EFFECTS OF INBREEDING ON THE SEX RATIO 5 



of pairing a female twice witli a litter brother and then twice 

 with an unrelated stock male was continued through the first 

 twenty-five generations of both inbred series. 



In each series litters having the desired sex ratio were reared 

 as possible breeding stock only when the young were of large size 

 and lusty at birth; all other litters were discarded regardless of 

 their sex ratio. At the time that the animals became sexually 

 mature the largest and most vigorous pairs were the ones taken 

 to continue the strain. Selection of breeding stock, it will be 

 noted, was based primarily on the sex ratio in the litters, not on 

 the size or on the vigor of the young. This means that the 

 animals in one generation that became the progenitors of the 

 succeeding generation were selected because of their parents, 

 tendency to produce young of a certain sex. A pair of rats that 

 produced two litters, each of which had the desired sex ratio, 

 was considered as having an unusually strong tendency to pro- 

 duce unisexual young; individuals from each of these litters were 

 used for breeding when possible. The basis of the selection, 

 therefore, was along the line in which Pearl ('12, '12 a, '17) has 

 obtained such marked success in increasing egg production in 

 poultry, i.e., according to the ability of the parents to transmit 

 to the offspring the quality desired. 



In the early part of this investigation the number of breeding 

 females was, of necessity, small, but in the later generations about 

 twenty females in each series were used for breeding, so that at 

 least 1000 rats w^ere obtained in each generation of the inbred 

 strain. Sex records for the first twenty-five generations are 

 given in the present paper; the data comprise 3408 litters con- 

 taining 25,452 individuals. 



2. THE NORMAL SEX RATIO IN THE ALBINO RAT . 



The normal sex ratio in any species can properly be determined 

 only by obtaining the sex data for the total number of offspring 

 produced by many females during the entire period of their 

 reproductive activity. Unfortunately, no such series of data 

 for the albino rat have been recorded, and only two sets of ob- 

 servations regarding the normal sex ratio in this animal have, 



