:fcN(i 



CHAPTER 30 



instability of various loci in other flower- 

 ing plants, terns, fungi, and bacteria may be 

 due to similar factors. 



* Segregation Distortion in Drosophila 



Drosophila melanogaster homozygous for the 



II chromosome mutants, cinnabar (en) and 

 brown (bw). have white eyes because en en 



and bw bw prevent the formation of the 

 brown and the red pigments, respectively, 

 which together comprise the dull-red eye 

 color of the wild type. When the test cross 

 4- + en bw ' by en bw en bw 9 is made, 

 the progeny typically occur in the approxi- 

 mate phenotypic ratio of one white to one 

 + . If, however, the unmarked II chromo- 



figure 30-1. The effect of Activator on the action of Dissociation. (A) 

 No Ac is present. The kernel 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 devel- 

 opment, leading to large colored sectors. (C) Two Ac factors arc 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.) 



