5 22 JOSHUA LEDERBERG 



would be heterozygous, and prototrophs carrying the markers Lac+Vi and 

 Lac~Vi T could be produced. The type Lac+Vi' would not be expected unless, 

 indeed, genetic material from all three types could combine in a sort of menage 

 a trois. As recorded in table 7, no instance of such a three-way combination 

 was found in 628 tests, a different class being vacant, as anticipated, in each 

 of the four parts of the experiment. It may be concluded that genetic factors 

 from different cells are not freely miscible, as would be demanded by the most 

 economical version of the interpretation of transformations. 



From all the experiments so far cited, it must be concluded that if trans- 



Table 7 

 Pairwise occurrence of recombination in mixtures of three components. 



PARENTAL TYPES 



RECOMBINANT PROTOTROPHS ' 



B+M+T+L+B^ or Be 



B-M-T+L+Bi* B+M+T'L-Br LarVf Lac~V x ' Lac+Vf Lac+Vf total 



.. Lac + Vi r , 



Lac~Vi T t -ir. l 73 49 4 ° 226 

 Lac V 1* 



usrt £% ,6 o 7 ,8 



LacV >' Lac-Vs ° I36 37 4 ° 2I3 



L T aC+ l l ' T LacrVi' 65 48 o 25 138 



Lac v i r 



Total 628 



* Mixtures of the three types indicated in each experiment were plated into thiamin-contain 

 ing agar. The prototrophs are therefore a mixture of B{~ and B]+ types, as indicated in table 4, 

 footnote. 



forming factors are operating in this system, 'the diverse factors (or genes) are 

 not independent of one another, but are grouped in separate and immiscible 

 parcels. Such parcels would also be potentially capable of transmitting all of 

 the genetic factors of a cell, so that there seems to be no compelling reason 

 why such a parcel, speaking purely genetically, could not be regarded as a 

 gamete. Muller (1947) has interpreted the pneumococcus transformation in 

 terms of "still viable bacterial chromosomes or parts of chromosomes floating 

 free in the medium . . . these have penetrated the capsuleless bacteria and in 

 part at least, taken root there, perhaps after having undergone a kind of 

 crossing over with the chromosomes of the host." It remains to be seen whether 

 this interpretation will be upheld by further studies on factor interaction in 

 bona fide transforming systems. 



Several attempts were made to determine whether "transforming activity" 

 could be separated from the living cell under conditions comparable to the 



160 



