132 



CHARLES A. THOMAS, JR. 



of two crosses, the first (Cross I) with the multiplicity of the parent 

 bearing the markers a, b, and c about 10 times that of the minority 

 parent a'b'^r*. A second cross (Cross II) was performed with exactly the 

 ftainc, but opposite multiplicities. In Fig. 7 the relationships are depicted. 



Fig. 7. The types of crosses which give a sensitive test for hnkage in phage. The 

 recombinant type indicated should be equal in the two crosses if the c marker were 

 unlinked from the linked a and b markers. The recombinant type shown in 

 Cross I was actually substantially higher than that shown in Cross II. (From Strei- 

 singer and Bruce, 1960.) 



In this figure the recombinant type is indicated (for convenience only) 

 by a copy-choice scheme. In these two crosses the progeny phage will 

 contain a certain number of phage of the recombinant type indicated. 

 If this number is divided by the total number of a 6^ recombinant types, 

 irrespective of whether they contain c or c^, the resulting ratio should 

 be identical in both crosses — if the c marker is truly unlinked. In all 

 experiments that were perfoniied in both T2 and T4, the ratio obtained 

 in Cross I was substantially higher than that obtained from Cross II — 

 a fact which indicated that the c marker is linked to the other two. 

 These conclusions are completely free of assumptions concerning either 

 the kinetics of replication or of recombination. Using this technique with 

 various markers thought to be unlinked, it was demonstrated that they 

 were all linked to each other. This unitaiy genetic map has been con- 

 finned using the many temperature-sensitive mutants which can be 

 found distributed over the entire map. The closer spacing of these 

 markers makes the sensitive test described above unnecessary (Edgar, 

 1961). 



By a continuation of the type of crosses done by Streisingcr and 

 Bruce, it was found by Streisinger, Edgar and Harrar (1961) that 

 markers previously thought to be at the extremes of the genetic map 

 were themselves linked to each other. Thus the genetic map in T2 and 

 T4 must be considered to be formally circular. This is in contrast to the 

 linear structure of the excised T2 or T4 DNA molecule. At present it 

 is not yet known whether any other smaller virulent phages have a 



