EFFECTS OF INBREEDING ON THE SEX RATIO 15 



Data given in table 4 show that in the half-inbred litters pro- 

 duced in the eighth to the twenty-fifth generations of the A series 

 the range of variation in the sex ratios was from 99 to 140.8 

 males for each 100 females, six of these ratios being slightly below 

 the norm. When the data were combined in generation groups 

 (table 6) , it ivas found that not a single group gave a sex ratio as 

 low as the norm. The sex ratios for the litters in the later genera- 

 tion groups were somewhat more uniform than those for the 

 litters in the earlier generation groups, but the uniformity was 

 not as striking as that in the corresponding groups of inbred 

 litters. For the total of 678 half -inbred litters the sex ratio was 

 115.6 cf : 100 9 . This ratio was some 11 points above the norm 

 and less than 7 points lower than the sex ratio in the inbred litters 

 belonging to the same group of generations (122.3 cf: 100 9). 

 While the litters produced by the mating of inbred females with out- 

 bred stock males thus tended to have a lower sex ratio than did the 

 strictly inbred litters, they did not give the sex ratio that was to be 

 expected according to the current view that chance alone deter- 

 mines whether a male-producing or a female-producting spermato- 

 zoon shall fertilize the egg. Such an hypothesis requires that the 

 sexes shall appear in approximately equal numbers when large series 

 of sex/data are examined. In the present case the proportion of 

 the sexes among the 5230 individuals obtained was very far from 

 equal. In only one group (ninth generation) out of eighteen was 

 there a nearly equal proportion of the sexes, in all other groups 

 there was a pronounced excess of males. 



The first twenty-five generations of the A series of inbreds com- 

 prised 1752 litters containing 13,116 individuals, 7116 males and 

 6000 females. The sex ratio for this series of animals was 117.4 

 cf : 100 9 • This ratio was over 12 points above the norm, and 

 since it was based on data for such a large group of animals, it 

 would seem to indicate that in the rat the sex ratio can be altered 

 by selection within a closely inbred line. In this instance the 

 relative number of males was apparently increased by selecting 

 breeding females from litters that contained an excess of males. 



The sex data for the inbred and for the half -inbred litters of the 

 B series, combined in generation groups, are shown in table 7. 



