82 MUTATION AND PLANT BREEDING 



16. Duplication 



An aberration in which one or more segments are duplicated. 

 They are also called repeats. In a triplication a segment is present 

 three times. 



17. Deficiency or deletion 



Alternate names for fragments but usually restricted to mean 

 small fragments which often are lost from the daughter nuclei by 

 virtue of the fact that they are acentric. (See also 4 above.) 



18. Dicentric 



Any chromosome or chromatid which contains two centromeres. 

 Tricentrics have three centromeres, quadri-centrics four. Those with 

 more than two can be referred to as multicentrics or poly-centrics. 

 A chromosome with two or more centromeres may form a bridge 

 (or bridges) at anaphase. 



19. Micronucleus 



A small nucleus which usually lies in the cytoplasm at some 

 distance from the main nucleus and in irradiated cells usually results 

 from one or more lagging acentric fragments. There may be one to 

 several per cell. Micronuclei can also result from other causes, such 

 as abnormal spindle behavior, and from the lagging of centric 

 chromosomes. 



20. Spontaneous breakage 



This and resultant aberrations are know to occur in cells exposed 

 only to background levels of radiation. Suitable experiments indicate 

 that it is very doubtful if more than a small fraction of these spon- 

 taneously appearing chromosome aberrations are the result of natural- 

 ly occurring radiation (108). They presumably have their origin in 

 the inherent instability of the nucleoprotein structural framework of 

 the chromosome. There are wide fluctuations and variations in the 

 rate of spontaneous breakage. 



The above description of cytological effects is necessarily brief. 

 More detailed considerations of the aberrations are given in many 

 places (21, 30, 39, 49, 78, 125, 137, 183, 198, 199). These sources 

 should be consulted for descriptive diagrams and pertinent references. 

 The "Bibliography on the Effects of Ionizing Radiations on Plants" 

 (160) is also useful in finding original references. 



