In general, the frequency with which crossing over occurs between two linked genes depends on what? see concept 15.3 (page 305)

It depends on their distance of the genes.

Recombination frequency is a measure of genetic linkage but it is not a direct measure of how physically far apart genes are on chromosomes (it is only the approximation of physical distance). So, genes with a larger recombination frequency are usually farther apart, while genes with a smaller recombination frequency are usually closer.

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