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Thus far we have not considered difference in selection strategy between males and females. However, in most species there is a difference. There are 3 main reasons for this difference:

1. Important reason for that is the limited reproduction capacity in females, especially in mammalian species. A general assumption in animal breeding is that the population size remains the same across generations. This has consequences for selection strategies, because it means that the selected animals should be capable to produce sufficient number of offspring to maintain population size! Males can produce more offspring than females, especially when AI is available. Selection in males, therefore, often is more stringent than in females. In other words: there may be a difference in selection intensity.

2. Another reason is a difference in information sources to estimate the breeding values in males and females. There are some traits that cannot be measured in both sexes, for example milk production. Consequently, males may be selected based on progeny performance, whereas females are selected on own performance, resulting in a difference in accuracy of selection.

3. Related to that, a third reason is the age at which animals can be selected and at which the average progeny is born. If males are selected based on progeny test results, then they on average will be older than the females, provided those are selected based on own performance, for example. On the other hand, in most species males mature earlier than females. In other words: there may be a difference in age at first progeny, and thus in generation interval.

Consequence of these potential differences in selection in males and females is that the difference in selection paths need to be considered when determining the genetic gain. The way to handle these different selection paths is fairly straight forward: just split the equation in a part for males (m) and a part for females (f):

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The intensity of selection and accuracy of selection and generation interval may be different in males and females. The genetic standard deviation, however, is a population parameter and is, therefore, the same between males and females.