1170 BIOLOGICAL EFFECTS OF RADIATION 



nonhomologous chromosomes. Both chromosomes involved in reciprocal 

 translocations are simultaneously donors and recipients. 



2. Inversion (Sturtevant, 131). A section of a chromosome may be 

 rotated 180 deg. without removal from its place. Thus, if the order of 

 genes in the original chromosome is ABCDEFG, the chromosome carrying 

 an inversion has the order ABCFEDG, or a similar one. Inversions are 

 merely special cases of intrachromosomal translocations. 



3. Deficiency. Bridges (13) defines deficiency as "the loss or inactiva- 

 tion of an entire, definite, and measurable section of genes and framework 

 of a chromosome." The deficiencies first discovered in Drosophila 

 (Bridges, 13; Mohr, 68) were too short to be visible cytologically, and 

 for this reason Bridges mentioned the possibility of inactivation rather 

 than a loss in his definition. Painter (92) observed in mice a cytologically 

 visible deficiency, and proposed for similar cases the term "deletion" 

 (used subsequently by Painter and Muller, 98, and other authors). 

 It is obvious now that deficiencies of Bridges and of Mohr were losses 

 and not inactivations. "Deficiency" and "deletion" are synonyms. 



4. Duplication (Bridges, 14). An individual carrying a duplication 

 has a normal chromosome complement plus an extra fragment homol- 

 ogous to a part of one of the chromosomes (the duplication). The 

 fragment may be either free (in which case the cells contain an extra 

 chromosome as compared with the normal condition), or it may be 

 attached to another chromosome. 



INFLUENCE OF X-RAYS ON DISJUNCTION OF CHROMOSOMES 



Bridges found (12) that normal, untreated, females of Drosophila 

 melanogaster occasionally produce eggs carrying two, and eggs carrying 

 no X-chromosomes. The offspring coming from these exceptional eggs 

 can easily be recognized, if a recessive female is mated to a male carrying 

 sex-linked dominants. Regular offspring consist of dominant females 

 and recessive males, and the exceptional offspring are recessive females 

 {XXY, coming from XX eggs fertilized by Y spermatozoa) and dominant 

 males (having one X- and no F-chromosome, coming from no-X eggs 

 fertilized by X-sperm). The production of the exceptional eggs is mostly 

 due to nondisjunction of the X-chromosomes at the maturation divisions. 

 The fact that the XX eggs are less frequent (1 : 2500) than the no-X ones 

 (1:600) indicates, however, that some of the latter are produced by 

 occasional loss of the X-chromosomes in female gametogenesis. 



The frequency of the production of exceptional eggs in Drosophila 

 melanogaster can be increased about 20 times if females are exposed to 



locations, implied in this expression a definite hypothesis of the origin of translocations 

 (by "illegitimate" crossing over between nonhomologous chromosomes). Although 

 this hypothesis may be correct, it is by no means conclusively proved, and reflections 

 of contentious hypotheses in terminology should be avoided. 



