genetic complementation problem
bacteriophage T4 has a single chromosome (approx. 200 kb in length). The chromosomes are terminally redundant,  so the genetic map is circular, even though the physical map is linear (as I recall).
A mutation in E. coli (or its viruses) that affects two adjacent protein-coding genes is said to be 'polar'. This indicates that the two genes in question form part of an operon. A strong loss-of-function mutation in the promoter for the operon will typically affect the expression of all the genes in the operon. In your example there are two cistrons, or genes, in the rII locus of T4.


It is possible that the data for this question are taken directly from the published literature. I suggest you search PubMed for papers by Seymour Benzer, from the 1950s.