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Crossbreeding are matings between animals of different breeds or lines.

A breed is an interbreeding group of animals within a species with some identifiable common appearance, performance, ancestry or selection history.

Selection lines are formed by pure breeding or by crossing different breeds. After the formation of the line the animals in such a selection line are selected for a limited number of breeding goal traits. After generations of selection they excel in these specific breeding goal traits.

When breeds or selection lines are crossed, the crossbreds not only combine the characteristics of each of the breeds or lines, but for some characteristics the performance of the crossbreds is higher than the average performance of the parent breeds or parent lines due to heterosis.

Heterosis or hybrid vigour is the extent to which the performance of a crossbred in one or more traits is better than the average performance of the two parents.

Heterosis has a positive effect because in the crossbreds many genes are heterozygous that were homozygous in the parent breeds. Alleles with a negative effect are often recessive. In the crossbreds these negative recessive alleles are ruled out.

Heterosis is one of the reasons to apply crossbreeding of breeds or lines. The second reason for crossbreeding  is to exploit the complementarity of breeds or lines: combination of the characteristics of two breeds or lines is favourable. The third reason is that crossbreds combine characteristics that cannot easily be improved simultaneously in a single breed. The last reason for crossbreeding is the protection of the genetic improvement in the selection lines of commercial companies.

In all crossbreeding systems before animals are crossed, they are first selected for the relevant traits. Crossbreeding does not make selection redundant.  Crossbreeding schemes require a strict implementation by all participants. Hence, crossbreeding is applied for several reasons and they can only be realized when the chosen crossbreeding system is strictly implemented.