A Small Change in DNA Has a Big Impact in Angus Cattle

Wayne R. Wagner, PhD.
Extension Livestock Specialist
West Virginia University
Morgantown, WV 26506

As you are probably aware, a lethal genetic recessive gene has been recently discovered in Angus cattle.  It is important to understand that this defect is the result of a mutation that occurred and no one is at fault.  Mutations occur naturally and are more often detrimental, as is the case with this one, than good.  This one was discovered because it occurred in a popular line of cattle, some inbreeding took place, and the defect was uncovered as a result of the close matings of relatives.  Most mutations will go unnoticed because most animals produce relatively small numbers of offspring.  This genetic abnormality is known as Arthrogryposis Multiplex (AM).  A normal individual is referred to as AM-Free (AMF) and an individual that has one copy of the defect is heterozygous for the gene and is called an AM-Carrier (AMC).  These cattle will physically appear normal.  If a fetus has two copies of the defect,  it is homozygous for the defect, the calf will be abnormal and it will not live. 

It is important to understand these genetic principals so you will not have to fear this defect.  Modern technology will allow us to detect carriers of the  defect through a DNA test.  A carrier animal (AMC) will transmit the defect to approximately 50% of its offspring and 50% of it’s offspring will receive a normal copy of the gene.  An AMF animal does not have the defect and will never transmit it even if it has an AMC parent.  To get an abnormal calf, both parents must be AMC and only 25% of calves from such a mating will be abnormal.

I believe seedstock producers should assume responsibility for eliminating this defect.  If a commercial producer uses only AMF bulls, they will never have a problem.  Therefore, seedstock producers need to DNA test any cattle that may be carriers and eliminate those diagnosed as  (AMC) so the commercial industry does not have to deal with this problem.

For many purebred producers it will be relatively easy to assure themselves that they are AM-Free.  However, it will take some effort for others.  Let’s take a hypothetical situation where a purebred producer bought a son in 1990 of the oldest bull that was identified in the release from the Angus Association.  That purebred producer used that bull as a clean-up sire behind an AI program in which about 50% of the herd conceived to AI.  Because he was used as a clean-up bull no DNA sample was collected.  Therefore, the breeder has no idea whether or not this bull had been AMC and over a three year period he kept five daughters.  Since we can not test the bull, the next choice is to test the daughters, but four of the five are no longer in the herd.  The producer can collect DNA from the one daughter  and if she is determined to be AMF, he will not need to test any of her descendants because they will be AMF.  However, this will not prove the clean-up bull in question is AMF and, in fact, he could be AMC.  If she is diagnosed as AMC, then her daughters will need to be tested and if a daughter is AMC her progeny will also need to be tested.

One daughter no longer in the herd has three daughters currently in this herd.  Since there is no DNA on the dam of these three, each of these will also need to be DNA tested to determine whether they are AMF or AMC.  Finally, there are three granddaughters of that clean-up bull out of the other  daughters and each of those will need to be tested and if any of them are AMC,  their descendants will also need to be tested.

This episode reminds us just how long a bull’s genes remain in a herd and how the bull we choose impacts the herd for many years.  Bull selection is pretty important and we need to take it seriously.  Remember, in the last 18 or 20 years, there have been many composite breeds of cattle that have incorporated Angus genetics into their cattle and some of those Angus genes used would include bulls identified as AMC.  A small change in the DNA of a single gene in a single animal, probably in the  1970’s,  is now having a big impact on the beef cattle industry.  I believe there are a significant number of cows in the current population that are carriers of this defect, simply because it took nearly 30 years to discover the mutation.

Contributed November 18, 2008