Chromosomes and Genes 133 



(1954), who described it as a "new transvection-efiFect" acting upon 

 dominant Bithorax in a specific pseudoallelic setup (the latter is the 

 only difference from our old findings); it was used on a larger scale 

 for discovering rearrangements. It should be stated also that Green 

 and Oliver (1940) found similar effects of translocations and in- 

 versions upon the vestigial phenotype. We might mention, in this 

 connection, the crossover-enhancing effect of inversions in a different 

 chromosome, assuming that mutants affecting crossover exist, though 

 it is by no means certain that in this case an interpretation in terms 

 of position effect is admissible. 



6. Many mutants act as dominant or recessive lethals. Fre- 

 quently these turn out to be deficiencies, but many cases are knowTi 

 in which lethals are not associated with visible rearrangements and 

 therefore may be called point mutants (see, however, the Notch 

 case). Visible rearrangements apart from deficiencies are frequently 

 lethal, a fact which has been much discussed for other reasons and 

 has already been touched upon in our discussion of the proportionaHty 

 of rearrangement breaks to the dosage of irradiation. Lea had calcu- 

 lated that the data on lethals near rearrangement breaks allow only 

 a small percentage of position effects, and that the majority must be 

 considered as real lethals or lethal deficiencies. Muller's opposition 

 to these conclusions, based upon a critical attitude toward the target 

 theory upon which Lea's calculations are based, has already been 

 registered. For the present discussion it does not matter how many 

 of these lethals are position effects so long as we acknowledge that 

 a position effect may also act as a lethal. 



7. Mutants have more or less pleiotropic effects, meaning that 

 frequently other phenotypic effects beside the most conspicuous one 

 can be seen. This is true also of position effects: Dichaete, for ex- 

 ample, acts upon wing posture and bristles. The white position effect 

 acts upon eyes and Malpighian tubules just as the mutant does. 



Altogether we may say that so far no difference in principle 

 between mutation and position effect has been found, a fact which 

 will be borne out in other observations to be mentioned. The most 

 obvious conclusion from this, as well as from the foregoing dis- 

 cussioris, is that so-called point mutations are really position effects 

 of rearrangement breaks below the level of visibility with present 

 techniques (to be discussed in detail later). But the cytological basis 

 for such a conclusion is not completely missing: Palay and Claude 

 ( 1949 ) found with the electron microscope a substructure of parallel 

 bandings within salivary chromosome bands. 



