ASA: New defect points to need for crossbreeeding -

When the gene associated with this condition is paired in a mating the results are either 1) early embryonic death or 2) calves are born with multiple limbs or polymelia. Polymelia occurs spontaneously in cattle of a variety of breeds as well as sheep, horses, and humans. Other than an increase in the occurrence of mortality associated with dystocia, calves born with these extra limbs often thrive (especially with removal of the limb[s]).

During the last four years, the incidence of polymelia rose above expected sporadic levels in purebred Australian Angus populations.

Drs. Laurence Denholm (the NSW Department of Trade and Investment) and Jonathan Beever (Agrigenomics, Ltd. and the University of Illinois) led the investigation into the genetic control of this condition and developed a test for the defective allele in the Angus population. Based on their research, developmental duplication is a reportedly a simple recessive trait like so many other genetic defects (AM, NH, CA, OS, etc.) meaning an animal must carry two copies of the defective gene in order to show this condition.

Dr. Beever tested 1,099 high-use AI Angus bulls and found 72 carriers of the defective allele. Based on this analysis, Dr. Beever estimates an allele frequency of 3 percent (considered moderately high). This corresponds to 6 percent carrier frequency among these U.S. Angus sires. Based on the allele frequency, the incidence of polymelia should be higher than is actually reported.

Due to this discrepancy, Dr. Beever hypothesizes a relatively large proportion of embryonic loss during gestation in homozygote embryos (possibly up to 80 percent). This would dramatically reduce the number of homozygous calves that survive to term and therefore result in a lower occurrence of the polymelia condition than the allele frequency would predict. Because the embryonic loss is thought to occur early in embryonic development, it would be interpreted as simply a failure to conceive.

In addition to the troubling news of yet another genetic defect hitting the beef industry, it is unsettling how quickly new defects are being discovered. In all reality, these defects and many more have likely existed for a long time — we are simply better able to detect, report, and, fortunately, develop markers to identify their incidence and location in the population.

As science progresses in this area, evidence is beginning to mount that there are a large number of genes in all populations of cattle that result in, at the very least, early embryonic death, when paired. The number is so large it is quite likely that all animals carry at least some of these genes. If this is in fact true, we will need to develop more effective tools and policies than we (the seedstock industry) currently employ to address the reality.

Certainly, improving our ability to manage defects by avoiding matings that will result in a significant possibility of defective genes being paired will be of value. In the commercial industry, we already have a system in place that works quite effectively — it's called crossbreeding!

As most of you know, ASA has a well-established policy and infrastructure in place to deal with genetic defects. Our Board will ultimately determine how we will best deal with DD. We will keep you abreast on our progress.

More detailed information on polymelia and a list of Angus sires that have been tested for the condition can be found on the Angus association's website.