SEX IN ANIMALS 547 



The presence of one such factor leads to the appearance of a sex-ratio 

 of 50 males to 100 females; if two sex-linked lethal factors occur in the 

 same strain the ratio is less than 50 males to 100 females, the exact ratio 

 depending upon linkage values of the two factors. Thus to take a particu- 

 lar case, Lethal I is situated at locus 0.7 and Lethal III at locus 26.5 

 in the sex chromosome. About 25 per cent, of crossing-over, therefore, 

 occurs between these two factors. A female of the genetic constitution, 

 (liLsX) (Lil 3 X), therefore produces gametes in the following proportions: 



When such a female is mated to a normal male, (LiL 8 X) F, presumably 

 all the females survive because of the normal allelomorphs received from 

 the sperm cell, but of the males only those which are of the genetic con- 

 stitution (LiLaX)Y, therefore, only one in eight survive. This leads to 

 a ratio of 12.5 males to 100 females in such a population. 



While this is a hypothetical case it may, however, be mentioned that 

 Morgan and his associates have discovered at different times actual in- 

 stances of two sex-linked lethal factors occurring in the same strain. 

 Furthermore, the occurrence of sex-linked lethal factors, by virtue of 

 their linkage relations with other factors is as well established as the exist- 

 ence of any other Mendelian factors. They provide one means of ex- 

 plaining definitely unusual sex-ratios without assuming any changes in 

 the mechanistic relations. 



The inheritance of unusual sex-ratios in mammals and the reasons 

 for such inheritance are in considerable doubt. About the only results 

 which have been reported upon at all which give definite positive evidence 

 are those of Miss King. These results have not been published in detail 

 but a very preliminary note states that more than 22,000 albino rats have 

 been reared in the course of the investigation. The experiments started 

 with two pairs of rats from the same litter, and two lines of selection 

 were made one for high proportion of males, the other for high proportion 

 of females. The method of experimentation was rather unique. Thus 

 the progeny of pair A, one of the original pairs were bred brother to sister 

 without selection for six generations in order to establish a homogeneous 

 race. After the sixth generation the brother to sister matings were 

 continued but selection was invariably made from litters which had a 

 high proportion of males. In Line B, the progeny of the other pair, the 

 same procedure was followed except that selection after the sixth genera- 

 tion was made from litters having a high proportion of females. Fifteen 

 generations of selection in Strain A give a sex-ratio of about 125 males 

 to 100 females; in Strain B a sex-ratio of 83 males to 100 females. This 

 series of experiments seems, therefore, to indicate that unusual sex-ratios 

 may be inherited. Judgment, however, must be reserved until the com- 



