/ 



The Genetic Control of Mutability 221 



mediums, lights, and nonreds are not really figure 25-8. Effect of distance of Mp from 



mutations at the P locus. These changes P i^pon transposition rate of Mp to P. 



are the phenotypic consequences of muta- 

 tions involving the transposition of Mp, and 

 are in this respect like the changes conse- 

 quent to the transposition of Ds. Transpo- 

 sition of Mp to another locus may change 

 the phenotype produced by the recipient 

 locus. Thus, in a particular medium red 

 carrying an allele on chromosome 9 that re- 

 sulted in the starchy phenotype, a "mutation" 

 to the waxy phenotype was observed. The ^».«~,^ 



waxy phenotype was unstable and frequently 

 "mutated" back to starchy. Tests showed 

 that Mp had been transposed to the starchy 

 locus which then gave the waxy phenotype, 

 and that the reversions to starchy were the ^__^- a 



result of Mp\ being transposed away from \ \ \ 



this locus. All the phenotypic changes con- 

 sequent upon the movement of Ds and Mp '"^^^ 

 strongly resemble position effects. > / >^ , 1. 1 



PER CENT RECOMBINATION VARIEGATED SECTORS 

 p _ ^ PER 1000 KERNELS 



2.6 15 



4.3 11 



7.6 8 



12.0 3 



So far, we have not offered any evidence 

 that the transposition of Mp is under genetic 

 control. If Mp transposes away from 

 P'' Mp 100 times in the absence of a trans- 

 posed Mp, the presence of one transposed 



Mp reduces this frequency to about 60, while 42.0 0.2 



the presence of two transposed M//s further 

 reduces this value to about 5. Thus, the 

 transposition of Mp from P'" Mp is con- 

 trolled by the presence of transposed Mp\ \ 

 Note that while breakability by Ds is regu- ^ 

 lated by a different factor Ac, breakability W» " 

 and its regulation are the consequence of a 

 single kind of factor in the case of Mp. 



Factors like Ds and Mp are known to be 



