716 



RADIATION BIOLOGY 



and chromosome types may occur in the same cell — or rarely, even in the 

 same chromosome (Sax and Mather, 1939)— chromosome types replace 

 the chromatid types and are present exclusively from about 31 hours on. 

 The frequency of chromosome aberrations rises rapidly to a maximum 

 shortly after this type appears, and this frequency is maintained for 

 several days after irradiation. The typical periods for observation of 



CHROMOSOME ABERRATIONS 



TYPE 



IS 



o 



in hi 



cc _J 



uj i±J 



I- Q 



Z 



INTERPHASE 



PRO- 

 METAPHASE 



ANAPHASE 



CHROMATID ABERRATIONS 



TYPE 



2 UJ 



O a: 



CC CD 



I 



o 

 1- 



CD 



PROPHASE 



PRE- 

 METAPHASE 



ANAPHASE 



UJ 



o _ 



2 8 



o 



cr 



cc o 



0^ 







z 



I- 

 z 



o 



vv 



Fig. 10-1. Diagrammatic representation of the origin and types of aberrations, both 

 chromosome and chromatid, induced in Tradescantia microspores by ionizing radia- 

 tions. Only the types of major importance which can be recognized at metaphase 

 and anaphase in cytological analyses are shown. Original break positions at inter- 

 phase or prophase are indicated by short diagonal lines. {Modified from Sax, 1940. 

 Reproduced by permission of the author and the editor of Genetics.) 



chromatid and chromosome aberration frequencies are 20-24 hours and 

 4-5 days following irradiation, respectively. The time and duration of 

 the transition period appears to depend rather markedly on environ- 

 mental factors. 



The various kinds of chromatid and chromosome aberration types pro- 

 duced in Tradescantia microspores have been diagramed and discussed 

 in considerable detail by Sax (1940), Lea (1946), and especially by 

 Catcheside, Lea, and Thoday (1946a) and Catcheside (1948). The 

 present discussion will consider in detail only certain selected types which 

 occur most frequently and which have been of major importance in the 



