38 SEX-DETERMINATION 



union of an X-bearing sperm with an X-bearing egg. Since 

 this egg is already endowed with female-determining pro- 

 perties of a certain valency, the symbol for male must be 

 (MX)(MX)F whilst that for a female is (MX)F. In order to 

 simplify the symbols it is convenient to leave out the X and 

 to indicate a male by the formula MMF and a female by 

 MmF (the small m indicating that only one X chromosome 

 is present). The female-producing tendency of the cyto- 

 plasm F can be overcome by MM but not by M, and so in 

 each generation equal numbers of normal males and females 

 are produced in the case of each pure race of Lymantria. 

 The explanation of the appearance of intersexes on crossing 

 different races turns upon the assumption that the intensity 

 of the action of M and F differs in different races. The M 

 and F in L. japonica are stronger, and exert their influence 

 earlier in the course of development, than the M and F of 

 L. dispar. 



The relative 'strength' and 'weakness' of the sex-deter- 

 mining genes can be indicated by assigning to the M and F 

 numerical values, e.g. M5 is much stronger than M3, and 

 M3 than Mj. F^ is much weaker than F3, and Fg weaker 

 than F5. Now, consider the mating of a 'weak' female 

 (MamFg) to a 'strong' male (M5M4F4). (It will be noted 

 that the male-determining factors of the male have a different 

 valency. This is possible since the two X-chromosomes 

 which carry these come from different parents which may 

 differ in respect of the valency of their sex-determining 

 factors.) The female will elaborate two kinds of eggs — one 

 M2F3 and the other mFg. The male will elaborate two kinds 

 of sperm — M5 and M4. It is to be noted that the female- 

 determining factors are restricted entirely to the female line. 

 These four kinds of gametes, two from each side, will pro- 

 vide the following types in the F^: M5M2F3; M5mF3; 

 M4M2F3; M^mFg. 



Now, sex is determined by that kind of sex-determining 

 reaction which is in effective excess. In the case of the 

 M5M2F3, M=7, F=3— that is, M is greater than F. The 

 sex-chromosome constitution is XX and therefore this 

 individual is a normal male. In the case of the class M5mF3, 



