Final answer:
The frequency of crossing over between two loci on a single chromosome is proportional to the distance between them. Geneticists use the proportion of recombinant gametes to measure the distance between genes on a chromosome.
Step-by-step explanation:
The frequency of crossing over between any two loci on a single chromosome is proportional to the distance between them. As the distance between two genes increases, the probability of crossovers between them also increases. Geneticists use the proportion of recombinant gametes as a measure of how far apart genes are on a chromosome, and this information is used to construct genetic maps.