GENETIC RECOMBINATION AND HETEROZYGOSIS 

 IN BACTERIOPHAGE 



A. D. Hershey and Martha Chase 



Department of Genetics, Carnegie Institution of Washington, 

 Cold Spring Harbor, New York 



In this paper we summarize the principal features of inheritance in the bacterio- 

 phage T2H, and describe some new experiments. 



The genetic structure of this virus has been analyzed in terms of mutational 

 patterns (Hershey, 1946) and by recombination tests (Hershey and Rotman, 

 1949). These two types of evidence agree in showing that mutational changes 

 occur in localized regions of a complex genetic system. Mutations producing 

 different effects usually occur at different loci, but one example of multiple 

 allelism has been found (Hershey and Davidson, 1951). In this instance, the 

 locus of the alternative mutations could be analyzed rather completely because 

 most of the host-range mutations selected in a particular way proved to belong to 

 a single allelic series. It was found that one pair of distinct mutants satisfied 

 all three criteria of allelism listed below, and that another pair satisfied none of 

 them. The criteria used were the following: 



(1) If the second of two successive mutations from wild type occur at the locus 

 of the first, reversion to wild-type in a single step is possible. 



(2) No genetic recombination can be observed between allelic mutant pairs. 



(3) The map position of the locus is independent of its allelic state. 



BiPARENTAL RECOMBINATION 



The production of new genetic types of phage by intracellular interaction be- 

 tween different bacteriophages was first observed by Delbruck and Bailey (1946), 

 who mentioned genetic recombination as one of two possible interpretations of 

 their result. The principle of genetic recombination was established by experi- 

 ments with genetically defined stocks of the bacteriophage T2H (Hershey and 

 Rotman, 1948, 1949). 



The main facts of genetic recombination in this bacterial virus can be illus- 

 trated by examples of the interaction between two classes of mutant. Rapidly 

 lysing (r) mutants are easily recognized by inspection of the plaques they produce 

 on an agar plate seeded with sensitive bacteria. The plaques are larger, and have 

 a sharper margin, than those of the wild-type. Host range {h) mutants are able to 

 infect a suitable bacterial "indicator" strain that is resistant to the wild-type 



Reprinted by permission of the authors and the Long Island 



Biological Association from Cold Spring Harbor Symposia on 



Quantitative Biology, 16, 471-479 (1951). 



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