Chromosomes and Genes 125 



sidered (a)* a stable change in the action of a gene or genes 

 lying next to the point of rearrangement, possibly within the 16A1-2 

 doublet at that point, and (b) an influence somehow exerted by the 

 close juxtaposition of two sets of identical loci." After mentioning 

 the Bar reversals without cytological change described by Dubinin 

 and Volotov (1940) and Sutton (1943), and probably due to a sup- 

 pressor mutant, Lewis continues: "In all of the above studies in 

 which reversals of the dominant Bar phenotypes appeared, it was 

 found as expected on the basis of a known Bar deficiency that loss 

 of the 16A region does not give a Bar effect nor does it enhance or 

 suppress Bar, that is B/Df-B resembled B/+ [Bridges, 1917]. Again 

 the problem as to the nature of a gene change which can produce 

 such results arises ... if the bands 16A1-2 are considered as repre- 

 senting two closely linked genes with similar effects . . . the problem 

 becomes a very complex one, indeed, in the Bar case." I add to 

 this: how simple and logical it becomes when our viewpoint, dis- 

 carding genes and their position, is adopted. 



We finally return to a problem mentioned above, that no mutant 

 locus is known in the Bar region which accounts for the position 

 effects of Bar and the baroid rearrangements. We mentioned the 

 possibility that a position effect, especially a dominant one, might 

 sometimes have nothing to do with a locus near the break acting as 

 if it had mutated, but might be a direct effect of the breakage, a 

 change in chromosomal pattern. As opposed to Bar, a number of 

 duplications with dominant position effect are known, usually small 

 ones, in which a recessive mutant locus near or within the duplication 

 is known (e.g., eyeless dominant and brown dominant). Others behave 

 like Bar (e.g.. Hairy-wing). Thus far only one recessive position 

 effect of a duplication has been described in the known presence of 

 a similar dominant mutant locus within the duplication, namely. 

 Green's Beadex-recessive ( 1953Z? ) , The behavior of the Bar deficiency 

 previously described seems to indicate for Bar the absence of an 

 unknown locus for Bar mutation, that is, the alternative of a breakage 

 effect pe/ se. 



It is strange that among the relatively rare dominant mutants 

 in Drosophila many are described as "inseparable from translocation 

 or inversion . . ."; that is, are position effects for which no mutant 

 locus of similar action is known in the section involved. Bar, Dichaete, 

 Curly, Upturned, Abrupt, Blond, Glazed, Xasta belong to this group. 

 It is possible that in some the missing mutant locus may be found. 



* From here on italicized in the original. 



