing stayers) was widely published by the late Marianne Haun in 1994.  After Secretariat died and the autopsy revealed that he had a 22lb heart, his pedigree was analysed and found to carry five lines to Pocahontas (1837). Haun’s theory was that large heart size traces to this mare and it is passed down via the X (female) sex chromosome.

There has been much discussion about it since then and in 2016, leading US pedigree analyst Anne Peters gave the theory the big thumbs down. She postulates that firstly, there is no concrete evidence that a large heart will translate to a superior runner, with many famous champions having average sized hearts on measurement including Secretariat’s own sire, Bold Ruler.

Secondly, it is difficult to prove the theory of individual mares carrying the large heart gene back through the generations, when they are not available for measurement.

And thirdly and most conclusively, Peters points out that recent equine genome mapping has proven that heart size is not passed on via the X chromosome.

Heart Score

Heart score proponents argue that there is a strong relationship between cardiac dimensions and peak oxygen uptake in Thoroughbreds, with the latter being a measure of performance (ie horses with high peak oxygen uptake scores are in general, better racehorses with higher earnings than those with lower peak oxygen uptake).

The ECG method of measuring heart score was popular in the 1970’s, however ultimately proved to be inaccurate in measuring cardiac output. Cardiac ultrasound scan is now the method ‘du jour’ and there does appear to be a relationship between heart size and performance, but as Matchem Genetics points out in their information on this method, “The larger heart that is pumping a lot more blood in every heart beat is better, but it is important to note that it is only relative to the size of the horse. A large heart in a small/light horse isn’t ideal as the cardio is a muscle that gets fit and in order to get a large heart fit you need to work the horse hard, which often breaks down the smaller/light horse. Equally, a really small heart in a big horse just about guarantees you have a slow horse!”


Italian Franco Varola’s dosage system was devised to assist in predicting the likely racing characteristics of a horse, particularly distance aptitude. American Steve Roman developed a new version of the system in the early 1980’s.

It was based on the influence of prepotent sires called Chefs de Race, in the pedigree. The influence of these sires has been categorised by their progeny’s distance aptitude, ranging from Brilliant to Intermediate, Classic, Solid and Professional.   Depending on how close up they are in the pedigree of the subject horse, they are given a mathematical point value which is added together to give an indication of that horse’s likely aptitude.

The system, which was developed in the northern hemisphere, doesn’t take a large amount of Australian and New Zealand pedigree influences into account, which can give a distorted assessment of an ANZ horse with young parents who may not have had the immediate sires or even grandsires in their pedigrees assessed for Dosage.

The system is also wide open to misinterpretation, as many people only look at the points total and think that the higher the score the better the horse will be.

The Dosage system has really been outmoded by genetic testing, which can much more accurately identify the potential distance preferences of an individual.

Genetic Testing

An improvement on the Dosage theory was developed by Irish-based geneticist Dr Emmeline Hill, with genetic testing systems launched in 2010. She is co-founder of the innovative company Equinome (recently acquired by fellow Irish company, Plusvital).  Based on a blood sample, their ‘Speed Gene Test’ is claimed to have a 90% accuracy and categorises the horse into one of three categories, C:C (sprint / mile types), C:T (middle distance types) and T:T (staying types).  The categorisation assists in not only breeding a desired type, but assists trainers in planning horses’ campaigns.  This concept had been further refined in recent years by the company, to predict a horse’s optimum distance down to within 400 metres.  Many studs are having their stallions tested and advertising the result, making it easier for mare owners to decide on which stallion best suits their breeding goals.

Advances in equine genetics has also allowed this company to offer products such as ‘Turf vs Dirt’, ‘Elite Performance’ (probability of achieving elite racing success), ‘Projected Height’ and ‘Raced / Unraced’ (potential to race at 2 or 3 years).

Nowadays there are other companies offering genetic testing as a part of their repertoire, including Performance Genetics in the USA.

Biomechanical Measurement

The measurement of a horse’s physical dimensions and their interrelationship is also said to have a bearing on potential success. These days, the old fashioned measuring tape has been superseded by photogrammetry and image recognition.  Based on the comparison of the measurements (including stride length) of successful racehorses, the system can only be effectively used on individuals older than 12 months and becomes more accurate on older (physically mature) horses.

With a number of breeding theories and methods still alive, it depends on the amount of available time and depth of the buyer’s pocket to determine how much research is invested in the process of elimination.  You could analyse and inspect an entire catalogue of yearlings for instance, using a combination of available methods and end up with a very short shortlist, however what we can’t yet predict is ‘will to win’.

As many an owner and trainer can attest, there are innumerable horses with above average ability that would rather hold up the white flag than push themselves out of their comfort zones once the pressure is on.

In the words of Federico Tesio, “A horse gallops with his lungs, perseveres with his heart and wins with his character.”

Written October 2017



  1. Thank you, about to do a presentation to breeders on applying genetics to their thought processes and this was helpful – love Federico Tesio’s book Breeding the Racehorse and have just read Thoroughbred Breeding, Pedidgree theories and the science of Genetics by Dr Matthew Binns and Tony Morris; food for thought. One of my old favourites is the late Anne T Bowling’s Horse Genetics. She and her husband Michael are that rare set of people who are both serious scientists of equine genetics and also practical horse breeders, although of Arabians. They practise line breeding and have bred highly successful horses in performance including winners of the 100mile Tevis Cup.

    • Thanks for the info, I have been breeding since 2014 and now the IRS is on attack–just because I have income from Mineral interests, I enjoy the info on these studies–

  2. So you think over an adelaide mare or adelaide over a so you think l mare

  3. David Ard says:

    Referring to the above ‘Line Breeding & Inbreeding’ paragraph the author has overlooked the obvious: Man O War belongs on any compilation of legendary horses – both as a racehorse & influential sire.

    • Hi Dave
      The article is not a compilation of legendary horses, it is an overview of breeding theories past and present. Any references to particular individuals in the article are examples of types of breeding theories and practices. If I were to refer to Man O’ War as an example in the article, it would have been under the heading of Line Breeding.
      The Author

    • Man O’ War owes very much to his dam-sire, Rock Sand, winner of the English Triple Crown in 1903,
      If you want to apply some of the theories expounded above, refer to Rock Sand’s pedigree and note the position therein of Pocahontas (‘Blue hen’ dam of 15 foals including Stockwell, Rataplan, and King Tom.)

      • Hi Paul
        Yes, Rock Sand is a terrific influence, not only carrying 5 lines to Pocahontas, but also line bred to Banter, Palmyra and Martha Lynn amongst others. A genetic hand grenade!


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