136 CHARLES A. THOMAS, JR. 



By employing tlie s^ucrosc gradient sedimentation technique, it was 

 actually possible to physically separate the unbroken molecules bearing 

 the h and i^ genes from the broken inolccules whicii can transfoi-in the 

 i^ gene only. This is shown in Fig. 9. 



If extensions of this kind of exiu'rinient can be made to the otiier 

 genes in lambda, and to other genetic systems, the results may prove 

 what everyone has hoped: that the map sequence and the nucleotide 

 sequence is the same, and that genetic structure is a simple manifesta- 

 tion of physical structure. 



4. The Structure of Heterozygous Phage Particles 



When a bacterial cell is infected with two bacteriophage T2, one of 

 which is a mutant and the other wild type, approximately 2% of the 

 resulting progeny will contain both alleles. These heterozygote particles 

 were discovered and analyzed by Hershey and Chase (1951). In general 

 these heterozygotes (HETS) will give rise to both types of phage on 

 further growth, and this is the way in which they are detected. So far 

 as is known it is possible to find particles which are heterozygous for 

 any marker tested, although the r (rapid lysing) marker is very con- 

 venient because HETS will produce plaques containing both r and r^ 

 phage. These plaques have a characteristic mottled appearance. Pairs of 

 markers which are closely linked are often heterozygous for both 

 markers simultaneously, while an unlinked marker shows only a 

 slightly higher than usual (2%) chance of also being heterozygous. 

 Further work showed that phage which were heterozygous for a central 

 marker were in general recombinant for markers on either side of the 

 heterozygous region (Levinthal, 1954; Trautner, 1958), a fact which 

 indicated that these structures could play an important part in the 

 formation of recombinants. These findings were extended by Edgar 

 (1957), who was able to demonstrate that there was a higher probability 

 of recombination between closely linked markers when cells were 

 infected with heterozygotes (for these markers) than was the case for 

 inixedly infected bacteria. Perhaps the most important of Edgar's finding 

 was that the heterozygote particle gave rise to a distribution of recom- 

 binants which suggested that the heterozygote persisted or multipled 

 within the infected bacteria in such a way as to produce recombinant 

 particles at any time during the latent period. Thus heterozygotes were 

 pictured as a more permanent feature of the phage DNA, and not as 

 structures which were obliged to segregate on replication. These findings 

 led to the investigation of the fine structure of the heterozygous region 

 by Doermann and Boehner (1962), who developed techniques to test for 

 six different closely linked r (rll) nuitations and a distant tu marker. 



