MUTATION AND PHENOTYPTC VARIATION IN PHAGES 295 



The effect is best seen in the experiments of Streisinger (1 956c) 

 who studied mixed infections with specially prepared stocks of 

 T2 and T4 that differ only with respect to a single genetic locus 

 controlling the difference in specificity of adsorption in these 

 two phages. Thus T2 adsorbs to B and to B/4, but not to B/2, 

 and T4 adsorbs to B and to B/2, but not to B/4. The same locus 

 controls the difference in serological specificity by which it is 

 possible to prepare antisera neutralizing the infectivity of T2 but 

 not T4, and vice versa. This presumably means that the same 

 component in the phage tail is responsible for specificity of 

 adsorption and neutralization. 



The issue of phage from bacteria mixedly infected with these 

 special stocks of T2 and T4 contains particles of only the two 

 expected genotypes: T2, forming clear plaques on the mixed 

 indicator B+B/4, and T4, forming clear plaques on B+B/2, 

 with none forming clear plaques on both indicators. One can 

 also determine the phenotype of the individual particles, both 

 with respect to specificity of adsorption and specificity of neu- 

 tralization by antiserum. Either method reveals three pheno- 

 types: particles resembling T2, particles resembling T4, and a 

 new kind of particle that adsorbs to both B/2 and B/4, and is 

 neutralized by both anti-T2 and anti-T4 sera. More remark- 

 able still, there is practically no correlation between phenotype 

 and genotype of individual particles. This result evidently 

 means that the tail of a phage particle contains a certain number 

 of unit structures, different in T2 and T4, and that a phage 

 particle formed in a mixedly infected bacterium receives tail 

 units of either kind, as well as a third structure determining the 

 tail-specificity of its offspring, by more or less independent acts. 



The phenomenon was first observed in crosses between T2 and 

 T4 (Delbruck and Bailey, 1946; Novick and Szilard, 1951a) 

 and is also seen in mixed infections with T2 and its h mutant 

 (Hershey, Roesel, Chase, and Forman, 1951) and with lambda 

 and its h mutant (Appleyard, McGregor, and Baird, 1956). It 

 has not been observed with respect to any characters not asso- 

 ciated with specificity of adsorption. 



