306 PHYSIOLOGICAL OENETIC8 



Dubinin and (heir collaborators; Muller, Patterson, and Stone; 

 Beadle and Sturtevant et ah). The work of the last-named 

 group of authors showed that breaks at or near the scute locus 

 were involved. The more refined study of such cases by the 

 salivary-gland method revealed that actually very small inver- 

 sions were present. Muller and Prokofieva (1934, 1935), 

 Muller, Prokofieva, and Raff el (1935) made such an analysis 

 for the left end of the X-chromosome. They showed that 

 different rearrangements involving the scute locus in Drosophila, 

 in the great majority of cases, result in phenotypically different 

 "allelomorphs," whereas nearly identical rearrangements (scute 

 4 and scute 68) have given essentially the same allelomorphs. 

 Of 27 scute and achaete mutations, 18 were proved to be caused 

 by rearrangements after breaks, and the others are suspected of 

 being the same. One case (scute 19) is a deletion of only a single 

 segment (in the salivary chromosome) with insertion in chromo- 

 some 2. Other scutes proved to be inversions of small fragments 

 in situ. In this and similar cases, two effects had been observed 

 (double mutations) which thus are referred to the two points of 

 breakage of an inversion. From such facts the authors are 

 inclined to assume that possibly all mutations produced by 

 X rays may be such minute rearrangements. 



But such effects are not necessarily bound to minute inversions. 

 Griineberg (1936) reported upon a very long inversion within the 

 X-chromosome which behaved like an allele of roughest and 

 produced a very rough eye surface. He w r as able later to get a 

 return arrangement to a normal order in the chromosome, and 

 with it the effect disappeared. What seemed to be a mutation, 

 then, was a position affect. 



3. A third type of position effect was described by Dubinin and 

 Sidorov (1934, 1935) for translocations between the fourth and 

 other chromosomes. In this case, the effect of the break does not 

 resemble a gene mutation but a change in dominance, w r hich, of 

 course, could also be described as the production of a mutant 

 dominigene. In flies heterozygous both for this translocation 

 and for the fourth-chromosome recessive cubitus interruptus, this 

 latter character appeared as semidominant to completely domi- 

 nant. Sturtevant and Dobzhansky (Dobzhansky, 1936) tested 

 some more such translocations, so that altogether 48 have been 

 tested, of which 22 showed the dominance effect. Dubinin 



