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Moving from Blood Typing to DNA


 

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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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