Pacemaker May, 2003
the breeding puzzle
Dr. Steve Harrison of The Thoroughbred Genetics Company explains what makes a successful Thoroughbred.
observing a top class sportsperson, it is obvious that he or she is something
special. Built to succeed. A natural. They have the gift to seemingly
effortlessly excel in their chosen arena. The aura is confident and they
appear almost invincible. Their power and stature is communicated not only
physically, but also telepathically. In summary, they are the finished
articles and seem to have inherited every useful gene version in the book.
Whilst, as humans,
we can all readily recognise and appreciate their attributes, it is also
apparent that some horses are blessed with the same qualities. Their breeders
may recognise it, the agent who purchases them may recognise it and the public
may come to recognise it. When it becomes obvious that something special has
arrived on the scene, one cannot but help wonder about the mechanisms which
arranged all the right genetic combinations within the perfect equine package.
A similar state of
wonderment can be applied to an examination of the unseen, negative genetic
factors that consign a once promising and apparently athletic, colt to gelding
and obscurity. Or those which contrive to produce a similarly disappointing
filly which will join the ‘also ran’ of breeding ranks. Picking out the
decent looking animal is one thing, successfully breeding it, or breeding from
it, is another.
differ from most humans is that they are artificially selected and can be bred
to emulate the achievements of their notable predecessors.
Breeders at the top end of the market have the luxury of realistically
attempting to reproduce similar genetic entities, others hope to get somewhere
near it. Many others still, have the modest objective of producing improved
progeny from less gifted mares. All things are relative and one man’s
failure at the top end may be the stuff of dreams for somebody further down
the ladder. Whatever the level, many do not fulfil their expectations,
realistic or otherwise.
The inevitable impact of commerciality and
environmental factors such as training, feeding and prepping can make it
difficult to assess the genetic contribution to certain trends. Although
environmental factors will help determine whether genetic potential is
reached, they will not affect the actual genetic status of a horse and
production of the successful racehorse is made easier if the raw genetic
material is present at the outset.
Breeding strategies and trends combined with a
perception of a horses ability, or potential, determine the shape of things to
come. If there are as many successful fillies on the track as there are colts
and given that there are probably less that 5% of colts and over 60% of mares
going back into breeding, it would seem natural to assume that the industry is
either missing out on a number of useful male animals or breeding from a
preponderance of mediocre to useless mares.
Whilst the former might be true to a lesser
extent, support for the latter assumption might be a good deal stronger. There
is an obvious imbalance and choosing the right mating combinations is
The fact that this imbalance exists indicates that
the selection of colts for breeding is based on success, whereas the
‘selection’ of mares is often based only on hope. This is not restricted
to smaller breeders as many of the mares of the larger organisations, rather
than being selected based on performance, are chosen on the basis of the
presence of black type further back in the pedigree. This would suggest that,
for most breeders, the more reliable source of genetic potential comes from
the sire. However, the ability of a sire to translate his racing performance
into breeding potential is not always guaranteed.
Varying stamina and maturation objectives ensure
that there are many genetic mechanisms that may influence success.
Single, major genes known as monogenes undoubtedly
affect certain desirable physical and physiological characteristics that
contribute to performance. Different variants or alleles of these, carried by
different horses probably contribute to variation in performance levels. On
the whole, they are relatively easy to select for because the characteristics
they control are likely to have obvious effects. However, getting the right
combinations of favourable alleles for a number of major genes in the same
horse is not so easy.
Groups of genes, known as polygenes, also work in
an additive manner to affect a trait. These are less predictable in their
inheritance as groups of genes are not generally inherited en masse.
They are sometimes referred to as Quantitative Trait Loci (QTLs). They affect
imprecise traits such as growth, height and stamina. They are likely to be
modified by environmental factors such as nutrition and training regime.
Further variation and interaction between genes is
also likely to contribute to precocity of young horses. Genes that are
activated or suppressed as a response to accumulation of chemicals within the
horse’s body may affect the maturation rates.
Whilst both parents of a foal contribute
equally to genes that are carried on chromosomes, the dam also unilaterally
contributes DNA via structures called the mitochondria. This can mean that the
dam can contribute up to 52% of total DNA and the sire only 48%. The mtDNA as
it is known is present in different formats in different female families. When
one considers that mtDNA plays a role in respiration and energy release, a
molecular basis for Bruce Lowe’s family classification is provided.
Consider all of these types of genetic
interaction and add the fact that some alleles may conceivably be ‘switched
on’ or ‘turned off’ depending on which parent the foal receives it from
and a rough explanation for variability in performance is provided.
It is likely to be a rare occasion when
absolutely all of these factors work in harmony to achieve the ultimate
objective. To get at least some
of them in synchronization would be a useful achievement. They not only affect
a horse’s performance, but the manner in which they are inherited determines
subsequent breeding potential and optimal choice of mating. Ultimately,
achieving the optimum mating is dependent on the correct gene dynamics between
the stallion and mare.
All horses carry two copies of each regular
gene, one from the sire and one from the dam. In the ideal situation, a horse
that has received a high number of duplicated alleles for beneficial traits,
whilst avoiding duplications of bad ones, is the best racing/breeding
proposition. Ideally, it would all be made easier if stallions were extremely
genetically homogenous. It would also be desirable if his alleles were
dominant over those carried by the mare. This would ensure that the stallion
was prepotent. Realistically, this is unlikely to happen. In recent times one
might envisage that only Northern Dancer may have been the closest to falling
into this category.
The very strategies that ensure the
relative genetic health of the breed and the success of many individuals
probably dictate that most successful animals are relatively genetically
variable or heterogenous. This makes it more difficult to predict the outcome
The cyclical choices of matings that
recognize the importance of outcrossing are likely to give rise to genetic
variability in progeny. Similarly, the use of pedigree theories based on
‘nicks’ and also based on an outbreeding theory, would result in genetic
heterogeneity in the foals.
Genes involved in producing sprinters may
be different to those influencing staying ability. The common policy of
crossing speed orientated stallions with mares influencing stamina is likely
to produce foals carrying a balance of genetic factors influencing both of
these extremes. They are also likely to be variable because of this.
In essence, these policies would give rise
to ‘hybrids’. It is probable, therefore that a number of successful
animals benefit from the hybrid vigour generated by these strategies. This
might help to explain why numerous successful individual on the track are
unable to consistently stamp their progeny.
The most prepotent stallions are unlikely
to be very genetically homogeneous because this would have arisen through
repetitive inbreeding which is also likely to have produced animals subject to
all of the undesirable disadvantages of this procedure. They would be less
likely to be genetically healthy. They should at least, however, have a
greater degree of genetic consistency than those with less successful stud
In many instances when the mare has
performed poorly or has limited potential a prepotent stallion is a useful
choice. However, because every horse is genetically different, the genetic
solution in mating one particular stallion or mare may not be the same for the
In some instances it is possible that a mare may
be more prepotent than a potential stallion. In the case of a successful track
mare with a high number of allele duplications, one might be better off
covering her with a stallion that is less prepotent. This would allow the
mare’s own genes to dominate. It is not necessarily always a good strategy
to cross the best with the best and hope for the best.
These two scenarios, where either the stallion or
the mare is the dominant genetic party help to explain two potential
mechanisms for success (or failure). The third mechanism would be where they
contribute equally to the genetic equation and there is a certain degree of
genetic complementation, a situation that some people may refer to as a
Genetic trends are not always apparent until a
number of matings have been made. The number of genetic possibilities for each
different horse produced is enormous. It is not always possible to predict the
genetic requirements for each animal from paper information alone. If it were,
then there would be no failures. If techniques and reasoning can be applied
which can give a better indication of the genetic status of horses they can
only clarify and improve the accuracy of knowledge of pedigrees and breeding
It is a
complicated situation. Thoroughbred breeding has been an inexact process.
There appears to be ‘more than one way to skin a cat’ and production of
successful horses has been seen to arise via a number of different approaches.
The objective of
this article has been to briefly outline the genetic complexities of the
Thoroughbred breeding puzzle and to provide the foundation for future articles
which will describe the application of genetic reasoning and new technologies
which might help to make the breeding process less of a riddle.
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