BACTERIOPHAGE GENETICS 343 



The parameter m is the average number of random matings per 

 progeny phage particle and the parameter p is the probabiUty 

 of recombination between two markers per heterozygous mating. 

 Visconti and Delbriick concluded that for phage T2 under 

 conditions of normal lysis the parameter m had the value of five 

 matings per progeny phage particle. Because the matings are 

 random in respect to both partner and time, some progeny 

 phage particles may have mated only with like particles before 

 maturation. These particles which have had no opportunity 

 for recombination dilute the population in which recombination 

 has occurred, and dilute both single and double recombinants 

 by the same factor. The net effect is an apparent increase in the 

 frequency of double recombinants over that calculated from the 

 frequency of single recombinants by the classical formula. 

 The Visconti-Delbriick theory accounts quantitatively for this 

 apparent excess of double recombinants, the "low negative inter- 

 ference" of phage genetics. When the parameter p, the prob- 

 ability of recombination per heterozygous mating, is used in 

 constructing linkage maps, low negative interference is not 

 observed. 



Levinthal and Visconti (1953) tested the assumptions of the 

 Visconti-Delbriick theory by making use of the phenomenon of 

 lysis inhibition to study genetic recombination over a longer 

 period of time than is possible under conditions of normal lysis. 

 Using the closely linked markers h and rl3 of phage T2 they 

 found that the proportion of recombinants in the total yield 

 increased linearly with time for about the first hour of intra- 

 cellular multiplication and leveled out when intracellular multi- 

 plication ceased. From this observation they deduced that the 

 recombinant frequency in the vegetative pool must also increase 

 as a linear function of time. Because the parameter p, the 

 probability of recombination per heterozygous mating, is in- 

 dependent of time, the average number of matings per progeny 

 particle must increase with time. From their data Levinthal 

 and Visconti calculated that one mating per progeny particle 

 must occur every 2 minutes in the vegetative pool. Phage 



