Three types of crossbreeding, for new breed synthesis, grading up, and repeated crossing between continuing purebred populations, have been identified and well illustrated for sheep in reviews by Rae (1952 and 1956). Bowman (1966 and 1967) updated the surveys of experimental data, and more results are published every year as workers in each country report on trials designed to investigate the benefits from crossing among their native breeds and between these and imported animals. It seems likely, however, that the greatest advances in the past decade have taken place in our understanding of the overall mechanisms of crossbreeding systems, rather than in the development of actual new breeds or crosses, or the extension or simplification of existing systems. 

M. Bichard

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 1, Madrid, Spain, 779–784, 1974
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