BACTERIOPHAGE GENETICS 347 



linkage relationship of the two markers used, because each singly 

 heterozygous particle gives rise to one parental and one recom- 

 binant segregant. Therefore if heterozygous particles were the 

 precursors of all recombinant particles, it would explain the 

 lack of correlation between the frequencies of the sister recom- 

 binant types in single cell bursts observed by Hershey and 

 Rotman (1949). Because each of six genetic loci tested con- 

 tributed 2 per cent of heterozygotes to the total yield and because 

 the heterozygous region is limited in extent relative to the total 

 phage genetic material, it follows that each phage particle in the 

 progeny is potentially heterozygous and is therefore actually 

 diploid for at least a part of its genetic substance. The sig- 

 nificance of heterozygosis as a key to the mechanism of genetic 

 recombination in phage was clearly visualized by Hershey and 

 Chase (1951), but the phenomenon was neglected for several 

 years. 



The genetic structure and function of heterozygous phage 

 particles was further considered by Levinthal (1954). He 

 proposed two variations of the structures suggested by Hershey 

 and Chase and performed three factor crosses to distinguish 

 between them. The proposed structures are shown in Figure 

 11. In model I the heterozygous phage consists of one full 

 parental chromosome plus an attached fragment from the second 

 parent to form the heterozygous region. Model H consists of 

 a chromosome fragment from each parent overlapping in the 

 heterozygous region. By using parents differing in three linked 

 markers it is possible to distinguish between the two models by 

 analyzing the segregants obtained from the heterozygous par- 

 ticles. The cross involved the parents Ar2+r7 and /i+r2r7+. 

 The characters r2 and rl are mutually epistatic which means 

 that a phage particle carrying either r factor will produce an r 

 plaque. Therefore if a heterozygote from this cross is to produce 

 a mottled plaque it must carry both r2+ and r7+ as well as one 

 of the r genes. For instance a heterozygote of the type r2+r7/r2 

 will not produce a mottled plaque because it will segregate to 

 produce rl'^rl and r2rl particles which are both pheno- 



