EFFECTS OF INBREEDING ON BODY WEIGHT 49 



in body size to stock animals reared under similar environ- 

 mental conditions. 



Brother and sister matings automatically tend to reduce 

 heterozygosis, and by the time that the animals have reached 

 the fifteenth generation they are 96.277 per cent homozygous 

 (Fish, '14). Such inbred individuals, according to Pearl ('13), 

 can by no chance possess more than 0.006 of 1 per cent of the dif- 

 ferent lines of ancestral descent which are theoretically possible. 

 In the present experiments selection was also a factor that 

 tended to decrease heterozygosis, since the mating of only the 

 largest and most vigorous pair of individuals in a litter presumably 

 brought together gametes of like genetic constitution, in the 

 majority of cases, and thus aided in increasing the proportion of 

 homozygotes in the progeny population. In spite of the very 

 high degree of homozygosis which they had attained, the animals 

 of the fifteenth inbred generation showed a considerable amount 

 of variability in body weight at different ages (table 17). How 

 much of this variability was due to genetic factors for body size 

 and how much was purely the result of environmental and nutri- 

 tive action is not known, since there is, as yet, no means of 

 determining to what extent environmental agencies can in- 

 fluence body growth. Nutritive conditions alone can greatly 

 alter body size in the rat, as is shown by the experiments of 

 Osborne and Mendel ('16). Temperature and humidity likewise 

 act upon body growth in rodents (Sumner, '09; '15), and housing 

 conditions are known to materially change their body size. 

 With all of these environmental agencies that are known to 

 affect body size made as uniform as possible under existing 

 laboratory conditions, the variability in the body weights of the in- 

 bred rats continued to decrease at a small, but fairly uniform, 

 rate in both males and females from the seventh to the fifteenth 

 generation. This indicates that the individuals were becoming 

 more homozygous with respect to the factors that determine 

 body size. It is evident, however, that even after fifteen gen- 

 erations of continued brother and sister matings the strain was 

 far from 'pure' in the sense in which this term is used by Johann- 

 sen ('09) and his followers, since the data for body growth in the 



THE JOURNAL OF K.XPERI.MENTAL ZOOLOGY, VOL. 26, NO. 1 



