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DNA Selection - The Basics

03 December 2009

Scientific discoveries, despite been mind boggling, are providing a number of opportunities in the livestock sector, writes Dr Scott P Greiner, Extension Animal Scientist, Virginia Cooperative Extension.

Mapping of the human genome has led to the mapping of the bovine genome (and other species). This genetic map provides the pathway for things to come related to genetic selection in beef cattle. The use of DNA technology is rapidly expanding in the cattle arena, and as a result our toolbox is getting bigger. A specific example is our ability to manage genetic defects through DNA tests which provide the specific genotype of an animal in question. DNA genotyping tests have very rapidly been discovered and applied commercially for the industry to deal with arthrogryposis multiplex (AM) and neuropathic hydrocephaly (NH), two genetic defects controlled by a single gene. Our ability to manage coat colour through DNA genotyping (identification of black and red alleles) has been available for several years.

Historically, genetic evaluations in beef cattle have been developed using pedigree information complimented by performance records. In simple terms, expected progeny differences (EPDs) originate from the average of an animal’s sire and dam EPDs, which are then adjusted for the animal’s own performance and the performance of its progeny. As more performance data is accumulated on an animal’s progeny, and its relatives, the accuracy of the EPD is enhanced. EPDs are reflective of the cumulative effect of the many genes influencing economically important traits (growth, maternal, carcase traits).

DNA selection offers the potential to identify individual genes or groups of genes which have an effect on a trait of interest. Research to date using DNA markers (markers have close association with region of genome affecting a trait) has shown that for most growth and carcase traits, each individual marker explains a relatively small proportion of the genetic variation in the trait. Consequently, it is possible for an animal to have a very strong EPD a given trait yet have a “poor” DNA test for the same trait. The reason for this is that EPDs reflect the cumulative genetic merit for all genes that affect a trait, whereas a single DNA marker only provides a snapshot of one (or a few) genes that affect that same trait. Such scenarios create challenges with incorporating DNA genotypes into selection decisions, particularly for traits which also have EPD information. Ideally, the two sources of information could be integrated - and DNA information could enhance EPDs. This is the precise direction the industry is headed with DNA.

Very recently, DNA technology has advanced to the point that it is now possible to determine the DNA profile of an animal utilising many markers simultaneously (50+). These DNA tests are likely to be more informative since they scan a larger number of genes impacting a trait. This winter, the American Angus Association will release the first EPDs enhanced by DNA information. Angus breeders are currently submitting samples to be DNA tested, with results reported to AAA. DNA profiles will be recorded for several traits, some of which EPDs are currently available and some which do not exist in EPD form (example is feed efficiency). The DNA information for carcase traits will be incorporated into the fall EPD genetic evaluation, and the EPDs released subsequently will be influenced by the DNA information. The primary impact of this new approach is likely to be enhanced accuracy of EPDs for young animals (those without progeny records).

Moving forward, DNA technology and the utilisation of genomic information in genetic evaluation provides great potential to offer selection tools for traits which are difficult and/or expensive to measure (and therefore there is limited performance data), such as feed efficiency. Will we cease to take performance measures? Likely not, since phenotypes are needed to associate differences in DNA with corresponding influence on the trait.

In summary, DNA technology is advancing rapidly and offers significant potential to enhance genetic evaluations in beef cattle. By incorporating DNA information into existing genetic evaluation systems to provide enhanced EPDs, the industry will be able to capitalise on this technology in a fashion which is already familiar to producers.

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