RECOMBINATION IN VIRUSES AND BACTERIA 



met ■*■ bio "*" thr~ leu~ lac~ 7'1-s 



139 



met bio thr"*" leu'*' lac'*' Tl-r 



Control 

 No prototrophs 



Prototrophic 

 met '*' bio "*■ thr ■*" leu '*' 



From exchange 

 mef*" bio'*' thr~ leu~ 

 met ~ bio ~\thr'*' leu "*" 



Test for unselected markers by streaking on indicator agar 



Control 

 No prototrophs 



Parents 



Prototrophic progeny 



FIGURE 5.16. The result of plating a mixture of thr leu and met auxotrophic 

 E. coli on agar devoid of the required growth factors (after Lederberg, 1947, Genetics, 

 32:505). 



structed in which the distances are roughly additive. The frequency of 

 triple exchanges expected (0.26 x 0.47 X 0.33 = 0.04) is almost the same 

 as that observed (0.02). Even numbers of exchanges within this region 

 would not, of course, be selected by the method used. 



A similar order of genes is observed by a completely different tech- 



