42 I'HYSIOLOCICAL GENETICS 



process dependent upon a series of multiple alleles. This, 



however, anticipates the discussions of a following chapter. 



C. Lethality 



It is one of the elementary facts of genetics that mutants with 

 rather Large somatic effects tend to be lethal. In terms of gene 

 action, this would mean that the change in developmental 

 processes produced by the mutant gene is of such a magnitude 

 that the proper coordination and integration of the different but 

 simultaneous processes are put out of gear. The study of the 

 phenocopies together with certain genetic facts brings these facts 

 into line with the general concept of gene action. 



In Goldschmidt's work on the vestigial wing in Drosophila, it 

 was found that the sensitive period for scalloping of the wings 

 occurred a short time before pupation. It was further found 

 that the maximal phenocopic effect produced at this time by heat 

 treatment corresponded to the effect of the alleles notched to 

 snipped or to grade V to VI, if the amount of scalloping is divided 

 into 10 classes with nicked as class I and vestigial as X. The 

 study of development of flies with different allelomorphs further 

 showed that no visible degeneration of wing area has taken place 

 at the time of pupation, if the maximal final result is class VI of 

 scalloping. This means that the mutant genes up to snipped will 

 act in such a way that the visible effect does not occur before the 

 sensitive period and presumably that their action occurs actually 

 within this sensitive period. We know, furthermore, that the 

 higher alleles produce a similar visible effect before this period 

 and the highest alleles even in early larval stages. We might 

 infer from this that other sensitive periods exist in which definite 

 gene action takes place, maybe before each molt, and the work of 

 Harnly points to such a period at the molt from the second to the 

 third instar. 



It is now knowm that the imaginal disk which produces the 

 wing also contains the material for the thorax segment and that 

 this material becomes separated from the wing material only 

 in the course of development. A degeneration of tissue produced 

 by the ^-allelomorphs will therefore affect the development of 

 the thorax segment also, if the action takes place very early. It 

 seems that the onset of the action of the y<7-gene occurs just near 

 this point in development; in some lines of vg, individuals with a 



