95. The following figure represents a physical map and a genetic map for 5 different genes (a to e)

Which one of the following statements based on the above is correct?
(1) The region between b and c is more recombinogenic than the other loci.
(2) In comparison to the region between a and b, the region between d and e is more recombinogenic.
(3) 1 CM is equal to 1 Kb
(4) If more markers were mapped between d and e. the genetic distance between d and e is likely to decrease

The correct option is (2) In comparison to the region between a and b, the region between d and e is more recombinogenic.

Introduction

In linkage mapping, genetic distance in centimorgans (cM) reflects how often recombination occurs between loci, whereas physical distance in kilobases (kb) measures actual DNA length in base pairs. Because recombination is not uniform along chromosomes, two intervals with the same kb distance can show very different cM values, revealing recombination hot spots and cold spots. The given MCQ compares such intervals to identify which region is more recombinogenic and to test conceptual understanding of the cM–kb relationship.​

Interpreting the figure

• Physical map (top):

  • a–b = 10 kb, b–c = 20 kb, c–d = 10 kb, d–e = 10 kb.

• Genetic map (bottom):

  • a–b = 9 cM, b–c = 2 cM, c–d = 11 cM, d–e = 20 cM.

Genetic distance in cM is proportional to recombination frequency: 1 cM ≈ 1% recombination between two loci in a large testcross population. Higher cM at similar kb therefore indicates a region with more crossing over (recombination hot spot), whereas low cM at similar kb indicates a cold spot.​

Option (1) – Region between b and c

Statement: “The region between b and c is more recombinogenic than the other loci.”

• Physical distance b–c (20 kb) is the largest interval on the physical map, but its genetic distance is only 2 cM, which is the smallest among all intervals.

• Since recombination frequency is directly reflected by cM values, 2 cM indicates fewer crossovers than in the a–b (9 cM), c–d (11 cM), or d–e (20 cM) intervals.​

Therefore, the b–c region is actually least recombinogenic; this statement is incorrect.

Option (2) – Region between d and e vs a and b

Statement: “In comparison to the region between a and b, the region between d and e is more recombinogenic.”

• a–b: 10 kb corresponds to 9 cM.

• d–e: 10 kb corresponds to 20 cM.

• Physical length is the same (10 kb), so any difference in cM must be due to different recombination rates, not DNA length.​

• Because 20 cM represents about 20% recombination and 9 cM about 9% recombination, the d–e region experiences more than double the crossover frequency compared with a–b, identifying it as more recombinogenic.​

Hence, option (2) is correct.

Option (3) – “1 cM is equal to 1 kb”

Statement: “1 cM is equal to 1 Kb.”

• A centimorgan is a genetic unit; it measures recombination probability, not an exact physical length.​

• Average kb per cM varies widely with species and chromosomal region: for example, around 150 kb per cM in Arabidopsis and about 200–1000 kb per cM in many plant and animal genomes, with large deviations in hot and cold regions.​

• Therefore, 1 cM ≠ 1 kb; there is no universal fixed conversion between cM and kb.

So option (3) is incorrect.

Option (4) – Effect of adding markers between d and e

Statement: “If more markers were mapped between d and e, the genetic distance between d and e is likely to decrease.”

• Genetic map distance between two flanking markers equals the sum of distances between all markers within that interval; adding more internal markers usually divides the interval into smaller segments whose distances sum to approximately the original length, or can slightly increase total map length when more crossovers are detected.​

• Modern high‑density linkage mapping often shows that increasing marker density tends to maintain or slightly expand total map distance rather than shrink it.​

• Therefore, introducing additional markers between d and e would not systematically reduce the total genetic distance of 20 cM; at best it would redistribute it across sub‑intervals.

Thus, option (4) is incorrect.

Key takeaway

• Physical map (kb) shows real DNA length; genetic map (cM) shows recombination frequency.​

• For intervals of similar kb, the one with higher cM is more recombinogenic; here, the d–e interval (20 cM in 10 kb) is more recombinogenic than a–b (9 cM in 10 kb), making option (2) the correct answer.