A number of steps are required to achieve response to selection, and thus to make genetic progress or genetic gain. First of all the animals need to be ranked in order of their predicted genetic potential. How to obtain the most accurate estimates of the genetic potential (the EBV) was subject of the previous chapter. Now that the animals can be ranked, the next step is to select the best for breeding. Success of the selection decisions depend on a number of factors:
How heritable is the trait under selection (i.e. the trait in the breeding goal)?
How much genetic variation for that trait is there in the population?
What is the average accuracy of the EBV, and thus the accuracy of selection?
What proportion of the animals will be selected for breeding?
In case genetic gain is to be expressed per year, rather than per generation: how long is a generation?
The heritability and the genetic variance are population parameters and cannot be influenced by the breeder. This is assuming that the phenotypes that were collected for estimating the heritability was of good quality, and the pedigree was recorded without errors.
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Even though the genetic progress PER GENERATION is increased, the genetic progress PER YEAR is not or even decreased. In other words: there is a balance between increasing accuracy of selection and the time required to achieve the information to achieve the largest genetic gain per year.
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Thus:
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To optimise the success of a breeding program it is important to balance the relatively short-term decisions: acquire high genetic gain, and the long term maintenance of the population: controlling rate of inbreeding. |
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