358 Action of the Genetic Material 



interpretation. This is that the sex-determining genes themselves act 

 as dosage modifiers. To disprove this, he mentions the following facts. 

 There is known a third chromosome recessive mutant, called "trans- 

 former," which in homozygous condition transforms 2X females into 

 what look to all purposes like males. If the eye color apricot is present, 

 the transformed males have the same color as genuine males with 

 apricot eyes. By introducing a duplication into a genuine male, it can 

 be made to have two doses of apricot instead of one with the result 

 of a darker eye color. Therefore, the conclusion runs, the XX males-by- 

 transformation should have this darker color if the sex determiners 

 were also the dosage modifiers. Actually, the color is the same as in 

 normal males, which means that not the sex determiners but the 

 dosage compensators within the X-chromosomes are responsible. This 

 argumentation clearly assumes that the normal IX males and the 

 transformed 2X males are otherwise the same, which means that they 

 have the same developmental system. If the problem had been looked 

 at, not from the point of view of modifiers, but from that of develop- 

 ment, it would have turned out that the males-by-transformation have 

 a developmental system which is female in a number of basic features. 

 As a matter of fact, the sex-reversal flies are not males but extreme 

 female intersexes ( = 2X intersexes ) in which some basic features of 

 growth and differentiation (actually those of early determination) are 

 still female. This is true for size and general growth, for time of 

 development, for many proportions, and for the rhythm of gonad 

 development. In addition, all sex-linked mutants which are sexually 

 dimorphic (i.e., without so-called compensation) show the female 

 phenotype, as Sturtevant ( 1945 ) noticed. Thus the behavior of eye 

 color is just what is expected in the 2X intersexes, and the dosage 

 compensation explanation becomes superfluous. 



These last facts really show in what direction to look for a better 

 interpretation, namely, in terms of development and not by invent- 

 ing hypothetical modifier systems where they seem to be needed. I 

 may point out that male and female differentiation take place in very 

 different developmental systems, laid down at the moment of fertiliza- 

 tion by the different balance of the sex factors. I point further to the 

 fact that developmental rates in the two sexes are different; that the 

 relative rates for the individual and consecutive phases of growth are 

 different; that times of determination as seen in temperature-effective 

 periods or in times for optimal production of phenocopies are different; 

 further, that the rhythm of differentiation of individual organs, like 

 the gonads, differs in the two sexes. Thus we can understand, on the 



