RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 51 



in the same functional gene placed in the trans position in a hybrid will 

 yield a phenotype which is wild, or intermediate between mutant and 

 wild. This phenomenon, called complementation, is probably due to 

 the fact that more than one step intervenes between the gene and the 

 end product measured— an enzyme, for example— and that interaction 

 between the primary products produced by the damaged genes gives 

 rise, in some instances, to a normal end product in small amounts. In- 

 vestigations of the nature of complementation in microbial systems will 

 no doubt furnish extremely valuable information concerning gene 

 structure and the mechanism of formation of specific gene products 

 (Giles, Partridge, and Nelson, 1957; Fincham and Pateman, 1957; 

 Woodward, 1959). 



Thus the gene has emerged from the status of an abstract notion 

 to that of a structure defined in terms of an extended nucleotide se- 

 quence. Since it is composed of a large number of chemically defined 

 units— the nucleotides— the complex phenomena of mutation and re- 

 combination can be harmoniously interpreted in terms of -chemical 

 subunits. 



Keeping in mind these essential concepts of the structural basis 

 of mutation and recombination, let us now consider quantitative as- 

 pects of genetic recombination. As mentioned, microbial genetics has 

 forced a radical reconsideration of the theory of the relationship be- 

 tween chromosome structure and recombination frequency. It is to this 

 aspect of recombination that the remainder of this report will be 

 devoted. 



We have seen that classical genetic theory predicts that the closer 

 two mutated sites are to each other, the more truly the recombination 

 frequency will reflect the linear distance separating them. Therefore, 

 as we place genetic markers closer and closer together, we would ex- 

 pect the observed recombination frequencies to decrease linearly as 

 soon as the distance becomes sufficiently small to render multiple cross- 

 overs very infrequent. In classical genetics, map distances usually are 

 additive when they are of the order of a few units of recombination. 

 What has been found in microbial systems, by selecting rare cross- 

 over events between two very closely linked mutated sites, is that 

 crossing-over is, in fact, very frequent on either side of the site of the 

 selected recombination event. In other words, recombination frequency 

 decreases proportionally with distance until these distances are of the 

 order of a unit of recombination or less, and then it increases instead of 

 decreasing. 



This phenomenon, called negative interference, was first clearly 

 demonstrated by Pritchard ( 1955 ) and has since been found in every 

 system in which selection of rare recombinants can be performed. In- 

 deed, using phage T4 and the extremely rich series of mutant alleles 



