Genetics of animal health

Published on December 1, 2013   52 min

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DONAGH BERRY: Hello. My name is Donagh Berry and I am an animals quantitative geneticist at Tegasc, Moorepark, in Ireland. I'm going to talk to you about the genetics of animal health.
With increasing world human population size and affluence, there is expected to be a large increase in the demand for food. To achieve this demand, animal health is fundamental. There's also increasing consumer concern, especially in the developed world, on food safety, especially when it comes to zoonotic diseases, which can be transmitted from animals to humans. And then finally, there's also increasing concern over animal health and welfare.
Genetics is particularly advantageous in that it is what we call cumulative and permanent. So in other words, we can introduce or introgress good genes into a population. And these can be built on with each generation. On the converse, however, if you introduce bad genes, they can be very difficult to breed out. So therefore, genetics must form part of an overall strategy to improve the animal health status of our world population.
I would argue that the heritability is probably one of the most misinterpreted statistics in quantitative genetics. The heritablity depicts the proportion of the field variation attributable to genetics. So in other words, if you went to a population of animals, what proportion of the differences or variation amongst those animals is actually due to differences in their genetic makeup? Now, a key point to remember about heritablity and variation is that a small proportion of a trait with lots of variation may actually exhibit more variation than a moderate proportion of a trait with little variation.
So in other words, 30% of these $100 bills on the left hand side of the slide is actually equivalent to 3% of the stack of 10 piles of these $100 bills.
There are two types of heritablity estimates. The first, broad sense heritablity, denoted as a capital H squared, includes both the additive and the non-additive genetic variation in its numerator. Additive genetic variation includes the additive effect of values within a single DNA locus and is generally passed on from one generation to the next while non-additive genetic variation includes the effects of interactions between DNA loci. The second type of heritablity estimate is the narrow sense heritablity. And this is denoted as a lowercase h squared. The numerator of the narrow sense heritablity is simply the additive genetic variation. And it is the narrow sense heritablity which is most commonly used in animal breeding. So looking at the literature as well as within this presentation, we generally refer to just a narrow sense heritablity estimate unless specified otherwise. A key factor to remember about heritability is that the remaining variation does not necessarily mean it's due to management. And this is particularly important when we discuss animal health. Because some of that remaining variation not attributable to additive genetic effects could actually attributable to errors, errors both in data recording, pedigree recording, as well as effects like the statistical model. And as I said, this is particularly important for health because misdiagnosis could actually contribute to the error term, thereby depressing the heritablity estimate.
Arguably, one of the most famous equations in quantitative genetics is this equation on the top where it says P is equal to G plus E plus G by E. So written out, what this actually means is P, the phenotype, is a function of G, the genetics, plus E, the environment, plus G by E, which is the genotype by environment interaction. Genetics here includes both the additive genetic effects and also the non-additive of effects such as dominance and epistasis. Environment here means the management of the individuals, but also, as alluded to in the previous slide, includes residual noise, such as a mis-diagnosis of health or inappropriate statistical models. Genotype by environment interaction refers to how the expression of the genotype differs depending on the environment in which the animal is exposed to.
This is also one of the most important equations in animal breeding where delta G here represents genetic gain. I represents the intensity of selection. So in other words, are the top 1% of animals chosen to become parents of the next generation, or the top 10% or 20%? Because it's in the numerator of this equation, all else being equal, the greater the intensity of selection, the greater will be generic gain. R represents the accuracy of selection. So in other words, how accurately can we identify the genetically elite animals compared to the genetically inferior animals? Again, all else being equal, the greater the accuracy of selection, the greater will be genetic gain. But we will come back again to accuracy in a second. The final numerator term is the variation. It would obviously be very difficult to achieve genetic gain if there was no variation present in a population. The denominator term is L, the generation interval. And this is the average age of the parents when its progeny were born. But just to come back again to the accuracy term and this is determined by both the information content available and also the heritability. So by information, I mean either that from the animal's pedigree or parents, from the animal itself, or from its descendants. For the same quantity of information, the lower the heritablity, the lower will be the accuracy of selection. But low heritablity, which we'll see later on, is common in health traits. Low heritablity can be overcome by increasing the information content available. So for example, a sire with several hundred progeny can still achieve accuracies of selection of almost 100%, even if the trait is of low heritablity. This is really important as we move on through the health slides and talk about the low heritability which is common for most of these health traits. By no means means that we cannot achieve genetic gain. It just means that we need more information on individual animals to which we can increase the accuracy of selection and thereby increase genetic gain.

Genetics of animal health

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