Thoroughbred is a genetically diverse breed. This is demonstrated by the
variety of physiques, conformation and performances in the population. Genetic
management of this diversity or ‘mongrelism’ is a key to the effectiveness
of our DNA and performance data assessments.
variability means that major performance characteristics, including
speed/stamina balance, are not controlled solely by any single gene.
Individual genes may certainly correlate with a particular trait but they are
generally part of a ‘mixture’, where strong correlations are also found
with many other genetic combinations.
approach to genetic testing and data provision is based on the fact that
stamina/speed components of performance are multi-factorial and it is likely
that it’s the existence of Thoroughbreds of differing speed and stamina
characteristics that contributes most strongly to this genetic variability and
the resulting unpredictability of breeding. Finding or breeding the desired
speed/stamina attributes of a horse is a key to success.
DNA analyses of stamina and speed are based on multiple gene marker approaches
incorporating the results of our scientific research. Our philosophy is
founded on exact scientific research (such
as our 2006 study of mitochondrial genes – the first scientific study to
show a genetic association with performance in Thoroughbreds….)
but has developed over the years to incorporate broader genetic
interactions including these and other genes.
chances of breeding a successful racehorse are clearly influenced by
probability-dependent genetic occurrences. Our aim is to manipulate and
identify genetic probabilities in a more structured manner. We seek to
help our breeding and racing clients produce or select ‘complete
equine packages’ that have been assembled with coordinated genetic
components to do a specific racing job and are likely to be genetically
adapted to higher levels of success in a particular speed or stamina niche.
employ a key series of DNA tests and/or genetic datasets, to help co-ordinate
or identify genetic combinations that support common speed, stamina and
precocity objectives. These help to produce progeny or identify horses that
are more complete, targeted and uniform ‘packages’. These have both
breeding and performance applications.
Assessment of respiratory/energy release genes: There are 4 respiratory gene complexes that affect performance and are of primary importance to our assessments – NADH Dehydrogenase, ATP Synthase, Cytochrome Oxidase and Cytochrome b.
There are two points to note regarding these different complexes:
firstly, they are all part of a wider metabolic cycle called Oxidative
Phosphorylation (OXPHOS) that converts glucose into the energy rich compound
ATP that provides the fuel for cellular respiration and energy release;
secondly, the enzymes involved are coded by mitochondrial genes passed on only
by the dam but also by chromosomal genes passed on by both sire and dam. There
are positive and negative interactions depending on which
mitochondrial/chromosomal combination of gene versions are inherited from the
parents. Part of our overall
assessments is based on determining these combinations for individual horses.
Importantly, following on from our 2006 work, it is clear that many stallions
produce better progeny when crossed with mares from specific mitochondrial
types. We, therefore, use databases that tell us which stallions are better
used with specific mares and also of the likely effect on the stamina of the
Multiple marker relatedness/cluster analysis: The OXPHOS genes are critical to our analyses but it is still
important to recognise the multi-gene nature of racing performance. It is
clearly not necessary to investigate thousands of genetic positions, since not
all of these are related to performance but we use what we consider to be an
optimum number of genetic markers. This generally takes into account between
75 and 750 genetic positions of relevance. These have been determined by
studies associating presence or absence of DNA markers with performance,
relatedness and stamina levels amongst a reference group of thousands of
horses varying in these factors.
The chart below illustrates such an approach. It is possible to
group horses into clusters based on similarity of DNA profile via a
statistical process called PCO. Except for the ‘subject horse’, each
symbol in the chart refers to a DNA score from a known horse of Gr1 level performance and tends to group them in line with stamina
range. This chart is for an Australian mare called Sacred Habit, which, using
our molecular selection tools, produced the multiple Grade 1 winner, Sacred
Inbreeding or Outbreeding? - Determination of heterozygosity and homozygosity: A better indication of the level of inbreeding of a particular
horse is provided by use of DNA analysis to measure gene duplication.
Inbreeding leads to an increase in homozygosity (gene duplication). Inbreeding
is the best way of fixing consistent, positive genetic characteristics but
only if duplication of the less desirable ones is avoided. Using DNA markers
we have been able to determine that there is an optimum amount of gene
duplication that occurs in the best performers and this also varies according
to stamina range. A pedigree might indicate that an animal is actually inbred
whereas often the converse exists and vice versa. We believe that
inbreeding is beneficial but only
when applied under the right circumstances. Through use of DNA assessment we
can obtain a clearer picture of how close this can be carried out and whether
it should be avoided or increased. It is more accurate to use a DNA sample to
determine this but we have also been able to extrapolate data that more
accurately indicate suitable on-paper inbreeding when this is required.
Taking account of a horse’s heterozygosity enables prediction of
the potential status of resulting foals. This allows us to aim for the optimum
level in the foal, giving the best chances of performance and breeding
success. Using the same example as above, the heterozygosity of Sacred Habit
was very low, at around 42% - i.e inbred. Prior to analysis she has produced a
series of foals that exhibited various conformational problems. Assessment of
her heterozgosity, stamina clustering and OXPHOS status indicated to us to
make the recommendation of Choisir as a potential mate and this successfully
produced Sacred Choice.
Australian Breeding & Racing Magazine, March 2010, Playing God (Link to article on Thoroughbred genetic testing.....)
Thoroughbred Times, August, 2006, Scientist links mtDNA, racing ability (Ink) (Link to article on mitochondrial genes and Thoroughbred performance.....)
Pacemaker, June, 2006, The foundations of stamina (Link to report on stamina and speed gene.....)
Mitochondrion, April, 2006, Mitochondrial DNA: An important female contribution to thoroughbred racehorse performance (Link to paper on genes and Thoroughbred performance.....)
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