chromosome aberrations in Tradcscantia 111 



quantitative data on radiation effects. The mode of origin of these 

 types is shown diagrammatically in Fig. 10-1, and photomicrographs of 

 the various types are presented in Plate I. 



There are three major chromatid aberration types: (1) chromatid 

 breaks, in which one of the two chromatids in a metaphase chromosome 

 has a transverse fracture, giving rise to a free acentric fragment at 

 anaphase (Plate I-E, F) ; (2) isochromatid breaks (chromatid dicentrics), 

 in which the two sister chromatids of a single chromosome are broken at 

 the same locus, the broken ends typically undergoing lateral union to 

 produce a dicentric and a U-shaped fragment (Plate I-B). At the suc- 

 ceeding anaphase a bridge and a free acentric fragment result (Plate 

 I-C). The interpretation of the manner in which ionizing radiations 

 produce isochromatid breaks has been a controversial one. For example, 

 Darlington and La Cour (1945) have maintained that this aberration 

 type arises as a result of chromosome breakage before effective division, 

 apparent sister union resulting from a reproductive error in the chromo- 

 some thread and not from true reunion. Catcheside (1948) has sum- 

 marized the evidence against this view and in favor of the alternative one, 

 that isochromatid breaks, in general, arise as a result of the simultaneous 

 breakage at the same level of the two chromatids of a single chromosome. 

 (3) Chromatid exchanges are derived from two chromatid breaks, either in 

 two separate chromosomes, yielding interchanges, or in the same chromo- 

 some, yielding intrachanges. The commonest category is the interchange 

 in which the union of broken ends gives rise to symmetrical (reciprocal 

 translocation, Plate I-D) or to asymmetrical (dicentric and acentric frag- 

 ment, Plate I-E) types. Additional exchange types, which are in general 

 much less frequent, involve recombination between two isochromatid 

 breaks or between one isochromatid and one chromatid break, and have 

 been described and diagramed by Catcheside, Lea, and Thoday (1946a). 



Chromosome Aberrations. This second major category of aberration 

 types is made up of those arising in chromosomes which are effectively 

 single at the time of exposure. There are also three major chromosome 

 aberration types: (1) chromosome breaks (terminal deletions), in which 

 the unsplit chromosome suffers a transverse fracture giving rise at meta- 

 phase to two shortened chromatids and two acentric fragments (Plate 

 I-G) ; (2) interstitial deletions (minute, isodiametric fragments), involving 

 two breaks close together in one arm of an unsplit chromosome, the broken 

 ends uniting to produce a small acentric ring which appears following 

 division as a pair of dotlike fragments at metaphase (Plate I-G, N). This 

 aberration is actually a type of intra-arm exchange (intrachange) but is 

 listed separately because of the frequency with which it occurs. (3) 

 Chromosome exchanges, which arise from two chromosome breaks either in 

 two different chromosomes, yielding interchanges, or in different arms of 

 the same chromosome, yielding large intrachanges. In general, only the 



