Final answer:
Gene distance underestimation between widely separated genes occurs due to double-crossovers in genetic mapping, which result in an apparent decrease in recombination frequency and thus inaccurately reflect the true genetic distance.
Step-by-step explanation:
Recombination between widely separated genes underestimates the gene distance due to double-crossovers. In genetic mapping, the further apart two genes are, the more likely a crossover event will happen between them during meiosis. Measuring the frequency of nonparental gametes (recombination frequency) provides an estimate of the genetic distance between two genes on a chromosome. However, when genes are far apart, multiple crossovers can occur, which can result in progeny that display parental-type genotypes instead of the expected recombinant types. This situation leads to an underestimation of the true distance between the genes because the double-crossovers mask the true recombination frequency.
A classic example used to demonstrate this concept is the Sturtevant genetic map, where the geneticist Alfred Sturtevant utilized recombination frequencies to map genes on chromosomes. Sturtevant's measure unit, the centimorgan (cM), correlates with recombination frequency, where 1% recombination is equal to 1 cM. While effective for closer genes, as the genetic distance approaches a 0.50 recombination frequency, prediction accuracy decreases, which can make it unclear if genes are very far apart or on different chromosomes. This phenomenon makes the accurate calculation of distances between widely separated genes on the same chromosome problematic.