DIFFERENTIALS IN CLOSELY INBRED GUINEA PIGS 95 



In a few experiments of the first series, in which brother-to-brother 

 transplantations were carried out, the grade was slightly above 1 ; likewise, 

 in transplantations from one strain to another the average grade was 1. In a 

 second series of transplantations, made within the same strains, the average 

 grade was 1.85 in six experiments. In six other experiments, in which 

 transplantations were made between different strains from Caworth Farms 

 guinea pigs to St. Louis guinea pigs, the average grade was 1.33. We may 

 then conclude that there is perhaps a slight indication of a beginning homo- 

 zygous condition in these guinea pigs, as indicated by grade 1.85 in trans- 

 plantations within the same strain ; but in the first series, there is no indication 

 of such a tendency. Therefore, after five or six generations of inbreeding, 

 there is not yet any definite advance in the direction towards a homozygous 

 state among these animals. 



The following are the principal conclusions suggested by all these experi- 

 ments with closely inbred guinea pigs. The difference between the relatively 

 rapid effect of inbreeding on the individuality differentials in guinea pigs as 

 compared to the effect in rats, which were studied in the preceding pages, 

 is striking. However, even in guinea pigs the ultimate goal of the inbreeding, 

 namely, a completely autogenous state of the individuality differentials, of 

 all the animals in an inbred family, has not yet been reached. The individuality 

 differentials of host and transplant were the more similar to one another the 

 larger the number of brother and sister matings which these individuals had 

 in common before the separation of these matings into different sidelines 

 took place, and the smaller the number of these separate and distinct brother- 

 sister matings had been in the preceding generation in host and donor. The 

 separation into side lines may cause an accumulation of unlike genes if a 

 perfect homozygous condition had not yet existed at the time of the separa- 

 tion; mutations may then add to the number of unlike genes. In family 2, 

 for instance, the grade was 2.50 in a case in which there had been six to 

 nine common ancestral matings, followed by 20 to 24 separate brother-sister 

 matings, while the grade was 3.25 with 12 and 16 brother-sister matings 

 in common and subsequent separation for only two or three generations. 

 Similar results were obtained in family 13. After 19 to 20 consecutive brother- 

 sister matings, there was usually an absence of any marked incompatibility 

 between the individuality differentials. Such individuals may behave like 

 identical twins, at least within the range of conditions which were used in 

 these tests ; however, a lengthening of the time during which the individuality 

 differentials of host and transplant had a chance to act on each other might 

 still have brought out a certain difference when a shorter period did not show 

 it, and some of our experiences indicate such an effect. Striking also is the 

 correspondence between the pedigree relationship of the various guinea pigs 

 and the degree of relationship of their individuality differentials as revealed 

 by transplantation ; this comes out especially in the brother-to-brother trans- 

 plantations and in the various types of hybrid transplantations. It is inter- 

 esting in this connection that in the experiments with inbred guinea pigs, as 

 well as in those with inbred rats, the greater similarity of the individuality 



