sparrow: cytogenetic effects of ionizing radiations 79 



may occur in a great variety of forms (59, 61, 62, 63). In the extreme, 

 tumors may be formed as a result of the radiation treatment (166). 

 It is not known whether the tumors originate as mutant cells or 

 because of a general physiological disturbance. The latter seems most 

 likely (151, 170). 



3. Reproductive capacity 



Reproductive capacity can be reduced to zero by gene or chromo- 

 somal changes and also in several ways which may or may not result 

 from direct cytogenetic damage, i.e., some may be due to physiological 

 disturbances. Sterility may arise (a) by severe stunting or growth 

 inhibition which prevents flowering; (b) flowers form but lack the 

 necessary reproductive structures; (c) reproductive structures are 

 present but pollen is aborted; (d) fertilization occurs but embryos 

 are aborted before maturity; or (e) seeds form but fail to germinate 

 properly or die after germination. 



4. Phenotypic appearance 



The phenotypic appearance of X T plants may change for any of 

 several reasons: (a) in haploid plants due to mutation; (b) in diploids 

 due to dominant mutation, or if the plant is already heterozygous, 

 due to recessive mutation or to deletion of the dominant locus; (c) 

 reversion of a chimera as explained above (Section IV, 9); or (d) 

 change of a characteristic presumably due to nongenetic physiological 

 change, e.g., short shoots apparently devoid of chlorophyll may devel- 

 op on chronically irradiated Tradescantia plants; however, they often 

 regain normal color at later stages of development (155). 



VI. Radiation-induced Chromosome Aberrations, 

 Abnormalties and Related Processes 



A. Terminology 

 It would seem appropriate to explain briefly a number of proc- 

 esses which occur during the production of or as a result of chromo- 

 some breakage and a description of the various types of induced 

 aberrations. (See also Section IV.) 



1. Fragmentation or breakage 



The process by which a chromosome, chromatid, or subchroma- 

 tid is broken into two or more pieces. Less commonly used synonyms 

 are fracture and shattering. 



