Below is shown a general formula for expected genetic changes by selection.
The changes are
proportional to the intensity of the selection, the accuracy of the estimated breeding value
and the additive standard deviation, sigmaA. The change take place
from one generation to the next, which means that the average
breeding value of the parents corresponds to the mean value of the population in
the next generation. If the changes are to be measured in time units, they are
inversely proportional to the generation interval.
The generation interval is defined as the average age of the
animals when they become parents. In the human population the
generation interval is said to be 30 years. I.e. the parent is on average 30
years old when they give birth to a child.
The general formula for selection response is shown below.
The general formula for delta G fits well with the results for accuracy of
individual selection, h,
given in Figure 8.1. The general formula for selection
response can be extended slightly when taking into account the different
roles the two sexes play in forming the new
generation. The formula is divided into two
components, one for the sires' contribution and the other for the dames' contributions,
see below.
An applet for calculating the selection response is found here
The formula for delta G is important for predicting what will happen if a specific breeding program is decided upon. Thus the model studies can be performed with an eye on design of an optimal balance between the generation interval and the intensity and/or accuracy of the selection. At short generation intervals fewer individuals are born to select among. This influences both the intensity and the accuracy negatively compared to a situation where longer generation intervals occur.