51 8 JOSHUA LEDERBERG 



point. The most economical hypothesis at this time, however, is that there is 

 a single unbranched chromosome as the physical basis of inheritance in E. coli. 



Attempts to Induce Aberrations 



Using a chromosomal theory as a working hypothesis, it was hoped that 

 some verification could be found by the study of types in which the normal 

 order of genes was disturbed. Since there is only one chromcsome (from the 



Table 6 



Segregation of Lac, Vi and V 6 . 



B-M-T+L+Bi+LactVfVfXB+M+r-L-BrLdc-Vi'V,' 



** See footnote to table 4. 



genetic evidence), the only types of rearrangements would be changes leading 

 to a series of inversion-transposition types. It was thought that such types 

 might be detected by genetical procedures by virtue of their effect on crossing 

 over. In particular, the occurrence of an inversion in the region Bi • ■ • [MB] 

 would be expected to have the effect of eliminating the recombination 

 classes involving interchanges in this region. In the cross B~M~T + L + Bi + 

 X B + M + T~L~Bi~ this would be equivalent to the suppression of prototroph 

 recombinants; Bi~ types, however, would be recoverable, and allow the in- 

 vestigation of the extent of the changes. 



Preliminary attempts to find such aberration types have, to date, been un- 

 successful. The procedure was as follows: 



Following treatment with nitrogen mustard (Tatum, 1946) or 20,000 r of 

 X-rays, cells of Y-40 and of Y-53 were incubated separately for 24 hours, to 

 allow the separation of cells or nuclei that might have been associated at the 

 time of treatment. The cultures were then streaked out on nutrient agar 



156 



