512 JOSHUA LEDERBERG 



another in the parents (as in reversed crosses) should lead to a corresponding 

 inversion in the ratios with which that allele is found in the prototrophs. The 

 tables cited show that in every case there is no agreement between the ratios 

 found in reversed crosses, unless the comparison is made with one of the ratios 

 inverted, in which case there is reasonably good agreement. This result is in 

 accord with the hypothesis that the genes in E. coli are arranged in one or more 

 linkage groups, and is in disagreement with the postulation of a diploid con- 



Table .3 

 Comparisons of IV segregations when introduced with alternative parents.* 



* See Lederberg (1947) for a statistical analysis of tables 3, 5, and 6. 



dition, or with a state of indefinite "ploidy" which would be characteristic of a 

 system of cytoplasmic inheritance. 



The results of these experiments seemed sufficiently secure that one could 

 adopt the existence of a linkage system as a working hypothesis and on this 

 foundation, an attempt has been initiated to "map" a number of markers in 

 E. coli. It was hoped at first that there might be found linkage groups which 

 would be independent of one another, so that recombination between bio- 

 chemical markers in one group could be used to detect recombinants, yet not 

 interfere with the segregations in the other group(s). There was, however, no 

 immediate prospect that these relationships could be found initially, so it was 

 decided to study linkage relationships in a single pair of mutant stocks, and 

 their derivatives. The stocks which were selected for this study were 58-161 

 (B~M~) and Y-53 (T~ L~ Br Lac~) and their IV mutants. Since Lac and Vi 

 could be so readily scored, using only a single streak from each prototroph 

 colony which appeared, it was hoped that the collection of an adequate volume 

 of data could be accomplished with greater facility than if biochemical markers 

 only were used. 



It was, however, necessary to determine the relationships of the biochemical 

 mutant loci of which at least four must be used to obtain recombinants. Mix- 

 tures were, therefore, plated into minimal medium supplemented with a single 



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