The realised genetic trend is not always the same as the predicted genetic trend. In practice it generally is not higher than expected, but it can be lower. If it is lower than expected, as breeding organisation it is essential to find out why this is the case. We have already discussed the potential issues with difference between predicted and realised selection response. If these issues remain across generations, this will have an influence on the genetic trend. But there are a few additional reasons that we will discuss.
An obvious reason is that the animals that were selected and used as breeding animals were not as good as expected. This may have been caused by the fact that some animals were scoring systematically better than others because of a treatment that was not included as systematic effect in the breeding value estimation (the details go beyond the scope of this course). This better scoring has been assigned to their genetic potential. Therefore, all these animals have an overestimated EBV, which placed them higher in the ranking then they should have been based on their genetic potential. When it has become clear what is the systematic effect causing this overestimation of some of the EBV, this can be fixed quite easily.
A change in the breeding goal in between the time of prediction and realisation most likely will result in a difference between the realised and the predicted selection response. The predictions were made using the old breeding goal, but the animals are selected already using the new, slightly changed breeding goal. Obviously this will result in a difference between predicted and realised genetic response to selection. Related to this, even though the breeding goal remains the same, a change in phenotype recording may have similar consequences. For example, introduction of new and improved equipment will improve the accuracy of the measurements, which may have an increasing effect on the heritability (see chapter on genetic models), which in turn has an increasing effect on the accuracy of selection, and thus on the realised response to selection. Solution to both of these issues is to adjust the predicted response to the new situation and compare the newly predicted to the realised selection response.
Thus:
Deviations from expected genetic trend may be due to a change in breeding goal or a change in phenotype recording