220 



CHAPTER 52 



FIGURE 25-7. Transposition of Modu- 

 lator and the origin of twin sectors. 



DIVIDED PARENT DAUGHTER 



CHROMOSOMES CELLS 



(Medium Variegated Phenotype) 



CO-TWIN 

 PHENOTYPES 



I Red 



I Light 



Variegated 



\ 



five and 15 units, and the remainder were 

 farther away. Hence, Mp tends to move 

 from the P locus by a short, rather than a 

 long, jump. This suggests that contact be- 

 tween old and new sites may be required for 

 shifts and transpositions of Mp. 



Sometimes reds revert to variegated. In 

 such cases, a transposed Mp is found to have 

 been transposed near to P\ In fact, the fre- 

 quency of such reversions from red to varie- 

 gated can be studied by introducing into a 

 chromosome containing P% a transposed 



Mp located various distances from P\ As 

 shown in Figure 25-8, the closer transposed 

 Mp is to P% the greater the frequency of 

 reversions. In summary, then, medium mu- 

 tates to red by loss of Mp from its position 

 near the P"" locus. In this process, comple- 

 mentary lights are produced which possess 

 an extra Mp, transposed Mp. The medium 

 type is reconstituted by the movement of a 

 transposed Mp back near the P"" locus. 



Two additional points need to be made. 

 The changes in phenotype involving reds, 



