1222 



BIOLOGICAL EFFECTS OF RADIATION 



the two allelomorphs in one chromosome, instead of their being in two 

 separate chromosomes, produces a much stronger developmental effect. 

 Sturtevant accordingly suggested the hypothesis that the position of 

 genes relative to each other in the chromosome is a factor in determining 

 their developmental effects. Were this so, breakages of chromosomes 

 which change the normal constellations of genes should produce effects 

 similar to mutations. Indeed, if the hypothesis is carried to its logical 

 conclusion, it becomes very difficult to devise a criterion which would 

 distinguish a true gene mutation from a position effect. The only 

 available method at present lies in the analysis of what happens at the 

 breakage points in chromosome rearrangements. 



In the first place, it should be noted that of the relatively few "spon- 

 taneous" chromosome rearrangements, a majority show phenotypic 

 changes located at the points of rearrangement. Bridges' (10) original 

 Pale translocation in Drosophila melanog aster was detected by virtue of 

 such an effect; similarly, with the "Blond" translocation of Burkart (11) 

 and Burkart and Stern (11a); while Muller's CI B inversion (107); the 

 "Curly" complex of Ward (187) ; Sturtevant's (33) translocation II-III E 

 afford further examples of rearrangements with phenotypic changes 

 located at the points of rearrangement. 



It is in the X-ray-induced rearrangements that the phenomenon 

 becomes really striking. Muller and Altenburg (116, 117) and Dob- 

 zhansky (27, 28, 30) found that the majority of such rearrangements were 

 lethal when homozygous, or else showed characteristic mutational 

 changes. Oliver (125) made a similar observation in the X-chromosome 

 regarding the correlation of lethal effects and chromosome rearrange- 

 ments leading to a reduction of crossing over. More recently, Patterson, 

 Stone, Bedichek, and Suche (142) have supplied extensive data on the 

 relative frequency of lethal effects in the homozygotes of translocations. 



Table 7. — Tests of Translocations for Viability in Homozygous 



Condition 

 (Patterson, Stone, Bedichek, and Suche, 142) 



Expected percentane of viable and fertile 2 to 3 translocations: 17.6. 



