Final answer:
The most accurate gene mapping occurs when genes are close together because the incidence of undetectable double crossovers is lower, reducing errors in estimating genetic distances based on recombination frequency.
Step-by-step explanation:
Mapping accuracy for genes is most impacted by the distance between them on a chromosome. The correct answer to the student's question is B: Double crossover events yield a result that looks the same as no crossover in a two gene mapping experiment and this throws off the calculations. When genes are close together on a chromosome, they are less likely to be separated by crossovers, leading to more accurate mapping of their positions relative to each other. However, as the distance between genes increases, the probability of crossovers goes up. This increases the chance of double crossovers, which may not be detectable, as they can restore the original order of the alleles, making it seem as though no crossing over occurred. This phenomenon can lead to an underestimation of the true genetic distance between genes.
On the other hand, single crossovers are easier to detect because they result in nonparental, or recombinant, genotypes. This makes the mapping of closely spaced genes more precise because single crossovers are the predominant type of event that can be consistently observed and quantified, resulting in more accurate estimations of recombination frequency and therefore genetic distance.