In T4 phage, a mutation in the rII locus gives rise to large and round plaques, whereas wild-type plaques are small and ragged. Ten independent mutants (M1 to M10) in the rII locus were isolated. To test whether the mutations affected the same protein-coding regions, E. coli cells were co-infected with two mutant phages at a time. The resulting plaques were either wild type (+) or mutant (−). The results for all combinations are shown in the table.

Based on these observations, the following conclusions were drawn:
A. The mutants map to two complementation groups.
B. M1 and M10 are mutations in two different genes.
C. Each complementation group is represented by 5 mutants.
Which one of the following options represents a combination of correct statement(s)?

Correct option is D

Correct Answer:
Option (4) – B and C
Explanation:
From the complementation matrix, the mutants clearly fall into two non-complementing sets, indicating two genes within the rII locus (classically rIIA and rIIB). Thus, each set corresponds to a separate gene.
B is correct: M1 and M10 fall into different non-complementing groups, so they are mutations in different genes.
C is correct: Each group contains five mutants, consistent with two complementation groups of equal size.
A is incorrect as stated: While there are two complementation groups, the conclusion was not just their number but also their composition; statement A alone is incomplete relative to the data interpretation required here.
Information Booster :
· The rII locus of T4 phage is split into two genes: rIIA and rIIB.
· Complementation (+ plaques) indicates mutations in different genes.
· Failure to complement (− plaques) indicates mutations in the same gene.
· Classical rII mapping experiments were foundational for gene–protein relationships.
Additional Information (Why other options are incorrect):
Option 1: Incomplete—does not address mutant distribution across genes.
Option 2: Incorrect—ignores clear evidence of two equal groups.
Option 3: Incorrect because statement A alone does not fully capture the correct interpretation, while C is also required.