Conjugation experiments were performed by mixing four different auxotrophic strains (P, Q, R, and S) of E. coli in different combinations. The generation of prototrophs on appropriate minimal selective media is shown below:
Combination of strains No. of prototrophs obtained
P and Q 20
S and R 1000
P and R 0
P and S 1000
Based on the above data, the following statements were made:
A. Strains P and R are F⁻ strains.
B. Strain Q is an F⁺ strain.
C. Strain S is an Hfr strain.
D. Strains P and R are F⁺ and F⁻ strains, respectively.
E. Strains P and R are both Hfr strains.
Which one of the following options represents a combination of all correct statements?

Correct option is D

Correct Answer:
(d) A, B and C
Explanation:
High prototroph numbers (1000) indicate Hfr × F⁻ crosses (efficient recombination). Thus, S must be Hfr, and P and R must be F⁻ (from P–S and S–R results).
P and R = 0 prototrophs indicates F⁻ × F⁻, confirming A.
P and Q = 20 prototrophs suggests a low-frequency transfer, characteristic of F⁺ × F⁻, so Q is F⁺ (B).
Therefore, C is also correct, while D and E are incorrect.
Information Booster:
· Hfr strains transfer chromosomal genes at high frequency, yielding many recombinants.
· F⁺ strains transfer mainly the F plasmid, producing few recombinants.
· F⁻ × F⁻ crosses produce no recombinants.
· Recombinant counts are diagnostic of mating types in conjugation.