L M. WINTERS 



University of Minnesota 



Chapter 23 



Rotational Crossbreeding 



and Heterosis 



It is well for all of us, including our most eminent scientists and philosophers, 

 to reduce our thinking to relatively simple terms. Genetics is, after all, basi- 

 cally rather simple. A fertilized zygote results from the union of two germ 

 cells, each of which carries a haploid number of chromosomes, and a haploid 

 number of genes which are resident in the chromosomes. By the very nature 

 of the procedure, genes are paired which are alike or not alike. As the pairing 

 of similar genes is increased, the population approaches increased purifica- 

 tion. As the pairing of dissimilar genes increases, the resulting population be- 

 comes more heterozygous. Increased heterozygosity has been generally as- 

 sociated with increased vigor which is generally spoken of as hybrid vigor. 



PLANNING THE MINNESOTA EXPERIMENTS 



I believe the best way to discuss rotational crossbreeding is to relate briefly 

 how the system was developed. When I was asked in 1928 to head the re- 

 search in animal breeding at the University of Minnesota, I brought with me 

 several proposed projects. One of these was a study of crossbreeding swine. 

 A review of the literature of crossbreeding experiments conducted previous 

 to 1928 shows that for the most part they were small-scale experiments. 

 When the data were all put together, however, the evidence was in favor of 

 crossbreeding. Yet, the general sentiment at that time among the stockmen 

 was overwhelmingly opposed to the practice of crossbreeding. The statement 

 frequently heard was that crossbreeding was quite satisfactory for the pro- 

 duction of one crop, but all of the crossbreds must be marketed because it 

 was absolutely disastrous to use any of the crossbreds as breeding animals. 

 By 1928, it was quite evident that corn breeders were revolutionizing the 

 system of breeding corn, and that hybridization was to become the rule rath- 



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