XLI 

 INDUCED CHROMOSOMAL ALTERATIONS 



T. H. GOODSPEED 



During the past six years evidence has accumulated to indicate the 

 cytogenetic significance of the alterations which chromosomes of plant 

 and animal species have long been known to undergo as a result of treat- 

 ment with high-frequency radiation. Before this period the extent of 

 occurrence of quantitative chromosomal alterations under natural 

 conditions had not been emphasized nor, in particular, had their possible 

 significance in the origin of mutational processes been fully appreciated. 

 It is now recognized that such chromosomal changes induced in plants by 

 high-frequency radiation differ in frequency of occurrence rather than in 

 kind from those found in nature. It is, therefore, possible to discuss 

 them in terms of such classifications of chromosomal alterations as those 

 of Bridges (8) or Belling (1). Thus, induced chromosomal changes may 

 involve portions of chromosomes, whole chromosomes, or chromosome 

 sets, and within these subdivisions examples under most of the specific 

 categories recognized as occurring under natural conditions have been 

 induced by high-frequency radiation. Thus instances of polyploidy and 

 haploidy which involve alterations in sets of chromosomes have also 

 occurred in radiation work. Similarly, the occurrence of polysomics, 

 monosomies, fragment chromosomes, and compound chromosomes is 

 not an infrequent effect upon whole chromosomes. Finally, the cate- 

 gories of alteration having to do with sections of chromosomes and involv- 

 ing chromosomal interchanges — translocation (simple and reciprocal) — 

 deficiency, deletion, inversion, and duplication are products of treatment 

 with high-frequency radiation. 



Although the majority of these various types of chromosomal altera- 

 tions in plants are primary products of the treatment, their occurrence is 

 also to be assigned to secondary effects of initially induced chromosomal 

 alteration. The accompanying chart (Fig. 1) indicates the relationship 

 between primary and secondary effects of high-frequency radiation and 

 the interrelations between them in the production of cytogenetic types. 

 Thus, irradiation may lead directly to extensive nuclear disruption or to 

 lethality, the latter being obtained secondarily via extensive nuclear 

 disruption, via initially induced gene mutation, or via chromosomal 

 reorganizations which are the direct products of treatment or occur as 

 consequences of the monosomic or polysomic condition. Aneuploidy 



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