290 BACTERIOPHAGES 



1. Definitions 



The term genotype refers to the genetic constitution of a phage 

 line, inherited from ancestors and transmitted to offspring. To 

 determine the genotype of a phage particle one examines a 

 progeny derived from it or, more exactly, to demonstrate a 

 genotypic difference between two phage particles one examines 

 progenies derived from each under conditions as nearly identical 

 as possible. Any constant difference in properties observed in 

 this way is said to be heritable and implies a difference in geno- 

 type. Other criteria, such as persistence under a variety of 

 conditions of growth, or genetic recombination when applicable, 

 may be used to further elucidate the difference. 



A genotypic difference arising within a single phage line can 

 be ascribed to mutation. Here further criteria can always be 

 applied. A mutation originates as a stepwise difference, pro- 

 ducing a mutant clone within the parental line. Such clones 

 should recur at random, producing a characteristic distribution 

 of clone sizes (Luria, 1951). A mutation should be identifiable 

 as a local change in genotype when analyzed by genetic recom- 

 bination (Chapter XVIII). The best studied phage mutations 

 show all these characteristics, but it is fairly certain that other 

 kinds exist (Sections 3h and 5 below) . 



The term phenotype, literally "what shows," refers to directly 

 observable characteristics, as opposed to genotype, which is a 

 somewhat more abstract concept and therefore easier to define. 

 With reference to a phage line, the phenotype corresponds to 

 the genotype (by definition), with the proviso that the phenotype 

 may vary with the conditions of observation whereas the geno- 

 type does not, but determines the phenotype under all conditions 

 (again by definition). Particular aspects of the phenotype of a 

 phage particle, therefore, may depend more on the conditions 

 under which the particle was formed than on pertinent aspects 

 of its genotype, though of course the general features of the 

 particle will always reflect its genotype. 



To sum up, the word phenotype is used to refer to observable 

 characteristics, usually a particular one, of phage particles, 



