The loz (lethal on Z) mutants of bacteriophage X are conditional lethal mutants that can grow on E. coli strain Y but cannot grow on E. coli strain Z. The results shown in the following table were obtained when seven loz mutants were analyzed for complementation by infecting E. coli strain Z with each possible pair of mutants. A “+” indicates that progeny phage were produced in the infected cells, and a “0” indicates that no progeny phage were produced. All possible cis tests were also done, and all cis heterozygotes produced wild-type yields of progeny phage.

(a) Propose three plausible explanations for the apparently anomalous complementation behavior of loz mutant number 7. 

(b) What simple genetic experiments can be used to distinguish between the three possible explanations?
(c) Explain why specific outcomes of the proposed experiments will distinguish between the three possible explanations.

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Mutant 3 4 5 6 2. + + + o +