MAPABILITY OF VERY CLOSE MARKERS OF BACTERIOPHAGE Λ

Recombinant frequency was compared with nucleotide distance in crosses involving markers in either the P  RM or the cy region of phage λ. For each pair of markers, we performed reciprocal four-factor crosses of the following types: (I) A  +  m  1  +  m  2  -  B  - × A  -  m  1  -  m  2  +  B  +; and (II) A  +  m  1  -  m  2  +  B  - × A  -  m  1  +  m  2  -  B  +. In crosses of type I, the frequency of A  +  m  1  +  m  2  +  B  + recombinants among total (selected) A  +  B  + progeny was directly proportional to nucleotide distance between m  1 and m  2 in the range from 3 to 160 nucleotides. When less than three nucleotides separated m  1 and m  2, the measured yields of m  1  +  m  2  + recombinants were significantly depressed.