The plaque morphology of wild-type and rII mutants of T4 bacteriophage following infection of different E. coli strains is summarized below: 

T4 Phage Strain	E. coli Strain B	E. coli Strain K
Wild type	Small and ragged	Small and ragged
rll mutants	Large and round	No plaques

​The following two experiments were carried out:

Correct option is B

• Wild-type T4 phages can grow on both E. coli B and K, forming small and ragged plaques.
• rII mutants:
  • Can infect E. coli B, producing large, round plaques.
  • Cannot infect E. coli K, meaning no plaques are formed.

Analysis of the Experiments

Experiment I:

• Co-infection of two independent rII mutants in E. coli K resulted in several plaques (all small and ragged).
• Since rII mutants alone cannot grow on E. coli K, the only explanation for the appearance of plaques is recombination.
• This suggests that at least some phages had wild-type recombinants, allowing them to grow on E. coli K.

Experiment II:

• E. coli B was co-infected with the same rII mutants.
• Phages from the resulting plaques (which would be a mix of wild-type and mutants) were used to infect E. coli K.
• Only a few plaques formed, all small and ragged, which indicates that:
  • Recombination occurred in E. coli B to produce some wild-type phage, which could then infect E. coli K.

Evaluation of the Statements

(A) Experiment I indicates that the two mutants are allelic.  (Incorrect)

• Allelic mutants mean that the mutations are in the same gene, so recombination would not restore wild-type function.
• However, plaques formed on E. coli K, which suggests that recombination did restore wild-type function, meaning the mutations were likely in different genes (not allelic).
  

(B) Experiment II indicates that the wild-type T4 phages that infected E. coli K resulted from a recombination event. (Correct)

• The appearance of a few small and ragged plaques on E. coli K suggests that some wild-type phage were recovered due to recombination between the rII mutants in E. coli B.
  

(C) In Experiment II, if the T4 phage isolated from E. coli B was used to infect E. coli B, all plaques would be large and round. (Incorrect)

• If recombination occurred, then the phage population would be a mix of wild-type and rII mutants.
• Wild-type phages would form small and ragged plaques, while rII mutants would form large and round plaques.
• Since a mixture of plaques would be observed (not all large and round), this statement is incorrect.
  

Information Booster

Key Concepts from This Experiment:

1. Plaque Morphology Indicates Phage Genotype:

   • Wild-type T4 phages produce small and ragged plaques.
   • rII mutants produce large and round plaques on E. coli B but *cannot grow on E. coli K (no plaques).

2. Recombination Restores Wild-Type Function:

   • If two different rII mutants infect the same cell, crossing over can restore the wild-type gene sequence, allowing growth in E. coli K.

3. Complementation vs. Recombination:

   • Complementation occurs when two mutations are in different genes and the combined gene products restore function in the same cell.
   • Recombination restores wild-type function by crossing over between mutations.

4. Interpretation of Experimental Results:

   • The presence of plaques on E. coli K suggests recombination occurred, producing wild-type phages.
   • This confirms Statement B and refutes Statement A (which assumes allelic mutations).