Equine Disease Quarterly | EQUINE
Equine Disease Quarterly –
January 2020
Commentary
The art of horse production is pattern recognition.
The most obvious pattern is that offspring resemble
their parents. These resemblances can be found in
phenotypes such as size, conformation, durability,
and performance. Using phenotypes and pedigrees,
breeders have applied selection, resulting in profound
changes in horses since their domestication 5,500
years ago. Today, hundreds of breed registries exist,
reflecting diverse goals among horse breeders. These
interests span divergent phenotypes related to racing,
pulling, jumping, dressage, performing special gaits,
and characteristics associated with coat color or size.
The commonality among these traits is they have a large
genetic component. Foundation Stock were identified
exemplifying traits of interest. The path to improvement
has been to identify quality breeding stock, cross
them to Foundation horses and select the best for
future breeding stock. As a result, Foundation Stock
are represented many times in horse pedigrees and
contribute to the distinctive phenotypic characteristics
within those breeds.
Recently, a reference whole genome sequence of the
horse was determined. Scientists identified genes for
discrete traits such as coat colors and some hereditary
diseases. However, despite a great deal of research,
no single gene has been found responsible for
complex traits such as racing, jumping, and gait. This
is not surprising since athletic performance involves
a combination of muscle strength, cardiovascular
capability, competitiveness, and coordination. Yes, a few
genes have been identified that influence performance
(e.g., DMRT3 for gait and MSTN for sprinting); however
the development of champions involves many other
genes as well as the art of the trainer and skill of the rider.
This relationship is remarkably complex since variation
exists despite generations of selection. The resilience of
genetic variation may reflect the diverse ways in which
a horse can become a champion. Northern Dancer and
Secretariat were champion Thoroughbred racehorses;
however, while Northern Dancer was relatively small in
stature, Secretariat was renowned for his large size and
stride length. While the old breeding adage is “breed
the best to the best and hope for the best,” the skill of
the successful breeder is to identify hereditary patterns
among the best and make judicious choices. There is
no single genetic test that will replace the skill of the
breeder.
Genomic tools have the potential to improve the
recognition of useful patterns. Horse genes are
distributed among 32 pairs of chromosomes (see
the image in the online version of this issue at http://
gluck.ca.uky.edu/equine-disease-quarterly).
These
chromosomes contain the genetic material under
selection. Genetic recombination occurs during
production of eggs and sperm in each generation
that divides these genetic blocks into subsets of
chromosomes. It is possible to identify the specific block
inherited from each parent and relate those blocks to
blocks found in the grandparents. Likewise, one might
• Volume 22 Issue 01 | March 2020 •
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