84 MUTATION AND PLANT BREEDING 



Although chromosome breakage predominates in the first divi- 

 sion following exposure, the cytological consequences of breakage 

 frequently also can be seen much later. In addition to chromatin 

 bridges and micronuclei which may persist and be visible for more 

 than one cell division, certain types of aberrations, such as inversions, 

 translocations, and rings, may persist indefinitely and express them- 

 selves by characteristic cytological configurations and by their con- 

 comitant genetic or physiological effect. Small duplications in the 

 form of centric fragments may also persist (15) and these, of course, 

 can produce genetic effects. In certain cases a series of phenomena 

 known as the breakage-fusion-bridge cycle can result whether the 

 initial breakage is of spontaneous origin, produced by ionizing radi- 

 ation or by a chemical mutagen. As reported by McClintock (100), 

 this cycle may go on for many cell generations. Other examples of 

 long-persisting effects are small dicentrics reported by Morrison (106) 

 and certain conditions such as translocations found in hexaploid 

 wheat by MacKey (96). For unknown reasons these are sufficiently 

 unstable to lead to secondary changes such as deficiency-duplication 

 or simple deficiencies. These, of course, can result in phenotypic 

 changes. 



2. The mechanisms of chromosome breakage 



Although much is known about the chemical changes produced 

 by ionizing radiation (7, 8, 38, 73, 91, 113), our knowledge of the exact 

 mechanisms by which chromosomes are broken is unfortunately 

 rather meager. This lack of understanding is due partly to our inade- 

 quate knowledge of the composition and structure of the chromo- 

 some threads themselves, and partly to a lack of adequate definition 

 and resolution which would enable us to see changes which are 

 produced at the submicroscopic level in the interval between the 

 initial chemical events and the final biological event recognized as a 

 chromosome or chromatid break. 



A summary of the possible interrelationships of various events 

 which occur preceding, during, and after chromosome breakage is 

 given in Figure 7. Possible pathways of various modifying factors or 

 processes are also given. The relationship of gene mutation to other 

 types of chromosome damage is obscure. 



There has been much speculation concerning the manner in 

 which damage to DNA protein or nucleoprotein molecules could lead 



