ROTATIONAL CROSSBREEDING AND HETEROSIS 373 



did any of us realize at that time that within twenty-two years we would be 

 in the midst of a flourishing hybrid boar business. None of us know much 

 about their true merits and demerits. Nevertheless, the crossbred boars were 

 not included in the experiment. 



Experimental Results 



The experiment did proceed as planned for the production of first cross 

 offspring from the mating of purebred females of one breed to purebred males 

 of another breed. Crossbred females were then retained as breeding animals 

 to be mated in one case to a boar of one of the two parental breeds and in the 

 other case to a boar of a third breed. The results of this experiment showed 

 that there was a very definite advantage in the production of firstcross pigs. 

 There was a slightly greater advantage in the production of backcross pigs 

 (that is, where crossbred females were mated back to a boar belonging to one 

 of the parental breeds). There was a still greater advantage where the cross- 

 bred females were mated to a boar of another breed. 



As I mentioned before, you bring together either genes that are alike or 

 genes that are not alike. There appears to be very little likelihood of bring- 

 ing about any more heterozygosity as a result of a three-breed or a four-breed 

 cross than there is in a two-breed cross. The advantages derived from the 

 backcross and from the cross to a third breed appear therefore to have been 

 derived from the fact that the female parents were crossbreds or in a more 

 hybrid state than their purebred half sisters. Why should this be the case? 

 The female produces the eggs, carries the fertilized eggs, and develops them 

 to the point where, after a period of about 114 days, they are ready for birth 

 and then nurses the little pigs for another 56 days. In general, the advantage 

 derived from the crossbred female in contrast to the purebred female is 

 about equal to that derived from having the progeny crossbred in contrast 

 with having progeny that are purebred. 



The above are the general deductions that I made at the close of our cross- 

 breeding experiment. Now I am not so certain that this interpretation is ab- 

 solutely correct. The reason for my questioning is that recently I had a long 

 visit with one of the largest hybrid seed corn producers in this nation. He is a 

 man who has had many years of experience in the field. lie told me that he 

 had not yet seen a single cross of hybrid corn that was as useful for com- 

 mercial corn production as the double hybrid. He elaborated further to the 

 efifect that the single cross hybrid often would yield as heavily as the double 

 hybrid, but that under adverse environmental conditions the double hybrid 

 fared better. This he attributed to the fact that the double hybrid de- 

 veloped from four inbred lines possessed greater genetic diversity toward 

 adversity. This appears to be somewhat in contrast to the experimental re- 

 sults and interpretations of those results in some of the ])resent-day funda- 

 mental studies of Urosophila genetics. Undoubtedly, with time and more 



