450 Genetics of Sex Determination 



destruction, is the mutant locus purple, so called for its effect on eye 

 color. Shall we now say that purple is one of the wing-determining loci, 

 or is it not rather true that the action of purple collides at some point 

 with one of the processes or reaction chains which take place in wing 

 determination? This collision may mean a common substrate or pre- 

 cursor substance or condition for the kinetics of the reaction. Exactly 

 the same situation is known from the realm of sex determination. Thus 

 Winge and his successors showed that certain mutant loci for pig- 

 mentation act as modifiers of sex expression in fishes. Such cases are 

 bound to be found whenever there is a labile balance which can be 

 pushed out of gear by effects of loci and the genetic background, 

 within which the balance system works, but works differently when 

 one or more of the variables of the developmental system are changed. 

 It is therefore to be expected that environmental actions like extreme 

 temperatures, hunger, and parasitism may have the same effects, 

 producing intersexuality. We have mentioned (in III 3 B) the pheno- 

 copies of intersexual features in Lijmantria. Examples, some very 

 remarkable, of similar action of other environmental factors (e.g., 

 parasitism) can be found in my book on sex intergrades (1931). 



With these facts as a background we shall have no difficulty in 

 assigning the proper place to genetic modifiers producing, if mutated, 

 intersexuality in the presence of the normal F/M mechanism. We 

 meet here with the same error of interpretation as the one mentioned 

 above. We remember, for example, that in Drosophila sex determiners 

 in the second and fourth chromosome were ruled out (see I 2 C d ee). 

 If a mutant locus which makes one sex intersexual is found in the 

 second chromosome, it is said that this shows that sex determiners are 

 nevertheless present in the second chromosome and that a multiple 

 factor sex determination is demonstrated. Actually, a mutant has been 

 found which changes the genetic system of development so that the 

 usual F/M balance works in a different system of coordinates and 

 therefore has a different effect. Thus it is not surprising that in Droso- 

 phila species a number of mutant loci which alone produce inter- 

 sexuality have been found; further, that combinations of ordinary 

 mutants exist which make the carriers intersexual, for example, the 

 low-grade intersexual males produced by the Beaded-Minute com- 

 bination (Goldschmidt, 1949a, 1951Z?). New cases of one or the other 

 type are being found continually. Whatever their specific features — 

 and these are expected, since they probably act through different co- 



