Five different strains of Salmonella (1, 2, 3, 4, 5) which can utilize lactose (Lac+) as the sole carbon source but cannot synthesize arginine (Arg-) are mixed with other strains (6, 7, 8, 9, 10) that cannot utilize lactose (Lac-) and can make arginine (Arg+). These strains are mixed in all possible combinations and plated on appropriate plates to get Lac+ Arg+ recombinants. The following results were obtained, where H represents 'high numbers of recombinants', L refers to 'low numbers of recombinants' and O represents 'no recombinants'.

On the basis of these results, the sex type (either Hfr, F+ or F-) of each of these strains was assigned.

• A. Strains 2, 3, 6, 7 are F-
• B. Strains 2, 3, 5,6, 7, 9 are F-
• C. Strains 1, 4, 8, 10 are F+
• D. Strains 1, 4, 8, 10 are Hfr

Which one of the following options represents a combination of all correct statements?

Correct option is B

The following symbols are used for recombination frequencies:

• H represents "high recombination."
• L represents "low recombination."
• O represents "no recombination."

The high recombination (H) suggests that the strains are likely F+ or Hfr strains, capable of donating genetic material. Low recombination (L) suggests that the strains are genetically similar and are F- strains (cannot donate the F factor). No recombination (O) means that there is no genetic exchange between the strains.

Statement Analysis:

1. Statement A: "Strains 2, 3, 6, 7 are F-"

   • F- strains cannot donate the F factor and typically show low or no recombination with other strains.
   • From the recombination table:
     • Strains 2, 3, 6, and 7 show low (L) or no (O) recombination, indicating that they do not carry the F factor, meaning they are F- strains.
   • Statement A is correct.

2. Statement B: "Strains 2, 3, 5, 6, 7, 9 are F-"

   • Strain 5 shows high recombination (H) with strain 1, suggesting it is an F+ strain and not F-.
   • Therefore, Statement B is incorrect.

3. Statement C: "Strains 1, 4, 8, 10 are F+"

   • F+ strains can donate the F factor during conjugation but do not have the F factor integrated into the chromosome as Hfr strains do. The recombination data (H and L) suggests that strains 1, 4, 8, and 10 can transfer the F factor, which aligns with F+.
   • However, Statement C is incorrect because Strains 1, 4, 8, and 10 actually show high recombination rates (H), which indicate that they are likely Hfr strains (high-frequency recombination), where the F factor is integrated into the chromosome. This makes them more efficient in transferring both the F factor and additional genes during conjugation.

4. Statement D: "Strains 1, 4, 8, 10 are Hfr"

   • Hfr strains have the F factor integrated into their chromosome, leading to high-frequency recombination. Strains 1, 4, 8, and 10 exhibit high recombination (H) with the other strains, indicating that they are Hfr strains.
   • Statement D is correct.