In a conjugation experiment between bacterial Hfr strain ‘X’ and F⁻ cell, lac gene enters the recipient in 4 minutes, but the F⁻ cells remain auxotrophic for Leu, Trp, Ura, Glu, Phe, and Gly. The mating is then allowed to proceed for 20 minutes and lac⁺ exconjugants are selected. Of the lac⁺ cells:

35% are leu⁺

98% are trp⁺

10% are ura⁺

65% are glu⁺

0% are phe⁺

81% are gly⁺

Select the correct order of the genes as they enter, from the choices given below:

Correct option is D

Explanation -

From the question -

lac⁺ enters at 4 minutes.
Mating continues for 20 minutes after that → Total 24 minutes of gene transfer.
Percentages of lac⁺ cells with each gene:
trp⁺: 98%        gly⁺: 81%                  glu⁺: 65%              leu⁺: 35%               ura⁺: 10%                      phe⁺: 0%

Interpretation:
These percentages are among lac⁺ cells, meaning genes that entered before lac are present in all lac⁺ exconjugants.
Genes that entered after lac will be in fewer lac⁺ cells, depending on how far after lac they entered.
Thus, the higher the percentage, the closer to lac (but still before it).
Hence, lac⁺ is not the last gene to enter — it entered at 4 minutes. The other genes (some of them) entered before lac.
Order Based on % (genes entering before lac will be present in nearly all lac⁺ cells):
trp⁺ (98%) → Entered just before lac
gly⁺ (81%)
glu⁺ (65%)
leu⁺ (35%)
ura⁺ (10%)
phe⁺ (0%) → Not transferred within 24 minutes
So the gene entry order is:
lac⁺ → trp⁺ → gly⁺ → glu⁺ → leu⁺ → ura⁺ → phe⁺
Since the lac⁺ gene enters at 4 minutes and others follow, lac⁺ is the reference point for selection, and thus, should appear first in the order for interpreting conjugation timing.

So, the correct answer is option d - lac⁺ → trp⁺ → gly⁺ → glu⁺ → leu⁺ → ura⁺ → phe⁺
This is because -

It  starts with lac⁺, which is consistent with selection
                    It follows decreasing order of appearance among lac⁺ exconjugants
                    It shows correct interpretation of mapping based on Hfr transfer timing