GenefJcs and Microbiology 33 



an example of fragmentary genetic transfer, that is, trans- 

 duction. Meanwhile, the chemical analyses by Avery and 

 his colleagues described the reagent in the vaccines as 

 principally, if not exclusively, desoxyribonucleic acid, 

 DNA (4). The genetic aspects of the transmitted fragment 

 have not been fully clarified, but in each of the several 

 examples of transduction, single genetic factors are the 

 rule, punctuated by occasional "linked transduction" of 

 two factors (20, 31, 44). As the number of factors examined 

 increases, so does the incidence of recognized linkages. 

 This suggests that the unit is a chromosome fragment, 

 rather than an absolutely delimited macromolecule, the 

 idealized "single gene." 



Transduction in at least two other bacteria. Hemophilus 

 and Neisseria^, was discovered by conscious emulation of 

 the Griffith and Avery procedures, and with some ad- 

 vantageous peculiarities, its general aspects are the same 

 as in the pneumococcus (31). In Salmonella^ on the other 

 hand, transduction was accidentally discovered in the 

 course of a fruitless search for sexuality as it occurs in E. 

 coli. In fact, too rigid insistence on the use of double 

 mutants to control the selection of recombinants nearly 

 obscured the initial discovery (49), but this emphasizes 

 the difference in mechanism. Transduction in Salmonella 

 differs from that in the pneumococcus primarily in the 

 function of temperate bacteriophage as the passive vector 

 of the hereditary fragments. 



To recapitulate, genetic transduction as much as sexual 

 fertilization is an agency of recombination but differs in 

 two principal features: morphologically, one reactant is a 

 subcellular fragment, and genetically, perhaps as a corol- 

 lary, a small fragment rather than the entire genotype is 

 all that is transmitted. In several bacteria, the fragment 

 may be transmissible after chemical extraction in essen- 

 tially native form, possibly pure DNA, but in Salmonella 

 a symbiotic phage effects the initial fragmentation of the 



