632 RADIATION BIOLOGY 



development of smear techniques, especially for spreading and staining 

 the microspores of plants (Taylor, 1924; Kaufmann, 1927), presented the 

 opportunity — used advantageously in the work on Tradescantia — of 

 irradiating large numbers of cells in a known stage of microsporogenesis 

 and obtaining, by subsequent inspection of the stained metaphases and 

 anaphases, extensive and comprehensive data on the frequency of induc- 

 tion of fragments and various types of chromosomal rearrangements. 

 Diagrams illustrating the kinds of aberrations detected in such studies of 

 Tradescantia have been presented by Catcheside (1945, 1946, 1948); 

 Catcheside, Lea, and Thoday (1946a); and Lea (1946). 



Comparable although less extensive studies have also been made by 

 direct examination of irradiated animal cells (for example, the studies of 

 Carlson, 1938a, 1941b, on neuroblast chromosomes of the grasshopper, 

 Chortophaga) . The effects of the treatments are detectable as breaks or 

 lesions along the chromosomes, or as new associations of the breakage 

 ends (represented diagrammatically in Figs. 9-3 and 9-4). An interpreta- 

 tion of the mode of origin of the types of breaks designated in these 

 diagrams as "chromosome," "chromatid," and "isochromatid" will be 

 presented subsequently. 



From these illustrations it is apparent that cytologic examination of 

 condensed chromosomes at metaphase or anaphase will reveal all types 

 of induced aberrations, including lethal as well as viable aberrations. 

 Differentiation of the two classes is desirable in some types of analysis. 

 It is also desirable to know more about the precise location of breaks and 

 the complexity of individual rearrangements than can be inferred from 

 observations of mitotic chromosomes. Studies on Drosophila have been 

 especially useful in supplying such information. 



In the method commonly used for detection of induced chromosomal 

 rearrangements in Drosophila, gametes are irradiated by treatment of 

 males, which are then mated with untreated virgin females. Experi- 

 ments with D. melanog aster by Muller and Settles (1927) and Demerec 

 and Kaufmann (1941) have indicated that doses of X rays approaching 

 the limit of tolerance of the adult fly (ca. 5000-10,000 r) do not usually 

 inactivate the spermatozoa, which fertilize the eggs and participate in 

 zygote formation. Some of the fertilized eggs fail to hatch, death of the 

 embryos being attributable in many cases to loss or duplication of sec- 

 tions of chromosomes in early cleavage mitoses (Sonnenblick, 1940). 

 The abortive embryos constitute a class of so-called "dominant lethals," 

 whose frequency can be determined by counting the number of eggs laid 

 and the number from which larvae do not emerge. 



Larvae hatching from eggs fertilized by irradiated spermatozoa may or 

 may not carry detectable chromosomal rearrangements. The relative 

 frequencies of the two classes resulting from any given treatment can be 

 determined by either cytological or genetical techniques. The cytologic 



