MUTATION AND PHENOTYPIC VARIATION IN PHAGES 291 



whereas the word genotype denotes the constant determiner of the 

 recurring phenotype of a phage line observed under standard 

 conditions. The word mutaiion, in general, refers to any observed 

 change in genotype that occurs in an otherwise pure clone. A 

 mutation is a heritable change ; a change in phenotype may be 

 heritable or not; finally, and unfortunately, the phrase pheno- 

 typic modification means specifically a nonheritable change in 

 phenotype. 



2. Nonhereditary (Phenotypic) Variation 



Nonheritable changes in the properties of phage particles occur 

 under a variety of circumstances. Most of the known examples 

 were reviewed by Luria (1953b) in a definitive manner. The 

 several kinds may be classified as follows. 



a. Host-Induced Modifications 



Nonheritable changes in the properties of phage particles in 

 response to growth in different bacterial hosts were called host- 

 induced modifications by Luria (1953b) and host-controlled 

 variations by Bertani and Weigle (1953). All known examples 

 have many features in common but fall into two classes in one 

 respect. 



The larger class is adaptive in the sense explained by the 

 following example. A phage grown on host A attacks host A 

 with high efficiency of plating but only a small proportion of the 

 particles is able to plate on host B. However, the progeny of 

 these few particles is able to plate on host A and on host B with 

 approximately equal efficiency. Host B has brought to light an 

 adaptive modification of the phage. The result described thus 

 far does not enable one to distinguish between a host range 

 mutation and a phenotypic modification of host range. When 

 the modified phage are grown again on host A, they revert com- 

 pletely and in one growth cycle to the original host range, with a 

 low plating efficiency on host B as compared with host A. This 

 manner of reversal of phenotype suggests a phenotypic modifica- 



