4(>S 



( IIAPTER 37 



chromosome I. the allele having moved less 



than 50 crossover units from /'. In the re- 

 maining 30 eases Mp was found transposed 

 to one of five different nonhomologous chro- 

 mosomes. Of the 57 eases where trans- 

 posed Mp was still linked to chromosome 1, 

 37 showed Mp within five crossover units 

 of /'; 10 showed Mp within 5 to 15 units; 

 and the remainder showed Mp to be farther 

 away. Hence. Mp tends to move from the 

 P locus by short rather than by long jumps. 

 This situation suggests that contact between 

 old and new sites may be required for shifts 

 and transpositions of Mp. 



Reds sometimes revert to variegated. In 

 such cases an Mp is found transposed near 

 P' . The frequency of such reversions from 

 red to variegated can be studied after intro- 

 ducing an Mp locus various distances from 

 P' in a P'-containing chromosome. As 

 shown in Figure 37-7. the closer to P' the 

 introduced Mp is, the greater is the fre- 

 quency of reversions. In summary, medium 

 mutates to red by loss of Mp from its posi- 

 tion near the P' locus. In this process, com- 

 plementary lights are produced possessing an 

 extra Mp — a transposed Mp. The medium 



type is reconstituted by the return of a trans- 

 posed Mp near the P' locus. 



Two additional points need to be made. 

 Changes in phenotype involving reds, me- 

 diums, lights, and nonreds arc not muta- 

 tions at the P locus. These changes are the 

 phenotypic consequences of mutations in- 

 volving the transposition of Mp and are, in 

 this respect, much like the changes which 

 follow the transposition of Ds. Transposi- 

 tion of Mp to another locus may change the 

 phenotype the recipient locus produces. For 

 example, a "mutation" to the waxy pheno- 

 type was observed in a particular medium 

 variegated individual whose chromosome 9 

 carried an allele for the starchy phenotype. 

 The waxy phenotype was unstable and fre- 

 quently "mutated" back to starchy. Tests 

 showed that Mp had been transposed to the 

 starchy locus, which then produced the waxy 

 phenotype; furthermore, the reversions to 

 starchy were the result of Mp's transposition 

 away from this locus. All the phenotypic 

 changes dependent upon the presence of Mp 

 (and Ds) strongly resemble position effects. 



No evidence has been presented thus far 

 that the transposition of Mp is genetically 



DIVIDED PARENT DAUGHTER 



CHROMOSOMES CELLS 



(Medium Variegated Phenotype) 



CO-TWIN 

 PHENOTYPES 



P Mp 



P Mp 



> 



| Red 



| Light 



Variegated 



figure 37-6. Transposition of Modulator and the origin of twin sectors. 



