The Genetic Control of Mutability 



215 



of Ds via breakage. This is not synonymous 

 with the change in color, though the change 

 in color is the phenotypic event which led to 

 the detection and proof of the mutational 

 event. The change in color is apparently a 

 position effect (Chapter 22); the presence 

 of Ds, next to the gene for color, suppresses 

 color formation; its absence permits the gene 

 for color to produce color. 



It was possible to prove that the very ca- 

 pacity of Ds to cause breakages nearby is 

 under genie control. These latter genes, 

 since they activate Ds to break the chromo- 

 some, thereby leading to the removal of Ds 

 from its original position, are called Acti- 

 vator, Ac, genes. Ac does not have to be 

 on the same chromosome as Ds, and usually 

 is not. 



FIGURE 25-1. TIxe effect o/ Activator on tlie action o/ Dissociation. (A) No 

 Ac is present. T/ie /<ernel is colorless due to the continued presence of Ds, which 

 inhibits the action of a neighboring pigment-producing gene. {B) One Ac factor 

 is present. Breaks at Ds occur early in kernel development, leading to large 

 colored sectors. (C) Two Ac factors are present. Time of Ds action is delayed, 

 producing smaller sectors which appear as specks. (D) Three Ac factors are 

 present. Ds action is so delayed that relatively few and tiny specks are produced. 

 (Courtesy of B. McClintock.) 



