While developing genetic maps, Alfred Sturtevant proposed that genetic distances are additive. From test crosses involving two genes, if the genetic distance between genes A and B was observed to be 15 cM and between B and C was 10 cM, then the percentage of recombinants observed between A and C would be 25, given that the arrangement is A—B—C. This will be observed only when there is:

Correct option is B

The correct answer is: (2) Complete interference.

Explanation:
Complete interference occurs when one crossover completely prevents the occurrence of another crossover in the nearby region. In this case, the genetic distances between A and B (15 cM) and between B and C (10 cM) are given. If the observed recombinants between A and C are 25% (the expected additivity of 15 cM + 10 cM = 25%), this suggests that complete interference is present.

In cases of complete interference, the number of observed double crossovers is zero (i.e., no double crossovers occur), and the expected recombination frequency is purely additive. If the observed recombination is exactly as expected (25%), it indicates that the crossovers between the genes are independent and do not affect each other due to interference, meaning the genetic distances are additive, which aligns with the concept of complete interference.

Additional Knowledge:

• No interference (option 1) would suggest that the crossovers are independent, but in this scenario, complete interference is the correct interpretation because of the observed recombination frequency.

• Negative interference (option 3) occurs when one crossover increases the likelihood of a second crossover, but it does not fit the scenario described here.

• Variable interference (option 4) is a more complex phenomenon where the degree of interference can change depending on the location or circumstances, but in this case, the situation points to complete interference.