1182 BIOLOGICAL EFFECTS OF RADIATION 



alleged compounding and fragmentation is practicable. A deficiency 

 for the spindle-fiber-attachment region of a chromosome may produce a 

 fiberless fragment which is cytologically indistinguishable from the 

 normal chromosome. Such a fragment may become attached to another 

 chromosome, thus simulating a permanent compounding of two whole 

 chromosomes. Formation of duplicating fragments including the 

 spindle attachment may increase the chromosome number (Painter and 

 MuUer, 98; Dobzhansky, 31, 33). If, then, a translocation from one 

 of the chromosomes to the duplicating fragment takes place, the resulting 

 chromosome complement may suggest a fragmentation of a chromosome 

 with a de novo formation of a spindle attachment. 



The only instance of an apparently real permanent compounding and 

 fragmentation of chromosomes is the case of the attached X-chromosomes 

 in Drosophila melanogaster. L. V. Morgan (69) and Sturtevant (133) 

 studied such spontaneous attachments of the two rod-shaped X's to form 

 a single V-shaped compound, and Anderson (4) apparently induced it by 

 X-rays. On the other hand, L. V. Morgan found that the attached. X's 

 sometimes fall apart spontaneously, and Muller and Dippel (79) increased 

 the frequency of this detachment by X-rays. The details of this phe- 

 nomenon seem to be, however, obscure at the present, since it is not 

 known whether the free X-chromosomes that emerge from the attached-X 

 compound are absolutely identical with the normal X-chromosomes. 

 The discovery of Kaufmann (56) that the spindle-fiber attachment 

 in the X-chromosome of Drosophila melanogaster is subterminal, and not 

 terminal, as was supposed previously, makes a further analysis of the 

 attached-X case especially imperative. (Such an analysis has been carried 

 through by Kaufmann (55). The detachments of the attached X's are 

 due to crossing over between the X- and F-chromosomes. Thus, the 

 attached-X case can no longer be cited as an instance of compounding 

 and fragmentation of chromosomes.) 



CYTOLOGICAL MAPS 



Chromosomal rearrangements are detected in Drosophila, as a rule, by 

 genetic methods. Linkages between genes not usually manifesting link- 

 age indicate the presence of translocations; suppression of crossing over 

 is in many cases evidence of the presence of inversions; suppression or 

 manifestation of genes sometimes shows that duplications and deficiencies 

 have arisen. Not only can the presence of chromosomal rearrange- 

 ments be detected but the details of their structure can also be studied 

 genetically. As has been shown, the loci at which chromosomes are 

 broken and reattached in translocations are determined by the same 

 methods which are used for studying the loci of new mutant genes. 

 Changes leading to gene mutations are, however, different from chro- 

 mosomal rearrangements in that the former produce no visible alterations 



