DNA to become a commercial breeding tool
DNA markers capable of accelerating genetic progress are becoming commercially available, offering breeders a powerful new tool
While all sheep inherit the same basic genome, with a similar complement of genes, small variations at specific sites along the DNA strand can account for differences between animals in the performance of a particular trait.
By identifying this variation, DNA markers allow breeders to track desired genetic differences and make earlier selection decisions in flocks.
Markers alone will not be a ‘silver bullet’ for achieving breeding objectives. Nonetheless, marker-assisted selection can accelerate genetic gain, and access to the technology is set to improve as markers become commercially available. In addition to DNA tests for parentage, Inverdale® and Booroola (reproduction), MyoMAX®, LoinMAX® (muscling) and i-SCAN® (blindness) markers are available to the sheep industry, and more will be released for commercial use in the next two to five years.
Since markers can readily detect genetic merit in an animal as soon as it is born, the technology is especially useful for predicting traits that:
- are difficult or expensive to measure, such as parasite resistance, staple strength, feed utilisation or footrot resistance;
- require the animal’s slaughter to measure characteristics such as meat yield and eating quality;
- are not expressed until adulthood, such as reproductive performance;
- are expressed only in one sex, such as milking ability; or
- are detrimental or require special management in homozygotes, such as spider lamb, dark fibres or the Inverdale® gene.
SheepGenomics program director Dr Rob Forage says using the markers simply involves taking a blood sample – typically on a blood card – and having it analysed by an accredited laboratory. However, he stresses that selection based on a single DNA marker, without reference to other performance characteristics, will not necessarily lead to improved performance.
Often it is necessary to take into account complex interactions with other genes that affect a trait. This extra information can be provided by selection indexes or breeding values (BVs), such as those reported by Sheep Genetics Australia (SGA).
"In some cases the advantage of markers is to improve the accuracy of currently available BVs," Dr Forage says. "The marker test is not a replacement or substitute for the ASBV."
Australian Sheep Breeding Values (ASBVs) that take marker and phenotype information into account will provide a level of accuracy above those generated using conventional measurement techniques.
Only when the DNA marker is for a gene that accounts for the majority of the variation in a particular trait is the ASBV of less use as a selection tool. These markers are called ‘major markers’.
However, many traits do not have ASBVs, either because they are expensive or impractical to measure in the living animal or on-farm. In this case, the traits may be measured in research flocks and the DNA markers arising through scientific discovery can be used later in commercial flocks as indicators of performance. These are termed ‘proxy markers’.
Some markers are not yet associated with observable traits, but can still be used to provide parentage information and increase the rate of genetic gain through more precise breeding decisions. These are described as ‘utility markers’.
SGA manager Dr Alex Ball is working closely with SheepGenomics and marker test providers to gain access to all DNA marker information, which will enable accurate calculation of ASBVs.
"SGA is developing its database to ensure that all DNA marker information can be uploaded and, where available, Type I performance markers will be integrated into existing ASBVs," Dr Ball says. "Breeders and producers will get the best value out of markers when they are properly validated and all the information is included in the ASBV calculations."
SGA is planning a series of Ram Breeder workshops in 2007 that will include a simple, practical explanation of the principles of markers and how they are being incorporated into ASBVs.
More information: Dr Rob Forage, 02 9463 9169, rforage@sheepgenomics.com, www.sheepgenomics.com; Dr Alex Ball, 02 6773 2948, aball@mla.com.au; Dr Troy Fischer, AWI program manager sheep productivity, 02 8295 3151, troyfischer@woolinnovation.com
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