80 MUTATION AND PLANT BREEDING 



2. Restitution 



The process in which broken ends unite back into the original 

 configuration. The result leaves no cytologically detectable evidence 

 of the break. 



3. Reunion or rejoining 



These words are used more or less interchangeably to describe 

 any union of broken ends other than the type referred to above as 

 restitution. 



4. Fragment 



Any portion of a chromosome separated from the main chromo- 

 some by breakage. With the exception of a few organisms which have 

 diffuse centromeres, fragments are commonly acentric. Terminal and 

 interstitial fragments are frequently referred to as terminal and inter- 

 stitial deletions or deficiencies. Small fragments or dot deletions which 

 are the same in length as in width are also called isodiametric frag- 

 ments and extremely small fragments are generally called minutes. 



5. Chromosome break 



The prerequisite to the production of fragments or other types 

 of aberration. It occurs when a chromosome is irradiated before it 

 has become effectively double (2-stranded). 



6. Chromatid break 



A break in one of the two chromatids of the duplicated chromo- 

 some, usually seen at late prophase or metaphase. 



7. Subchromatid break 



A break in a subchromatid or half-chromatid at a time when the 

 chromosome is 4-stranded. 



8. Isochromatid break 



A break in both chromatids of a chromosome at or near the same 

 position. The broken ends of the chromatids may rejoin in such a 

 fashion as to form a dicentric and a U-shaped fragment. The dicentric 

 then usually forms a bridge at the first anaphase following its forma- 

 tion. If both centromeres go to the same pole the dicentric will persist 

 until the next anaphase or possibly longer. 



