Racing Review May, 2002
Breeding, gene dynamics and DNA
Dr. Steve Harrison of The Thoroughbred Genetics Company .
Frequently, when 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 packet.
A similar state of wonderment can be applied to an
examination of the 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.
Where horses 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.
However, commercial aspects apart, environmental factors will not affect the
genetic status or potential of a horse and production of the successful
racehorse it is made easier if the raw genetic material is present at the
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 precarious.
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 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
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 repressed as a response to accumulation of
chemicals within the horse’s body may affect the maturation rates.
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%. As it is passed on only down the female line, the
mtDNA as it is known, is present in different allelic formats in different
female families. When one considers that mtDNA plays a role in respiration, a
molecular basis for Bruce Lowe’s family classification is provided.
all of these types of genetic interaction and add the fact that some alleles
may be ‘switched on’ or ‘turned off’ depending on which parent the
foal receives it from and a rough explanation for variability in performance
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.
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.
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 of matings.
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’ also based on an outbreeding theory
would result in genetic heterogeneity in the foals.
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.
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
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 would at least, however, have a greater degree of genetic
consistency than those with less successful stud careers.
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 next one.
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 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 and not be cancelled out or
over-duplicated. 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 ‘nick’.
Genetic trends are not
always apparent until a number of matings have been made. The number of
potential genetic scenarios for each different animal 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
are available 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 theories.
Through use of DNA analyses we are accumulating
information that is allowing us to look at the genetic patterns that result
from breeding policies.
Bands in a pattern ,
like the ones shown in the photograph, represent fragments of DNA carrying a
gene or genes spread over chromosomes, sex chromosomes and mitochondrial DNA.
The position of bands is dependent on the alleles present in a particular
horse. As a band represents an allele, it is described as a genetic
marker. Examination of the
genetic constitution of individual horses, modes of DNA inheritance and
identification of beneficial genetic types can be achieved through an
examination of ‘genetic profiles’ of horses created using a wide selection
of genetic markers.
Using broad range tests it is becoming possible to examine the basis of such phenomena as ‘nicks’ and damsire trends. In later articles we hope to refer to specific applications of this work in greater depth.
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