1172 BIOLOGICAL EFFECTS OF RADIATION 



of treated flies rather numerous mosaic individuals which had a part of 

 their bodies monosomic for the fourth chromosome. It is very probable 

 that not only the X's and the fourth chromosomes but also the large 

 autosomes of Drosophila undergo nondisjunction and elimination due to 

 X-ray treatment. The detection of these processes is, however, made 

 difficult by the fact that monosomies and polysemies for these autosomes 

 are inviable even in mosaics. Some of the "dominant lethals" induced 

 by X-ray treatment (MuUer, 72) are presumably just such monosomies 

 and polysomics (Schultz, 115). 



METHODS OF DETECTION AND OF STUDYING CHROMOSOMAL 



REARRANGEMENTS 



The first translocations which were observed in Drosophila arose 

 spontaneously (Bridges, 16; Bridges and Morgan, 19; Stern, 126, 127). 

 The induction of translocations by X-rays was discovered by Muller (72, 

 73) and Muller and Altenburg (77, 78). Their results were soon corrob- 

 orated by findings of Weinstein (138), Serebrovsky and his collaborators 

 (120), and other authors. 



The technique of finding induced translocations is based on the results 

 of Bridges (16) who has shown that translocations produce linkages 

 between genes located in different chromosomes, which, therefore, are 

 normally independent. This technique was described by Muller and 

 Altenburg (78) and by Dobzhansky (24, 26). Males having chromo- 

 somes "marked" by dominant genes are treated with X-rays and crossed 

 to untreated females^ homozygous for the corresponding recessives 

 (Fig. 1). In the Fi generation males that show the characteristics of the 

 marking genes are selected and back-crossed to unrelated recessive 

 females. The offspring of such back-crosses consists normally of several 

 classes representing all possible combinations of the marking genes (in 

 the case shown in Fig. 1 four classes, AB, ah, Ah, and aB are expected), 

 all classes being equal in frequency. Males carrying translocations 

 produce, however, some gametes and zygotes having deficiencies and 

 duplications for certain sections of the chromosomes involved in the 

 translocation. As shown in Fig. 1, these deficiency and duplication 

 zygotes are exactly those which carry the recombinations of the marking 

 genes {Ah and aB). Since, at least in Drosophila, deficiencies and 

 duplications either possess special somatic characteristics not present 

 in normal flies, or are altogether inviable, the recombination classes Ah 

 and aB are either visibly abnormal or completely absent. 



In an experiment of the writer (Dobzhansky 24, 26) males carrying 

 the dominants Bristle {Bl, second chromosome) and Dichaete {D, third 



' Males rather than females are used because they are able to withstand a stronger 

 irradiation without becoming completely sterilized. It is usually advantageous to 

 treat wild-type males and to cross them to females having their chromosomes marked 

 by mutant genes. The experimental procedure is similar in both cases. 



