Final answer:
The distance between two linked genes is inversely related to the frequency of crossing over; the further apart the genes, the more likely crossing over will occur.
Step-by-step explanation:
The relationship between the distance between two linked genes and the frequency of crossing over is an inverse relationship (b). This means that as the distance between two linked genes on a chromosome increases, the frequency of crossing over between them also increases. Geneticists use recombination frequency as a measure of genetic distance to create genetic maps. When two genes are close together, they are less likely to undergo crossing over, and thus their alleles are usually transmitted together during meiosis. However, as these genes are separated further apart, crossover events become more likely, resulting in a higher proportion of nonparental gametes, which helps to map their precise locations on chromosomes.