29. The additive nature of a genetic map as suggested by Alfred Sturtevant and T. H. Morgan is possible if there is:
(1) no interference in crossovers.
(2) complete interference in crossovers.
(3) partial interference in crossovers.
(4) variable interference in crossovers dependent on the genetic distances.

Introduction:
The additive nature of genetic maps is a foundational concept in genetics that allows researchers to precisely determine the relative distances between genes on a chromosome. Alfred Sturtevant and T. H. Morgan pioneered this idea in the early 20th century, building on the relationship between recombination frequency and genetic distance. This article explains when the additive nature of genetic maps holds true and how crossover interference influences it.

Explanation of the Options:

1. No interference in crossovers:
   If there is no interference, crossovers occur independently and randomly along the chromosome. This allows multiple crossovers, including double and triple crossovers, which can mask actual distances as some recombination events go undetected. This breaks the additivity of the genetic map because the recombination distances between genes do not sum linearly due to hidden crossovers.

2. Complete interference in crossovers:
   Complete interference means that only one crossover occurs within a specific chromosomal region, preventing double crossovers in that area. This ensures that recombination frequency directly corresponds to genetic distance without hidden crossovers, maintaining the additivity of the genetic map. Sturtevant and Morgan’s additive genetic maps rely on this condition to accurately reflect gene distances.

3. Partial interference in crossovers:
   Partial interference allows some double crossovers but at a reduced rate. Since not all double crossovers are prevented, some recombination events remain hidden. This partial interference improves but does not guarantee full additivity of genetic maps.

4. Variable interference in crossovers dependent on genetic distances:
   Interference level often changes with genetic distance, complicating the relationship between crossover frequency and distance. Variable interference makes the recombination distances inconsistent and non-additive across different chromosome regions.

Correct Answer:
The additive nature of genetic maps, as suggested by Alfred Sturtevant and T.H. Morgan, is possible if there is complete interference in crossovers. This prevents multiple crossovers in the same region, preserving the linear and additive properties of genetic distances. Hence, the correct answer is option (2) Complete interference in crossovers.​

This principle is critical for constructing accurate linkage maps, which remain fundamental tools in genetics research and breeding programs.