BACTERIOPHAGE GENETICS 299 



In trying to assess the role of the heterozygous particles in phage recom- 

 bination, one additional experimental finding must be taken into account. 

 If lysis is delayed, a very large burst of phage is produced and in tliis burst 

 the frequency of recombinants for closely linked markers increases at ap- 

 proximately the same rate as does the burst size. However, during this 

 increase the frequency of mottled plaques remains michanged (Hershey and 

 Chase, 1951; Levinthal and Visconti, 1953), suggesting that, during the 

 additional growth of phage, heterozygous particles are being lost by segrega- 

 tion into their two daughter types at the same rate at which they are being 

 formed. If the heterozygous particles multiply at the same rate as the rest of 

 the population, then one can show (Levinthal, 1954) that when they segre- 

 gate they produce most of the recombinants which arise in a cross. This is 

 seen most directly by considering the two particles in a hundred which 

 produce mottling for rl3. One-quarter of these are pure with respect to the 

 closely linked h marker and produce the combinations hr 13 and hr 13+ or 

 A"*" r 13 and h+ r 13^ when they grow in a new bacterium. Thus, the two re- 

 combinant types h+ r 13 and hr 13+ together comprise one-eighth of the 

 progeny of the r 13 mottlers. This is simply a restatement of the experimental 

 findings w^th regard to the character of certain mottled plaques; however, if 

 we assume that the heterozygous particles of this same type are being formed 

 and are segregating continuously in the vegetative pool, then they would be 

 a major source of recombinants. For any one generation in the pool this 

 source would add approximately one-quarter of 1 % (one-eighth of 2 %) to 

 the fraction of recombinants between h and r 13, and the several generations 

 in a normal T2 growth cycle would then be sufiicient to account for all the 

 1*5 % of the progeny which are recombinants for these two markers. The 

 weakest point of this analysis is the assumption that the heterozygotes are 

 formed and segregate in the mating pool. One alternative is that they 

 represent some kmd of aberration connected with the maturation process. 

 However, if this were the case, it would have to be a very cormnon aberration, 

 since most of the progeny phage contain at least one heterozygous region 

 somewhere in the genetic map. So far it has not been possible to construct a 

 plausible theory as to how these particles could be formed during the matura- 

 tion. 



C. Radioactive Tracer Experiments 



In addition to producing reciprocal recombinants in the mating event, the 

 cross-over model also requires the disruption of the particles which enter the 

 mating. The parental sequence of genetic markers no longer exists on a single 

 structure after the crossover has occurred, and likewise the physical structure 

 which contains the genetic information no longer exists as a single entity. 



