Elements Of Genetic Change

The amount of genetic change caused by selection depends on several factors. The key equation[2] to predict response to selection, R, is

Lf where rm and rf are the accuracies of evaluating the genetic merit of male and female animals, respectively; im and if are the intensities of selection (e.g., the top 5%) for males and females, respectively; s is the genetic variability in the population; and Lm and Lf are the generation intervals for males and females, respectively. A generation interval is the average age of a male or female parent when a progeny of that individual can replace it in the breeding population under a particular testing scheme (Table 1).

In many livestock species, males can have larger progeny groups than females. Consequently, males can be more accurately evaluated from their more numerous progeny than females. Because fewer males are needed for matings than females, the intensity of selection on males

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