EFFECTS OF INBREEDING AND CROSSBEEEDING. 



Tills renewed inbreeding has been continued in a small second 

 generation of brother-sister mating, Experiment C2, in which both 

 parents and offspring are, to some extent at least, again inbred. 



In Experiment CA, inbred females were mated with crossbred 

 males derived from two other inbred famihes. Experiment AC is the 

 reciprocal, crossbred females mated with unrelated inbred males. 

 The progeny of these two experiments should have approximately 

 the same heredity. Differences in the results must be attributed to 

 the direct influence of the dam or sire. 



The nature of these experiments is indicated in Table 1, in which 

 the result of prolonged inbreeding is represented by 0, crossbreeding 

 by 1, and the first and second generation of renewed brother-sister 

 mating by ^. It will be shown later that these figures represent 

 accurately the differences to be expected among the experiments. 



Table 1. — The breeding of the sire, dam, and young in the various experiments. 



Items. 



Heredity of sire . . . 

 Heredity of dam . . 

 Heredity of j'oung 



Inbred, 



CO. 



CA. 



AC 



CC. 



CI. 



02. 



Two selection experiments were begun in 1918 among the crossbreds 

 derived from the inbred stock. In Experiment CG only animals 

 which were exceptionally heavy at weaning at 33 days were used 

 (average weight, males, 307.7 grams; females, 298.1 grams). These 

 naturally came in the main from small litters (average 1.7). In 

 Experiment CL only animals from large litters were mated (average 

 4.3). These were generally light in weight (average, males, 214.1 

 grams; females, 207.0 grams), and seemingly much less thrifty than 

 those in CG. In both cases, animals were chosen from any cross- 

 breeding experiment (except B) and in a few cases from among the 

 inbreds. The experiments were discontinued after 1919. While 

 not carried on long enough to be satisfactory as selection experiments, 

 they are of interest as throwing light on the question as to how far 

 any conscious or unconscious selection of young on the basis of vigor 

 could account for the observed difference between experiments. 



DESCRIPTION OF TABLES. 



The results in the various experiments during the years 1916 to 

 1919 are shown in detail in Tables 14 to 21 and are summed up in 

 Table 29. 



Table 14 gives data on the fertility. The number of litters and the 

 average size of litter are given for each experiment. The number of 

 mating years was calculated as described in Part I.^ Each mating 



3 U. S. Department of Agriculture, Bulletin 1090. 



