EFFECTS OF INBREEDING ON THE SEX RATIO 7 



Table 1 shows that there was a relatively large excess of males 

 in the first litters cast by this series of stock femalyes (115. Qcf: 

 100 9 ), and that in succeeding litters the sex ratio tended to fall 

 considerably. A similar change in the sex ratio of successive 

 litters of mice was noted by Copeman and Parsons ('04), and was 

 found also by King and Stotsenburg ('15; table 7) in a series of 

 stock albino rats. Large groups of statistics for human births, 

 as summarized by Ahlfeld ('76), by D using ('83, '84), by Punnet 

 ('03), and by Newcomb ('04), all show that the sex ratio is very 

 high among the first children of young mothers and then tends to 

 fall with succeeding births until the mother is about thirty years 

 old. Whether a similar change in the sex ratio is characteristic 

 of other mammals has not been determined as yet. 



Among the 2818 individuals comprised in this series of stock 

 litters there were 104.6 males to each 100 females. A sex ratio of 

 105 cf : 100 9 was, therefore, taken as the norm by which to judge 

 the sex ratios obtained in the various groups of inbred rats. This 

 sex ratio, it will be noted, is very close to that given by Cuenot, 

 and is lower, by over two points, than the sex ratio found in the 

 large group of stock Albinos born in The Wistar Institute colony 

 during the years 1911-1914 (King and Stotsenburg, '15). 



3. THE SEX RATIO IN INBRED LITTERS OF ALBINO RATS 



The A series of inbred rats may be designated as the 'male 

 line,' since after the sixth generation all of the breeding females 

 in this series were taken from litters that contained an excess of 

 males. Table 2 gives, by litter groups, the sex data for the 13,116 

 individuals obtained in the first twenty-five generations of this 

 series. 



Table 2 is inserted chiefly for reference, and a detailed analysis 

 of the data, as given, wiU not be attempted. The summary of 

 the data for the various litter groups shows that the sex ratio for 

 the first litters produced was much higher than that for the 

 second, third, and fourth litters. A similar change in the sex 

 ratio was noted in the litter series of stock animals given in table 1. 



The B series of inbreds is called the 'female line,' since, after 

 the sixth generation, all breeding females in this series came from 



