So what do you do with your carriers of genetic defects?




Uploaded by purduebeefteam on 29.06.2009

Transcript:
I’m Terry Stewart professor of Animal Sciences with Purdue University Department of Animal
Sciences. The area that I am responsible for is beef cattle breeding. I also do some undergraduate
teaching. Today we are going to be talking about what do if you have a carrier of a genetic
defect in your herd. First we ought to really discuss how do you know if you have a carrier
or not. Well the most obvious is that the animal actually express the genetic defect
itself, of course this would only apply to the non-lethal defects. If it was lethal the
animal would be dead. You might not know why it died, a little bit of an unknown scenario.
But the most obvious way of knowing if you have a carrier is when it has produced an
affected offspring or has a subsequent descendant. It may be a grand offspring or even a great
grand offspring. Since most genetic defects are recessive traits, it takes two genes for
them to be expressed and there may be a generation or two before it shows up. The other thing
is to realize you have potential carriers whenever they are offspring or further descendants
of animals themselves are carriers. You’d know this first by pedigree inspection to
know if they are a potential carrier and then it cold be potentially confirmed with a DNA
test if that DNA test is available for the particular trait that you are concerned with.
The reason that we have to be concerned about carriers is that we are breeding two animals
that are carriers together; we have a 25% chance of producing an affected offspring
and about half of the offspring themselves will be carriers so the problem will persist
in your herd. You have several options once you have a carrier animal. One is you could
just cull the animal outright and be done with it. You could use the animal to produce
feeder calves so that the offspring would not be coming back into the breeding herd,
he would not be perpetuating the gene, but if the animals have some breeding value for
other traits that are important to you, you could screen the offspring of that animal
to find an animal that is defect free but still carrying the other positive characteristics
of that animal, so how do you decide? Well first is what is the breeding value of that
animal for other traits that are important to you? Is it a superior animal? Is this an
animal that you’d really like to have remaining in your herd? What is the availability of
the other animals of similar value that are clean, that are not carrying that genetic
defect? And then lastly, what is your ability or your willingness to take on the extra management
details of managing those carrier animals. One of the things to always keep in mind is
that whenever you breed a carrier animal 50% of those offspring are going to be carriers
too. When you breed that animal to a non-carrier mate, you would expect to produce two offspring
out of four that are homozygous clean but the other two out of four would be carriers
of the defect. So after you produce those offspring what you would then need to do is
do a DNA test assuming the test is available for the trait you are concerned with. First
you should consult with your breed association for the list of approved tests and approved
laboratories that can perform that test. You would submit either a hair or a blood sample
in most situations. The fees for doing these tests range from about 20 to 40 dollars depending
on the test and whether you are going to be doing multiple tests and discounts if you
are doing large numbers of animals you can actually get some discounts, further deeper
discounts than the 20 to 40 dollar range. After you submit your sample and wait a few
weeks for the lab to send you back a report, you’ll receive back a report from the lab
that indicates the DNA genotype of those animals for each trait. There is really three possible
outcomes. It will tell you that the animal is free of the genetic defect and that is
really the positive result. It will tell you that the animal is a carrier and those are
the animals that you are going to have to decide whether you want to keep them and further
manage them or whether you just want to remove them from your herd, or you may get a re-submit.
You need to realize that there will be a few times that they are not able to extract sufficient
DNA from your sample and then you would have to resubmit another DNA sample so that they
can confirm the test. The labs however will usually do that free of charge so there is
no additional cost. So in the end after you have your DNA results back and you make a
decision, you have to decide is that animal worth the extra management headache of keeping
it in your herd and trying to manage the gene, realizing that if you happen to breed two
carrier animals, you will be perpetuating the defect in your herd. Probably a different
decision for bulls versus cows, bulls producing a large number of offspring, fewer bulls in
your herd, you probably really don’t want to deal with any carrier bulls. My recommendation
would be to make sure you remove any carrier bulls from your herd. On the carrier cow side,
if you have the option of using them to produce offspring that would go to feeder calves you
could still utilize those animals in your herd. If you are producing breeding stock
again you probably would want to remove them from the herd or do further testing of their
offspring to make sure that you don’t perpetuate the gene.