Estimated breeding values (EBV) are an important resource for animal and plant breeders. The genetic improvement that has been realized in animal and plant breeding has been largely due to the use of estimated breeding values. These values play a pivotal role in genetic selection, driving improvements in populations over successive generations. This post serves as a brief introduction to the significance of breeding values and the process of estimating them.
Animal Breeding
Animal breeding is the practice of improving a population for traits of interest by means of genetic selection. The idea is to select animals from the current population to become parents of the next generation. If these parents are selected carefully, the next generation of animals will perform better than the previous one. If you repeat this process, it will result in a gradual increase in the performance of the population, generation after generation. But how do we know which animals to select?
The most simple strategy is to measure the performance of all animals in the population and select the best-performing ones to become parents. For example, if I want to improve racing performance in horses, I could measure the speed at which each animal runs, and select only the 10% fastest horses. However, this strategy (known as mass selection) has a couple of important pitfalls.
First, we do not know whether the faster horses performed better because of their good genes, or because they enjoyed a much better environment. The fast horses may have had better food, a better stable, or a better rider. Second, we can only select horses that have finished their training and are competing in races. It would be much better if we could already make a first selection at a young age, so we can avoid going through the trouble of training every single horse. Finally, mass selection does not work for traits that can only be measured on one of the sexes. For example, we cannot measure milk yield in dairy bulls, simply because they do not give milk.
The Breeding Value
In practice, breeders often work with breeding values. The breeding value is a number that reflects the genetic merit of an animal for a particular trait. The key word here is genetic. A breeding value does not reflect the superiority of an animal due to a better environment. It only reflects the superiority that is due to better genes. In other words, the breeding value is the part of the observed performance that is heritable: it measures the expected performance of the offspring of an animal.
It is important to realize that offspring receive half of their genes from each of the two parents. Hence, offspring also receive only half of the parent’s breeding value. For example, suppose that a chicken has a breeding value for egg weight of +4 gram. That means that the female offspring of this individual are expected to lay eggs that are (on average) 2 grams heavier than the average of the population.
Estimated Breeding Values (EBV)
Breeding values cannot be measured directly, so they need to be estimated. Breeders therefore work with estimated breeding values. Estimated breeding values are typically calculated from multiple sources of information, using a statistical model.
There are several benefits of using EBV. First, the EBV of an animal can be calculated without any performance information of the animal itself. For example, we can estimate the breeding value for milk yield of a young bull by using milk yield records collected on its female relatives (mother, aunts, (half-)sisters). These female relatives have genes in common with our bull, so it makes sense that we can use their performance to estimate the genetic merit of that bull.
Second, the estimated breeding values use all the data that is available. For example, in addition to milk yield records of a cow, the milk yield records of the sisters of that cow also give information about her genetic merit. EBV take into account all this information, resulting in the highest accuracy possible.
Online Tool for Calculating Estimated Breeding Values
Estimating breeding values can be challenging, especially if you have no experience in animal breeding or data analysis. I have therefore developed a simple online tool that can compute breeding values from a small dataset. This tool may not be very useful in practice, but it can be used to teach the general principle of estimating breeding values to students of animal or plant breeding.
Conclusion
Estimated breeding values are instrumental to genetic selection. The development and use of these values has dramatically increased the response to selection. The process of estimating accurate EBV can be challenging, because it requires to use sophisticated statistical models that can combine multiple sources of information. A simple online tool can provide helpful insights into the estimation of breeding values.
