156 Nature of the Genetic Material 



the germ-line mosaics produce colorless individuals. In the light of 

 our former discussions, these are very different events, as Stadler's 

 experiments also show. He states that A is a very stable locus in which 

 he could produce no mutation to a in "fairly extensive trials." (See 

 I 3 C c bb.) (The split locus in Drosophila behaved similarly, though 

 rearrangements involving it were produced easily.) We have noted 

 that ai appeared only twice spontaneously. The A alleles produced by 

 Dt — "Dt-induced mutation" — behave difiPerently. To avoid misunder- 

 standings, I repeat that these are extracted different A types from 

 variegation affecting the germ line, which, as explained above, are very 

 diflFerent from ordinary mutation. Now these alleles "are not strikingly 

 mutable, but they yield mutations to a at rates comparable to the 

 mutation rates found in the various R stocks studied. These mutations 

 have been found in 4 different Ai alleles, each tracing to a separate 

 Dt induced mutation of a." 



Stadler proposes some interpretations of the* fact, one in terms of 

 subunits in the A gene, another assuming that Dt present in the 

 derived line may cause mutation from A to a. I am convinced that the 

 correct explanation must be derived from the assumption that a is a 

 position ejEect, which works phenotypically in the direction of A -> a; 

 further, that the colored patches which may enter a next generation 

 are at least in part based upon subsequent rearrangements, working 

 phenotypically in the opposite direction, as shown above. Therefore, 

 the offspring of these derived colored plants will show different kinds 

 of segregation for these additional position effects, some of which 

 remove the secondary effect, leaving the primary ai position effect. 

 Thus all the facts fall in line and, as said before, the mutable locus 

 disappears completely, while position effect establishes its rule over 

 this interesting group of facts. 



ee. Theory of position effect and genie structure of the chromo- 

 some 



aaa. Breakage and mutant effects. — The facts on position effect 

 recorded thus far cannot leave any doubt that no distinction can be 

 made between point mutation and position effect except visibility or 

 non-visibility in the salivary chromosomes of Drosophila. The con- 

 clusion seems inevitable that so-called point mutants are rearrange- 

 ments below the level of microscopic visibility. These invisible 

 rearrangements may be deficiencies, inversions, duplications. We have 

 mentioned one-band inversions in the salivaries, which may be rec- 

 ognized only in rare instances. A similar inversion in maize, that is, on 



