EFFECTS OF INBREEDING ON THE SEX RATIO 17 



As far as can be judged from the results of this part of the inves- 

 tigation, close inbreeding, even when the animals are poorly 

 nourished, does not increase the proportion of male offspring to 

 any extent. 



The breeding females in the seventh generation of the B series 

 of inbred were all taken from litters that contained an excess of 

 females; among their offspring the sex ratio was 94.3 d". 100 9. 

 In not one of the subsequent generations was the sex ratio in the in- 

 bred litters as high as the norm, the nearest approach to the norm 

 was in the twelfth generation, where the sex ratio was 101.3 d* : 

 100 9 (table 5) . In these inbred litters, as in the corresponding 

 ones of the A series, the sex ratios were more uniform in the later 

 than in the earlier generations, but there was no cumulative 

 effect of selection in either case. In the B series, after the thir- 

 teenth generation, there was very little change in the relative 

 proportion of the sexes from one generation to the next, and some 

 of the variation found, as stated for the A series, can doubtless be 

 ascribed to environmental action. 



When the data for the inbred litters of the eighth to the twenty- 

 fifth generations of the B series were combined in generation 

 groups (table 7), it was found that the sex ratios for the various 

 groups showed even greater deviations from the norm than did those 

 for corresponding litter groups in the A series, but that this deviation 

 was in the reverse direction, i.e., the number of females born greatly 

 exceeded the number of males. The highest sex ratio for any 

 group in the B series was 92.5 cf : 100 9 : f or the entire group of 

 794 litters the Fex ratio was 81.8 cf : 100 9 , or 23 points below the 

 norm. This latter ratio is far too low to be considered as a chance 

 variation, and it certainly cannot be attributed to the action of 

 environment. For both series of inbreds were reared simul- 

 taneously under the same environmental conditions, and if one 

 ventured to suggest that environment swung the sex ratio in the 

 B series towards the female side it would be necessary to assume 

 that the same environment acted on the animals of the A series 

 in a reverse direction and so swung the sex ratio towards the 

 male side. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 27, NO. 1 



