200 W. Hayes 



expression in uncomplicated systems which can be reasonably 

 well synchronized. The only available systems of this kind 

 are the parasexual ones in which genes can be transferred 

 from one bacterial cell to another of different genotype, and 

 in which the subsequent behaviour of the resulting recom- 

 binant chromosome can be studied. 



There are three such systems, which differ from one another 

 mainly in the method whereby the genetic transfer is effected. 

 In each, a part of the chromosome of a donor cell can be 

 transferred to a recipient cell and then incorporated into the 

 recipient chromosome to form a recombinant chromosome. In 

 transformation, already described by Dr. Hotchkiss (this 

 symposium, p. 183) the agent of transfer is DNA extracted from 

 the donor population. In transduction (Zinder and Lederberg, 

 1952; Zinder, 1953), bacteriophage (i.e. virus) particles of low 

 virulence, derived from the donor population, act as vectors 

 of small fragments of the donor chromosome to those recipient 

 cells which they infect. The frequency with which any par- 

 ticular gene is inherited by the recipient population in trans- 

 duction is usually low (ca. 10"^). 



The third system, conjugation (Lederberg and Tatum, 

 1946; Lederberg et at., 1951; Hayes, 1953; Wollman, Jacob 

 and Hayes, 1956), which is found in Escherichia coli, differs 

 from transformation and transduction in three respects: 



(1) Genetic transfer from donor to recipient cell is effected 

 directly, by cellular fusion. 



(2) A large part of the donor chromosome, comprising 

 many linked genes, is usually transferred to the recipient cells 

 and may appear in recombinants. 



(3) When a special type of donor strain called Hfr (for 

 high frequency of recombination) (Cavalli, 1950; Hayes, 1953; 

 Wollman, Jacob and Hayes, 1956) is employed, the frequency 

 with which certain recombinant types appear may be as high 

 as 10-20 per cent of the recipient population (Hayes, 1957). 



The work described here has involved a donor Hfr strain 

 isolated by the present author which, under the experimental 

 conditions employed, transfers a specific part of its chromo- 



