EFFECTS OF INBREEDING AND CROSSBREEDING. 3 
This 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 families. 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 3. 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. re. ‘Inbred. CO. | CA. AG 1; CG} Ct. | ee 
SUL Soe oe ee 
Heredity of dam 
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 
J 
3U. &. Department of Agriculture, Bulletin 1090. 
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