EFFECTS OF INBREEDING AND CROSSBREEDING. 31 



The results of more recent tests of the resistance to tuberculosis 

 of guinea pigs of the control stock and five of the inbred families have 

 been discussed in a paper by the writer in collaboration with Dr. 

 Paul A. Lewis. 3 In these tests the controls did not live so long on 

 the average as the five inbred famihes used (2, 13, 32, 35, and 39). 

 These inbred famihes, however, were markedly differentiated among 

 themselves in resistance and probably do not represent the average 

 of the entire inbred stock as well as did the inbreds of the earlier 

 test, made at a time when most of the families were still on hand. 



SUMMARY. 



There has been an average decline in vigor in all characteristics 

 during the course of 13 years of inbreeding of guinea pigs, brother 

 with sister. The decline is most marked in the frequency and size 

 of litter, in which it is so great that it would have to be accounted for 

 even though the decline in other respects were assumed to be due 

 wholly to a deterioration in the environmental conditions. The 

 decline is greater in the gains after birth than in the birth weight, and 

 greater in the percentage raised of the young born alive than in the 

 percentage born alive. The ability to raise large litters has fallen off 

 much more than ability to raise small litters. 



A comparison of the inbred guinea pigs with a control stock, raised 

 under identical conditions without inbreeding, and derived in the 

 main from the same line-bred stock as the inbred families, indicates 

 that the inbreds have suffered a genetic decline in vigor in all char- 

 acteristics. The decline in fertility is again shown to be most marked. 

 Experimental inoculation with tuberculosis has shown that the in- 

 breds were inferior on the average to the controls in disease resist- 

 ance. A study of sex ratio yields results in marked contrast to those 

 obtained in connection with the other characters. There are no 

 significant fluctuations from year to year, no contrast between inbreds 

 and controls, and no indications of change due to inbreeding. 



In addition to the points brought out in this bulletin which in- 

 dicates genetic decline during inbreeding, extensive experiments 

 have been made in which different inbred families have been crossed 

 together. These are described in another paper (Bulletin No. 1121), 

 in which it is shown that crossbred guinea pigs born of unrelated 

 inbred parents are distinctly superior to their inbred relatives in 

 nearly all elements of vigor. A slightly larger percentage are born 

 alive, in small Utters at least, and a distinctly larger percentage of 

 those born alive are raised. The young are slightly heavier at birth 

 in a given size of litter and gain much more between birth and wean- 



3 Wright, S. and Lewis, P. A. 1921. Factors in the resistance of guinea pigs to tuberculosis with especial 

 regard to inbreeding and heredity. Amer. Nat. v. 55, p. 20-50. 



