484 The Determination of Sex 



crosses between individuals of the same race or strain, all ZW 

 individuals are females and all ZZ moths are males. The prob- 

 lem of intersexes and sex reversal arises only when races or 

 strains from different localities are crossed and even then not in 

 every such cross. To explain such results, Goldschmidt has as- 

 sumed the presence of a female-determining substance located 

 in the cytoplasm and a male-determining substance found in the 

 Z chromosome. The relative strengths of these substances within 

 any one race are such that two doses of male-determining sub- 

 stance as found in the two Z chromosomes in a male overcome 

 the action of the female-determining substance produced by its 

 cytoplasm. This female-determining substance, however, is suf- 

 ficiently potent to overcome the action of the single dose of male- 

 determining substance in the female. If we assign the symbol 

 "F" to the female-determining substance in the cytoplasm and 

 "M" to the male-tendency genes in one Z chromosome, the 

 formula of a female is F / M and of a male is F / ]MM, where 

 MM > F and F > M. 



Both the F and M sex determiners differ in potency or valency 

 in different races, although their relative strengths are approx- 

 imately the same in every race. Thus there are strong and 

 weak F's and strong and weak M's. If a strong F and a weak 

 M are combined in a race, which is crossed with races that have 

 M's of different strength, individuals that are ZZ and therefore 

 genetically males will result which might not be phenotypically 

 males. If the M-determiner introduced is strong (of highest 

 valency), it may be sufficiently powerful to overcome the strong 

 F-determiners in the cytoplasm, and the offspring will be males. 

 If the M-determiner is intermediate, the offspring may be an 

 intersex; but if the M-determiner is very weak, the strong female- 

 determining substance may be sufficiently powerful to overcome 

 the male-determining substances of both the weak M's even when 

 their action is combined. Such individuals will be genetically 

 males but will be phenotypically females and will therefore be 

 sex reversals. In one such test a strong F from a Tokyo race 

 was combined with a very weak M from a race from Hokkaido, 

 Japan. They were crossed with races from various parts of Eu- 

 rope and Asia which varied with respect to the valency of M. 

 These crosses produced a very interesting series of intersexes. It 



