37. Using interrupted mating, four Hfr strains were analysed for the sequence in which they transmitted a number of different genes to a F- strain. Each Hfr strain was found to transmit its genes in a unique order as summarized in the table (Only the first five genes were scored)

Order of transmission	Hfr strain 1         2          3           4
First	A	B	M	F
	B	A	K	H
	D	L	J	G
	F	M	G	J
Last	H	K	H	K

Which one of the following correctly represents the gene sequence in the original strain from which the Hfr strain were derived as well as the place of integration and polarity of the F plasmid?

The correct option is (3), which gives the right circular gene order and correctly shows the four Hfr origins with their polarity on the bacterial chromosome.

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Question data and gene orders

From the question, the first five genes transferred by each Hfr strain are:

• Hfr 1: A → B → D → F → H

• Hfr 2: B → A → L → M → K

• Hfr 3: M → K → J → G → H

• Hfr 4: F → H → G → J → K

In every Hfr, genes closest to the F-origin enter first and genes further away enter later, along a continuous segment of the same chromosome in one direction.

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Deducing the circular gene map

Step 1: Overlap of Hfr 1 and Hfr 4

Hfr 1 and 4 share the block A – B – D – F – H – G – J – K but in opposite directions:

• Hfr 1 segment (in transfer direction): A → B → D → F → H

• Hfr 4 segment: F → H → G → J → K

Placing them on a circle to satisfy both sequences gives a consistent stretch:

A – B – D – F – H – G – J – K (clockwise).

Hfr 1 must have its F-origin just before A (moving clockwise), while Hfr 4 has its origin just before F (moving clockwise but opposite to Hfr 1 on that shared arc). This orientation matches the arrangement shown in option (3).

Step 2: Position of M, L using Hfr 2

Hfr 2 transfers: B → A → L → M → K.

On the partially built circle A – B – D – F – H – G – J – K, the only way to see B immediately followed by A in the transfer direction is to move counter‑clockwise across the A–B segment. Thus Hfr 2 must start slightly past B (towards D) and move counter‑clockwise:

B → A → L → M → K (counter‑clockwise).

This fixes L between A and K, and M between L and K on the opposite side of the circle from F, consistent with the map in option (3).

Step 3: Confirming M and K block with Hfr 3

Hfr 3 transfers: M → K → J → G → H.

On the same circle, moving clockwise from M gives: M → K → J → G → H, exactly matching the observed order. This confirms the placement of M and K and the direction of transfer of Hfr 3, again exactly as shown in diagram (3).

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Why each option is right or wrong

Option (3) – Correct

• Shows a single circular order A – B – D – F – H – G – J – K – M – L – A (or its rotated equivalent), which accommodates all four Hfr segments with consistent directions.

• Hfr 1 origin lies just before A in the clockwise direction, producing A → B → D → F → H.

• Hfr 2 origin lies just after B but transfers counter‑clockwise, giving B → A → L → M → K.

• Hfr 3 origin lies just before M in the clockwise direction, giving M → K → J → G → H.

• Hfr 4 origin lies just before F in the clockwise direction, giving F → H → G → J → K.
  All gene orders and polarities match, so this is the only consistent consolidated map.

Option (1) – Incorrect

• The circular gene sequence in (1) places G adjacent to F and J in an order that cannot yield both A → B → D → F → H and F → H → G → J → K simultaneously without breaking continuity.

• When Hfr 2’s order B → A → L → M → K is overlaid, either B and A are not adjacent in the needed orientation or K does not lie in the correct relative position to J and M, so at least one Hfr segment becomes internally inconsistent.

Option (2) – Incorrect

• In this diagram, the relative positions of L and M around A and K do not allow B → A → L → M → K to appear as a single continuous segment in any direction on the circle.

• Trying to fit Hfr 3 (M → K → J → G → H) on this map forces a direction that then contradicts the Hfr 4 order F → H → G → J → K, so not all four strains can be explained with one chromosome.

Option (4) – Incorrect

• Option (4) typically rotates the same letters but reverses one of the crucial arcs; it places D and F so that A → B → D → F → H cannot appear without a discontinuity or reversal mid‑segment.

• Additionally, in this layout M and K are not situated to give both B → A → L → M → K and M → K → J → G → H with consistent directions, so at least one Hfr strain’s data is violated.

  Introduction

  Interrupted mating experiments with multiple Hfr strains are a classic CSIR NET genetics problem used to map bacterial genes and infer F‑plasmid integration sites. In the CSIR NET December 2018 paper, candidates had to use the first five genes transferred by four different Hfr strains to reconstruct the original circular gene sequence and identify the correct integration site and polarity of the F plasmid among four diagrams. This article walks through the logic behind the consolidated map, explains why option (3) is correct, and systematically rules out the other options to strengthen conceptual understanding and exam performance.