SYLLABUS log 



T2r means a mutant giving larger plaques than the wild type, 

 with a clear halo. 



T2T7 means a seventh distinct mutant of this type. 



T2r7ri3 means a phage known to carry the r mutation in the two 

 indicated loci. (Obtained by a suitable genetic cross.) 



T2hr7 means a phage carrying both the h and the rj mutation. 



If it is desired to indicate specifically that a phage does not carry 

 a mutation in a given locus, this is indicated by adding to the symbol 

 of the strain the symbol of the mutation with a superscript +, thus, 

 T2h+ri3 is a mutant which carries the wild type allele at the h locus 

 and the mutant allele at the ri3 locus. 



There are several wild type strains of T2 in use which are distin- 

 guished by capital letters. The principal ones are T2H and T2L. 



Another class of mutations, "biochemical mutations," has been 

 noted (Anderson, 1948; Delbriick, 1948). The mutational patterns 

 have not yet been worked out. They have also not yet been used system- 

 atically in recombination studies. 



The idea of distinct "genes," or "loci," on "genetic sites" in phages 

 originated with Hershey's studies of the mutational pattern of T2 

 (Hershey, i946a,b). Hershey utilized several mutations besides those 

 listed above and demonstrated that each of these mutational steps occurs 

 independently of the state of the phage with respect to other mutations. 



9. Taxonomy. — The principal index of relatedness between 

 phages is their serological cross reaction. By this criterion the seven 

 phages of the T series fall into four groups, as follows: 



Ti; T3, T7; T5; and T2, T4, T6 (the even numbered phages). 

 Phages in different groups do not cross react at all. Within a group 

 they do cross react (see 7). Within each group, the phages are mor- 

 phologically alike, phages belonging to different groups are morpho- 

 logically distinct (see Table I, p. 147). 



Contrary to earlier beliefs, host range is not a useful taxonomic 

 feature. Thus, T3 and T4 have a very similar host range, although 

 they are unrelated by every other index. On the other hand, T2 and 

 T2h, which differ in host range, are obviously very closely related. 



Plaque type is of taxonomic value if applied cautiously. Generally 

 speaking, small phages give large plaques, but plaque sizes are affected 

 also by mutations. 



