IN a dramatic shift from previous policy regarding genetic defects, the American Angus Association announced Aug. 14 that the Association would recognize Developmental Duplication (DD) as a genetic condition carried by some Angus seedstock, but would not require mandatory testing as a precursor for animal registration.
Originally described as polymelia, according to University of Illinois professor Jonathan Beever, the majority of calves exhibiting the condition are born with additional limbs, most usually a duplicate set of front legs originating from the neck or shoulder region. With the exception of calving-related mortality, affected calves generally thrive once the additional limbs are surgically removed.
The Association’s move to recognize the condition without mandating testing marks what president Phil Trowbridge called “an evolution” in the organization’s policy regarding genetic defects.
“Rather, the policy assumes that members will follow sound breeding decisions and make strategic use of DNA testing in dealing with this genetic condition,” he wrote in a letter to breeders. “There are alternatives to mandatory testing and, over the past five years, our members have shown a willingness to embrace them.”
Trowbridge said that earlier instances of genetic defects – including discovery of arthrogryposis multiplex and neuropathic hydrocephalus in 2008 – represented “catastrophic, once-in-a-lifetime events,” as both conditions were lethal. As such, early policies mandating genetic testing were predicated on a goal of eliminating those conditions from the gene pool.
In addition to DD, a simple recessive trait, the Association currently recognizes and maintains policies on five genetic conditions known to be carried in certain genetic lines. Beever’s initial testing of 1,099 Angus sires found at least 72 carriers of the condition, including some of the breed’s top carcass quality bulls.
“The allele frequency among U.S. sires is moderately high at approximately 3%; this corresponds to a carrier frequency of approximately 6%,” Beever wrote. “Given the moderate allele frequency, the rarity of affected calves, particularly as reported in the U.S. is somewhat puzzling.”
Beever noted that in work with colleagues in Australia
over the past two years, he has concluded that calves presenting with polymelia at birth are rare events that survive embryonic death. Instead, early DD events prevent many embryos from developing to term, reducing the frequency of live births of affected calves.