From
Blood To Hair
As we begin a new century, DNA testing is about to replace blood
typing for almost all horse breed registries. DNA testing has a
number of advantages over blood typing. For the standard test of 12
DNA systems most commonly used today, the probability of detecting
an incorrect parentage is 99.99%. Efficacy is based upon the
variability of the systems tested (reflected in the number of
variants at the tested systems and the frequency of the variants).
The higher the variability of the systems tested, the greater the
ability to detect an incorrect parentage. DNA systems used for
equine parentage testing have greater variation than blood typing
systems. This is partly because the DNA systems tested are
non-coding DNA. That means that they are not part of genes that
actually have a function, thus they are free to accumulate mutations
which are the source of variability.
Blood typing is based upon testing proteins, which are gene
products. Variation causing mutations are much less common in
functional genes and their products because mutations usually result
in proteins with diminished function.
Another advantage of DNA is that only one laboratory procedure is
required for testing, while several different procedures are
required for typing the different blood typing systems.
Simplification of testing will help to prevent cost increases.
Also, DNA can be obtained from almost any bodily tissue. The easiest
tissue for horse owners to collect is pulled hair with the follicle
(or root bulb) attached. It is the follicle, not the hair shaft,
that has the DNA. Hair samples can be mailed to the laboratory with
little chance of spoilage, as can happen with blood.
With these advantages it may seem surprising that many registries
did not change to DNA typing sooner. The primary reason is related
to DNA testing technology. The methods developed in the early 1990s
utilize genetic markers called microsatellites. Microsatellites are
highly variable and fairly easy to test using an
electrophoresis-based technology.
An alternative technique is to use a marker type known as a single
nucleotide polymorphism (SNP). SNPs are not as informative
individually as microsatellites so more must be tested to obtain the
same efficacy, but they could be tested using a microchip
technology.
The microchip technology offers several advantages to the
electrophoresis-based technology, but it is not yet available for
horse parentage testing while microsatellite techniques are
well-established. Those registries that have not changed to DNA
testing have chosen to accept the existing technology rather than
wait an unspecified time for a new technology.
Another issue is that DNA testing involves a different set of gene
markers than does blood typing. This means that all blood-typed
horses involved in a parentage case must be DNA typed, which is a
considerable expense for registries that have a major investment in
blood typing. In the long term, there should be enough cost saving
resulting from DNA testing to offset the cost of re-testing the
breeding stock.
The re-testing issue also influences which DNA technology to use.
SNP markers are different from microsatellite markers and if there
is a decision to change from microsatellites to SNPs for horse
parentage testing, the breeding stock will need to be re-tested
again. The costs and advantages of an SNPs-based test will determine
if this is a viable alternative.
DNA-based parentage testing of horses has arrived. The Jockey Club
plans to have the 2001 foal crop DNA typed and most horse breed
registries will be using DNA rather than blood typing within the
next two years.
Blood typing has served the industry well and there are still
aspects of blood typing that may continue to be used (such as
testing related to neonatal isoerythrolysis). However the future of
equine parentage verification is in DNA and the integration of
genomic science into the horse industry.
CONTACT
Dr. E. Gus Cothran, (859) 257-3337, gcothran@pop.uky.edu
Equine Bloodtyping and Research Laboratory
Article appeared in the Equine Disease Quarterly -
University of Kentucky, College of Agriculture, Department of
Veterinary Science: April 2000 Issue; Volume 8, Number 3
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