In pig and poultry breeding programs, animals from genetically distinct purebred lines are mated to produce crossbred animals, which provide food products to consumers. A challenge in such breeding programs is to improve the performance of crossbred animals, by selecting the genetically superior purebred animals to become parents in each generation. Selection of purebred animals based on purebred performance data can lead to suboptimal improvement in crossbred performance, because purebred and crossbred performance are usually genetically different traits. The results of this thesis illustrate how interactions between genes can lead to differences in genetic trait expression between purebred and crossbred performance. In addition, results show that when these differences are relatively large, the use of crossbred instead of purebred data can accelerate the improvement in crossbred performance. The results in this thesis are highly relevant for the understanding of the genetics of crossbreeding, and for the optimization of crossbred breeding programs.