Swine breeding in the U. S. A. has been patterned somewhat after the methods used to produce hybrid corn. The work of the Regional Swine Breeding Research Laboratory for example, started out in 1937 with a program of inbred line formation (C r a f t , 1953). Approximately 100 inbred lines within 7 breeds were started, as well as several breeds from crossbred foundations. Crosses of these lines and breeds performed well, and crossbreeding for commercial production of swine was readily accepted by the early 1950’s. At the same time studies of selection within lines showed that progress was less than had been expected (D ic k e r s o n et al, 1954). Results of relative ineffectiveness of mass selection in corn and swine led plant and animal breeders to speculate about the importance of overdominance in economically important traits. Since 1949 when C o m s t o c k (C o m s t o c k et al.) proposed a breeding procedure that would capitalize on both additive and non-additive genetic variance by selecting directly for crossbred performance in parent lines (Reciprocal Recurrent Selection), a large amount of experimental evidence has been published on this subject. This paper is concerned only with the experimental results obtained with swine and will not deal with theoretical aspects of the problem or with results from other economic or laboratory species.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 1, Madrid, Spain, 849–858, 1974
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