Identification of rare gene will help Angus breeders avoid problems
August 26, 2013
American Angus Association officials announced this week that a genetic condition known as Development Duplications (DD) or polymelia has been discovered in some Angus sires.
Early embryonic death has been observed to be the most common result of DD. However, in a rare number of mildly affected cases, calves with additional limbs have been born and survived, usually exhibiting duplication of the front legs originating from the neck or shoulder region.
Dr. Johnathan Beever at the University of Illinois and Dr. Laurence Denholm of Australia's NSW Department of Trade and Investment have conducted research to identify the genetic issues related to the defect. The condition is known to occur across many species and breeds of cattle.
In a statement this week, Angus Australia CEO Dr. Peter Parnell noted that initial reports of the condition were reported to Angus Australia some four years ago.
"It has taken until now to obtain sufficient number of affected calves to conduct the necessary research to understand the genetic nature of the condition," Parnell stated.
"With the exception of mortality associated with calving difficulty, these calves (born with polymelia) can often thrive, particularly if the extra limbs are successfully removed," Beever's report says.
Recommended Stories For You
The genetic condition was first observed in Australia where a small number of calves (approximately 20 over a four-year period) exhibiting duplication of front legs were reported. About three years ago, at the request of the Angus Society of Australia, Beever began investigating the condition, utilizing the calves from Australia and several that were born in the U.S.
"It has recently been determined that the condition is inherited as a simple recessive trait," Beever's report says. "We have successfully developed a DNA diagnostic test to identify carrier animals."
At the beginning of his study, Beever worked with DNA samples from affected calves as well as several DNA samples from their sire and dam.
"Even though pedigree analysis of the calves clearly showed evidence of line breeding, the molecular analysis revealed nothing that would lead us to believe that the condition was genetic," Beever reports. "Throughout 2011 and 2012, Australia continued to receive reports of polymelia and again in 2013 was able to provide us with additional DNA samples for analysis."
Examination of the most recent DNA samples led to the conclusion that the deformity resulted from recessive gene inheritance.
"Following this (early 2013) analysis, four calves displaying the same morphological features were reported here in the U.S.," Beever's report says. "Samples from these calves were collected and the analysis once again demonstrated that all the affected calves, both Australian and U.S., shared a common region of homozygosity, thus confirming the presence of a recessive allele responsible for the phenotype."
Beever's research has revealed that the allele frequency among U.S. Angus sires is moderately high at 3 percent, however there are no Angus sires in the present population that will, by themselves, produce calves with DD.
"Of course, one would not expect a calf born with five or six legs to become an AI sire," Beever says in his report. "However, given the moderate allele frequency, the rarity of affected calves, particularly as reported in the U.S., is somewhat puzzling. Additionally, given the use of specific U.S. sires within Australia, now known to be carriers, the frequency of reported calves is also unexpectedly low."
Beever went on to report that he has concluded that calves presenting with polymelia at birth are "rare events that survive embryonic death."
"In support of this conclusion, we also genotyped a case of conjoined twins that were submitted by Dr. (Laurence) Denholm (Australia) as part of this project," Beever reports. "Indeed, these conjoined twins are also homozygous for the same mutation. We hypothesize that early development duplication (DD) events prevent many embryos from developing to term resulting in embryonic death and reduced frequency of live births that are being observed."
Eric Grant, American Angus Association Director of Public Relations, notes that genetic mutations are found in all living species and leading geneticists in the bovine academic world suggest that all breeds have hundreds of mutations in their genome.
"Most agree that an association's approach to genetic conditions should reflect the likelihood – indeed, the certainty – that disovery of such conditions will continue in the future and at a pace accelerated by new scientific tools," Grant says. "As these tools and scientific understanding evolve, so too will continued detection of genetic conditions in all breeds of cattle."
Beever's report includes a listing of 1,099 Angus bulls tested to determine if the bulls are carriers or are free of the DD genetic condition. The listing is available to U.S. beef producers through the American Angus Association. Results for more current bulls in several breeds are pending and will be released as soon as they become available.
Parnell notes that beef producers who observe significant reductions in calving rates might consider testing animals for the recessive trait.
"An important part of the breeding program for any registered seedstock operation is identification and elimination of undesirable recessive genes in the population," Parnell states. "The recessive nature of (polymelia) means that in most situations they are rarely observed, and only occur when an individual animal inherits two copies of the recessive gene, from either side of its pedigree. When an animal contains just one copy of the gene it is considered a 'carrier' and does not display the particular condition."
Parnell has further advised that bloodlines carrying the recessive gene have contributed "great benefits to the Angus breed over past decades through increased growth, fertility and carcass performance. It is simply a matter of identifying and eliminating carrier animals from the population and continuing to utilize the positive attributes of these genotypes," Parnell adds.
Grant says the American Angus Association takes genetic conditions very seriously because they affect breeders and their customers. The American Angus Association works to equip producers with information and technologies essential to sound breeding decisions.
"Over the past five years, our members have shown willingness to embrace both genomic technology and other practices to reduce losses associated with genetic conditions," Grant says. "These include a better understanding and acceptance of the ability to manage around a known genetic condition by avoidance of breeding a carrier to another carrier and by use of voluntary, strategic DNA testing. Our commercial breeders also understand and embrace these management principles."