EFFECTS OF INBREEDING ON THE SEX RATIO 23 



An examination of the sex data for successive litters cast by 

 many hundreds of female rats does not indicate that there is "a 

 change of sex tendency from litter to litter" in the female, as 

 Papanicolau ('15) states is the case in guinea-pigs. Such a 

 tendency is not shown in any of the cases given in table 8, and 

 while the sex-proportions in the littei-s do change in many cases, 

 the change is not general or striking enough to warrant the con- 

 clusion that there is a definite sex-determining factor involved. 



4. THE SEX RATIOS IN THE LITTERS OBTAINED BY THE MATING OF 

 STOCK FEMALES WITH INBRED MALES 



As a check for the results obtained by the mating of inbred 

 females with stock males, series of stock females were bred to 

 males from various generations of the inbred strain. The num- 

 ber of such experiments was small, considering the scale on which 

 the main series of experiments was conducted, but the results 

 obtained were uniform enough to be significant. 



The stock females used in these experiments were reared under 

 the same environmental conditions as the inbred rats. When 

 they were about three months old they were paired with males 

 from the A or from the B series that had sired inbred litters. In 

 order to make this series of records more strictly comparable to 

 that obtained in the inbred strain, only four litters from any 

 one female were recorded. 



The data for the litters obtained by the mating of stock females 

 with inbred males are given in table 10. 



Stock females paired with males from the fifth generation of the 

 • inbred strain produced litters in which the sex ratio was below 

 the norm, whether the sire of the litter belonged to the A or to the 

 B series of inbreds. The litters sired by males from A series, 

 however, had a much higher sex ratio than did those sired 

 by males from the B series, although at the fifth generation 

 there was no selection of breeding animals in the inbred strain 

 according to a definite plan. The eighteen litters in this series 

 gave a sex ratio of 94.7 cf : 100 9 , or 10 points below the norm. 

 This ratio might seem to indicate that inbred males tended to 

 produce an excess of female offspring, but the number of litters 



