20 HELEN DEAN KING 



norm. The sex ratios in the Utters of the fifth to the twenty- 

 fifth generations varied from 95.7 to 119.5 males to each 100 

 females. Variation, it will be noted, was around the norm, eight 

 of the twenty-one ratios being at or above the nonn, the rest 

 below it. When combined in generation groups the sex data gave 

 a very uniform series of ratios, as the last column of table 8 shows 

 — not one of these ratios varied more than 6 points from the norm. 

 A variation as great as this w^ould doubtless be found in the sex 

 ratios of any other large series of albino rats, regardless of the 

 manner in which the animals were bred. For the 3256 individ- 

 uals comprised in the first seven generations of the inbred strain 

 the sex ratio was 108.6 & : 100 9 . This ratio is sufficiently 

 close to the norm, I think, to indicate that, in the rat, inbreed- 

 ing per se does not produce a marked increase in the number of 

 male offspring. The sex ratio in the 22, 196 individuals in the re- 

 maining eighteen generations was 101 cf : 100 9 : for the entire 

 series of 25,452 animals in the inbred strain the sex ratio was 

 102.7 & : 100 9 . While these last two ratios are slightly below 

 the norm, it is evident that in the inbred strain as a whole the 

 sex ratio was not greatly influenced either by inbreeding or by 

 selection. The very different sex ratios obtained in the two series 

 of the inbred strain seem to show, however, that through selection 

 the one inbred strain was separated into two distinct lines, one 

 line (A) having a tendency to produce an excess of males, the 

 other line (B) tending to produce a preponderance of females. 



Unfortunately, one cannot predict with certainty what the sex 

 ratio will be in the litters cast by any given inbred female, neither 

 does the sex ratio in the litters cast by one female give a clear 

 indication regarding the proportion of the sexes that will be 

 found among the offspring of a sister rat. It is only by taking the 

 averages for a large number of litters in a given series that the 

 change in the sex ratio is made manifest. As an illustration of 

 the individual differences in females regarding their tendencies to 

 cast young of a certain sex, four sets of data for litters cast by 

 sister females are shown in table 9. In each case given, sister rats 

 were first paired with the same litter brother and later with the 

 same stock male. 



