Geneticist Jonathan Beever offers some insight into the issue of genetic defects in cattle.
Listen in on just about any conversation nowadays and sooner or later, the issue of genetic defects will likely come up. BEEF sat down with Jonathan Beever, a University of Illinois in Urbana-Champaign geneticist and one of the nation's top authorities on genetic defects in cattle, to get his perspective.
BEEF: What causes genetic defects?
Beever: Mutations in the genetic material (DNA) occur with a predicted frequency (somewhat rare) in every generation in all breeding populations. In addition, mutation is more or less random. In other words, most mutational events are spontaneous and not caused by something like an environmental agent or exposure.
The majority of these mutations have no effect because they either occur in non-gamete cells (cells other than sperm or eggs) or occur in non-functional regions of the DNA (only about 95% of the genetic material in mammals is functional). The remainder that occurs in functional regions can have both positive and negative effects. In fact, constant mutation rate contributes significantly to continued genetic variation and thus, genetic progress.
However, in some instances, mutations can have a damaging effect. Fortunately, many of these mutations will be recessive in inheritance pattern, meaning that because we all have two copies of every chromosome, there is an “evolutionary safety” mechanism protecting us from being affected with these phenotypes.
This is why it takes so long for us to detect recessive defects — initially, all matings result only in animals that are heterozygous for the mutation. Unfortunately, however, as the frequency of these mutations increase, there is also an increased possibility that breeders might select and mate two individuals that carry a defect, thus producing an affected offspring. It can take decades from the time of mutation for this to happen.
BEEF: Are there more genetic defects in cattle now than in the past?
Beever: Not really; it is merely the recognition and reporting that has increased over the years. Additionally, changes in the industry, particularly in reproductive technologies, can bring them to light faster. That is actually a good thing in the long run, but can hurt in the short run.
For instance, the increased use of artificial insemination (AI) allows a single bull to produce thousands of offspring in a single calving season. If that bull is a carrier of a genetic defect, the frequency of the mutation can increase quite rapidly and again increase the probability of mating two carriers in future generations.
BEEF: Why are genetic defects a problem?
Beever: They have the potential to cause significant economic impact on an individual breeder, even though they may seem relatively negligible when considering an entire breed or population of cattle. Here is an example, which is actually not far from a true story.
A guy runs 200 tiger stripe (Hereford-Brahman) cows. He uses AI to breed the cows to a prominent Angus sire and gets a spectacular calf crop. So far, no problem.
The heifers are so good he decides to use them as replacements. They're bred to another popular calving-ease Angus sire, and he pays some good money to purchase some additional good-quality Angus bulls for cleanup (the bulls are of highly similar pedigree to the first AI sire). He just can't wait until calving season.
However, come calving time, he has eight calves born dead of arthrogryposis multiplex (AM or Curly Calf Syndrome). Add up the lost revenue of carrying a cow for nine months and the value of the calf he was going to sell, then multiply by 8. Be sure to include the premium over the salvage price of the clean-up bulls he has to ship to avoid having the problem again. The losses start adding up real quick.
BEEF: Is the gene frequency for genetic defects increasing in populations?
Beever: It depends. In most instances with DNA tests, the frequency is being reduced. However, I mentioned about how mutations are actually one of the continued sources of genetic variation in populations. We do have documented examples of where mutations in certain genes cause “better performance” in animals that are carriers of some of these mutations.
Thus, in some instances, it would not be unreasonable to see a rapid increase in the frequency of a particular mutation if it were under selection. Otherwise, the increase in frequency can simply be due to the coincidental heavy use of a popular sire or line.
BEEF: How concerned should commercial producers be about using untested bulls or bulls from lines known to have genetic defects?
Beever: There are many different levels of risk, depending on what a guy has to work with. In the earlier example, you can see that using the first Angus bull (an AM carrier) worked great. If the breeder didn't keep replacements or had used another breed with which to cross them, he would have avoided any loss for several generations.
So, for instance, if a guy is buying untested Angus bulls to breed his Charolais cows, no problem. He just needs to be aware of what he has to do if he chooses to save or sell any replacements. On the other hand, if he is running a very heavily Angus-influenced female base of somewhat unknown genetics, I don't know if I would take the chance.
I have started using the analogy of disease vaccines. Few ranchers have seen bacterial or viral diseases in their cows or calves, yet they vaccinate faithfully every year because they know “it could happen” and the financial consequences could be severe. The cost to vaccinate and worm a cow for the year is $8-$13/head. The cost to make sure your whole herd is protected against having a calf affected by a recessive genetic defect is $25 multiplied by the number of bulls you buy; but it's free if you're using an AI bull that someone has already tested for you.
BEEF: Where in the production chain should these defects be managed?
Beever: In my opinion, the burden should be shouldered by the seedstock producer. A “top-down” approach is the most efficient way to move these problems out of a population. If everyone uses only tested-free bulls, all affected calves disappear immediately. After several generations, the frequency in the cowherd can be diluted out without any additional cost to the commercial producer.
BEEF: Can DNA marker technology help manage defects?
Beever: It's the only long-term way to deal with them. If a genetic defect exists in a population, we have a number of different choices. We can ignore it; we can try to eliminate it by pedigree, which was very successful in the 1950s with dwarfism; or we can try to find outcross genetics and breed away from it. All of these will reduce the frequency of loss due to affected calves in the herd.
The problem is none of these are permanent solutions. We've had multiple examples of that over the last 30 years, where different breeds have selected away from pedigrees that have had known genetic defects. And here they come in 2008 or 2009, visiting us again.
So the only alternative is using the technologies we have in hand to begin actively identifying individuals that are carriers of these genetic defects.
BEEF: How accurate are DNA tests?
Beever: There are two distinct components that contribute to the accuracy of any DNA test. The first involves the scientific data that underlie the test. The second is the design and execution of the diagnostic assay performed as part of the testing procedure.
From a scientific standpoint, I'm confident the tests are nearly 100% accurate. Using the test for idiopathic epilepsy as an example, we estimate the assay may have an error rate of around 1 in 10,000.
However, as with any task requiring human intervention, errors can occur. Of course, we do our best to limit any errors by automating portions of the test and appropriately incorporating testing controls. But the testing process starts with sample collection and ends up with reporting. Any time humans are involved, you get sampling and labeling issues. But functionally, the tests are as accurate as the quality-assurance process that is present in the lab that analyzes the test.