26 HELEN DEAN KING 



5. DISCUSSION 



As a basis for discussion the results obtained in this investi- 

 gation are briefly summarized as follows: 



1. The inbreeding of litter brother and sister for six consecu- 

 tive generations, during which there was no selection of animals 

 for breeding, did not increase the number of male offspring to any 

 extent. The sex ratio in the 3256 young obtained was 108.6 cf : 

 100 9 , or less than 4 points above the sex ratio taken as the 

 norm (105 d^ : 100 9). 



2. Beginning with the seventh generation all breeding females 

 in the A series were taken from litters that contained an excess 

 of males. After this time the females in this series tended to 

 produce an excess of male young, whether they were paired with a 

 litter brother or with an unrelated stock male (table 6). The 

 litters sired by inbred males gave a sex ratio of 122.3 cf : 100 9 , 

 or over 17 points above the norm; while the litters sired by stock 

 males showed a sex ratio of 115.6 cf : 100 9 , or nearly 11 points 

 above the norm. 



3. From the time* that the breeding females in the B series 

 were selected from litters containing an excess of females (seventh 

 generation), the litters produced showed a reverse proportion of 

 the sexes to that shown by corresponding litters in the A series 

 (table 7) Litters sired by inbred males had a very low sex ratio 

 (81.8 cT : 100 9 ) ; in the litters sired by stock males the sex ratio 

 was 9 points higher than that in the inbred litters (91.1 cf : 100 



9 ) , but it was still significantly lower than the norm. 



4. On combining the data for the two inbred series it was found 

 that among the 25,452 individuals comprised in the inbred strain 

 the sex ratio was 102.7 d" : 100 9 , or less than 3 points below the 

 norm (105.0 & '■ 100 9 ). It thus appears that through selection 

 the inbred strain was separated into two distinct lines: one (A) 

 showing a high sex ratio, the other (B) a low sex ratio. Selection 

 had the greater influence on the female line, since the sex ratio 

 for the litters of the B series shovred greater deviation from the 

 norm than did that for the litters of the A series. 



5. Stock females mated with inbred males tended to produce 

 litters in which the sex ratio was below the norm, regardless of 



