142 Nature of the Genetic Material 



mutants and rearrangements. With Muller's method of measuring 

 mutation rate by sex-Unked lethals (in Drosophila), these lethals 

 were treated as one class, just mutations, though it was clear that 

 they included gross rearrangements, more or less small deficiencies, 

 and point mutation. This suggested that point mutations may, after 

 all, be invisible rearrangements and deficiencies. The deficiencies 

 were considered as actual absences of a gene (Stadler), while other 

 rearrangements must have been position effects (Muller, Gold- 

 schmidt). Meanwhile it turned out, as we saw, that deficiency breaks 

 also may produce position effects so that all rearrangements can be 

 treated alike. The conclusions from such facts differed: either they 

 were considered within the theory of the gene (e.g., Stadler's de- 

 ficiencies in maize); or they gave rise to speculation on whether the 

 gene is not a more complex structure with subunits (Muller); or 

 whether the concept of the gene should not be replaced by that of 

 a definite order of parts, the change of which produces the mutation 

 and position effect (Goldschmidt). 



The facts underlying such conclusions are very diversified and 

 are derived from different types of experimentation. As a first type, 

 comparable to the analysis of sex-linked lethals, we may mention 

 the Minutes (see I 2 C d dd), that group of possibly heterochromatic 

 dominant mutants scattered over all chromosomes and having similar 

 effects (see Brehme, 1939, 1941). Many of these have been found 

 cytologically to be deficiencies of different size, but many show 

 normal salivary structure. It is hard to believe that this could be 

 a genuine difference. Either we must assume that the deficiencies 

 have nothing to do with the M effect and accompany it only by 

 chance, which is very improbable, or we must assume that the 

 Minutes with normal chromosomes actually are based upon deficien- 

 cies below the level of a salivary band. The latter assumption seems 

 to be the logical conclusion. 



In the same category belongs another remarkable fact. The 

 dominant Notch is considered a deficiency for the facet and split 

 loci in the X-chromosome, and frequently a number of nearby loci 

 are included in the deficiency. For many of the numerous N alleles, 

 deficiencies of different length have been demonstrated in the salivary 

 chromosomes. Gottschewski (1937) first found a Notch genetically 

 deficient for a number of loci but with a completely normal chromo- 

 some cytologically. Demerec (1941o) and Barigozzi (1942) have 

 since added a number of similar cases. All these authors agree that 

 in these cases the normal genes had not been deleted but inactivated. 



