Correct option is B
Explanation -
Option b: Complete interference in crossovers
Complete interference means that no double crossovers occur in a given region — only one crossover happens at a time.
When there is complete interference, all crossover events are independent and non-overlapping. This ensures that every recombination event reflects the actual genetic distance. As a result, the distances between genes add up accurately (i.e., genetic map becomes additive).
Additivity is preserved because no recombination events are "hidden" by multiple crossovers.
So, the correct answer is option b -Complete interference in crossovers.
Incorrect options-
Option a: No interference in crossovers → Incorrect
No interference means crossovers occur randomly and independently. Double and even triple crossovers can and do occur. Double crossovers between two genes can cancel each other out — leading to underestimation of the recombination frequency. This breaks the additivity of genetic maps:
The recombination frequency between A and C may not equal the sum of A–B and B–C.
Hidden double crossovers lead to non-additive map distances.
Option c: Partial interference in crossovers → Incorrect
Partial interference means that double crossovers can happen, but their likelihood is reduced. It’s an intermediate case between complete and no interference. Since some double crossovers can still occur, the observed recombination frequencies may still fail to reflect the actual genetic distances. Therefore, additivity is still not guaranteed.
Partial interference may help but doesn't fully ensure additivity.
Option d: Variable interference depending on distance → Incorrect
The level of interference changes with genetic distance (common in real-life data). Non-uniform interference makes recombination unpredictable. Additivity cannot be assumed consistently across different gene intervals.
Variable interference breaks consistent assumptions needed for a clean additive map.
