BACTERIOPHAGE GENETICS 363 



damage which causes marker elimination and the nature of 

 the rescue process. The hits are damages of small dimensions 

 located on the linkage structure. Markers hit directly cannot 

 be rescued. Markers which are not directly hit may be res- 

 cued during matings with liv^e phage as a result of recombina- 

 tions occurring between the marker and the nearest hit on either 

 side. The probability of such recombinations decreases with 

 decreasing distance from the marker to the nearest hit, and 

 hence decreases with dose. The matings between the irradiated 

 phage and the viable vegetative population occur with essen- 

 tially normal mating kinetics. This accounts for the observa- 

 tion that for ultraviolet doses sufficiently high to ensure only 

 one rescue event in a given infected cell, the frequency distribu- 

 tion of the partial burst size of phage carrying the rescued 

 marker is like that for recombinants between very close markers 

 in normal crosses. 



Rescue of markers from phage killed by the decay of incorpor- 

 ated P32 also occurs (Stent, 1953; Stahl, 1956). In detail the 

 effect of P^2 differs in a number of ways from that of ultraviolet 

 light. The genetic consequences of a single P'^- disintegration 

 are greater than that of a single ultraviolet damage; whereas an 

 ultraviolet hit is likely to eliminate only those markers rather 

 closely linked to it, a single P^^ decay may prevent the contribu- 

 tion even of markers unlinked to the decay. The kinetics of 

 the interaction with the viable vegetative population are also 

 different for the two agents. Whereas rescue from ultraviolet 

 damage occurs with essentially normal mating kinetics, the 

 rescue from P^^-inactivated phage occurs immediately fol- 

 lowing infection about as often as it does at any later generation 

 of the vegetative population. 



d. Effects of Ultraviolet Light on Genetic Recombination 



Jacob and Wollman (1955) discovered an interesting effect 

 of radiation on genetic recombination. They studied genetic 

 recombination between different mutants of the temperate 

 phage lambda and found that if the parental phages were ex- 



