A Chromosome's Worth Of DifferenceTue, 23 Mar 2021 08:29:16 CDT
Sean McGrath, Genetics and Beef Consultant and Rancher writes in an article for simmental.org on EPD's.
While EPDs have been around for several decades there is still a lot of confusion about where they come from and their use. A lot of comments I hear relate to the confusion between the actual performance of a feedstock animal and the EPD for said animal. So letís go back to the beginning.
An EPD is an expected progeny difference and it uses all the available information we have to describe an animalís DNA for a specific trait. The EPD uses the animalís own performance in its calculation but it is not the performance of the animal, it reflects the predicted performance differences of the OFFSPRING. Letís look at the following example. We know that DNA is arranged in chromosomes and that chromosomes come in pairs. (Cows have 30 pairs or 60 chromosomes total). For this example letís simplify and only look at one chromosome. chromosomes from the parents, and thus they could have completely different DNA for various traits. A good example of this in the real world is that of bulls and heifers. We know that bull calves get an X and a Y chromosome at pair number 30 and heifers receive two X chromosomes. We could have full-sib calves, but a heifer and a bull have obvious differences caused by having a completely different chromosome.
To further complicate things, when the sire and dam are replicating their DNA, parts of the various chromosomes can swap places. If we use our example, we could end up with a calf that looks like Calf 3. The multitude of potential combinations runs into the billions, even if we are talking about a single sire/dam mating. This is the reason that we need EPDs.
On average, a calf is somewhere in the middle of the sire and dam; however, obtaining more information allows us to better predict what DNA that calf is carrying and thus can pass on to their offspring.
As we add information, we can do a better job of predicting differences between cattle and are more sure of the results. We express the amount of information included in an EPD in terms of accuracy. Since an EPD is a reflection of DNA that can be passed on to progeny, measuring progeny is the ultimate information, but measuring the actual animal is a good first step. As we add progeny measures, the impact of pedigree, DNA, and the animalís own performance declines in overall importance.
So to back up a step, each animal is the result of the DNA they inherit, interacting with their environment. We call the actual performance/appearance of the animal the phenotype. Initially, we may know pedigree information on each bull. If we take performance measurements of phenotype we can determine their performance relative to other animals that are managed in the same way (the same environment).
This same environment is what we refer to as a management group. A contemporary group is a further refinement. It includes animals of the same sex, of roughly the same age and raised in the same environment. If the environment is the same, then a lot of the differences we see between phenotypes are due to genetics or differences in DNA and interactions in the DNA. changes in an animalís EPD that are predicted from pedigree alone. Once we know the genetic component of those performance differences, we can compare these
differences across herds by using pedigree ties.
We can now further refine this, by looking at DNA directly using a genomic panel. This obviously also tells us a lot of information about what DNA the animal received from its parents and thus can change the EPD significantly as well.
Finally, since the EPDs are ďexpected progeny differencesĒ once an animal becomes a sire/dam and passes on its DNA, we can measure the offspring and see what differences are expressed in the progeny.
Letís go back to Calf 1, 2 and 3 again. From their pedigree we would predict that they would have identical EPD, but we can see that they have different DNA. Letís for a second assume that they were in the same herd, are all bull calves, born in 2020 and were raised together. They are a contemporary group. Their weaning information is shown in Table 1. Based on the weaning information we would predict that Calf 1 has DNA that is superior for growth from either Calf 2 or 3, and that Calf 2 has a genetic combination that is even lower for growth. A DNA test may further confirm this and provide more accuracy or certainty to that prediction, see Table 2 for an example.
Information in an EPD
Where to From Here?
A bull cannot pass on their own performance to his progeny necessarily. The performance of an individual is merely an indication of what DNA they might be carrying that they can pass on to their offspring. While individual performance is useful, management can impact the expression of genetics, so performance alone can be somewhat misleading. We are really interested in what DNA the animal possesses that can be passed onto offspring.
I have heard the statement made that order buyers donít pay for EPDs, and this is correct. Since we are not expecting progeny from our steer calves or market animals, we care about their performance since that is what we are paid on. In the same vein, we are not paid for the performance of a sire, we are paid for the expression of his DNA in those market offspring or retained females. EPD can provide us insight into the DNA that each animal possesses and can pass on to their offspring.
Sean McGrath is a rancher from Vermilion, Alberta and also a genetic consultant for a number of Canadian beef breed associations including Canadian Simmental and Canadian Gelbvieh Associations. Sean is a very engaged contributor to the IGS collaboration and works closely with IGS staff.
to see all the graphs and pictures that go along with this article in Simmental.org, click here:
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