JC2 F. B. SUMNER, M. E. MCDANIEL AND R. R. HUESTIS. 



INBREEDING. 



A comparison of "inbred" and " outbred " stock (as defined 

 below) was made at a preliminary stage of these studies, when 

 the total number of individuals dealt with was about 1,700 less 

 than at present. The results of this comparison were so com- 

 pletely negative and the amount of work involved is so great, 1 

 that it has not been thought worth while to revise them. 



For the purpose of comparing the sex ratios, our material 

 was divided into two classes, " outbred " and " inbred." The first 

 class included offspring from matings between unrelated individ- 

 uals or individuals not related more closely than first cousins ; the 

 second included matings between parents and offspring or brothers 

 and sisters. 



In the first class, we have 2,346 individuals, comprising 1,171 

 males and 1,175 females. The sex ratio is 99.66 2.70. In the 

 second class, we have 1,087 individuals, comprising 547 males and 

 540 females. The sex ratio here is 101.30 + 4.07. It is obvious 

 that this difference must be regarded as accidental. When we 

 restrict the " inbred " group to those individuals which were de- 

 rived from the matings of full brothers and sisters, the sex ratio 

 becomes somewhat larger, being 103.56 4.82. But even here, 

 the difference between this figure and that for the "outbred" 

 group is much less than its probable error. 



Various investigators have offered data which they believed to 

 indicate an effect of close inbreeding upon the sex ratio. It is 

 of interest (and quite characteristic of the literature of "sex 

 determination " in general) that some writers have found a higher 

 proportion of males in inbred strains, while others have found 

 the females to be in excess. 



Miss King (1918), from extended experiments on the white 

 rat concludes that inbreeding per sc does not affect the sex ratio. 

 Her experiments yielded, however, one important positive result, 

 namely that strains could be selected, giving preponderant num- 

 bers of male and female offspring respectively. 



1 It is necessary, with every brood, not only to look up the birth records 

 but to trace out the pedigrees to some extent. 



