For loci that are close enough together that multiple crossovers are uncommon, recombination frequency provides an estimate of their genetic map distance.

A genetic map measures relative separation by recombination, not physical distance in base pairs.

The centimorgan

One centimorgan ($\mathrm{cM}$), also called one map unit, corresponds approximately to a recombination frequency of $1%$ for a short interval:

$$1\ \mathrm{cM}\approx1%\text{ recombinant gametes}.$$

Thus if two linked loci show

$$r=0.18,$$

the simplest short-interval map estimate is

$$18\ \mathrm{cM}.$$

The chromosome can be drawn schematically as

A ------------------ B
        18 cM

The map is relative: it tells us that the interval behaves as an 18-cM recombination interval, not that the loci are a particular number of nucleotides apart.

Map distances can be combined along a chromosome

Suppose three loci have known order

$$A-B-C$$

with

$$A-B=7\ \mathrm{cM}$$

and

$$B-C=12\ \mathrm{cM}.$$

Then the genetic distance along the map from $A$ to $C$ is

$$7+12=19\ \mathrm{cM}.$$

This additivity is one reason genetic maps are useful: neighboring intervals can be assembled into a linear chromosome map.

Genetic distance is not a universal conversion to DNA length

A 10-cM interval in one genomic region need not contain the same number of base pairs as a 10-cM interval elsewhere. Crossover rates vary among chromosome regions, sexes and species.

Therefore

$$\text{genetic distance}\neq\text{physical DNA distance}.$$

Genetic distance measures the cumulative tendency for meiotic recombination across an interval.

The simple percent-to-cM rule is local

For short intervals, recombinant frequency is a good approximation to map distance because double and higher-order crossovers are rare. For long intervals, multiple crossovers can make the observed recombinant fraction smaller than the true cumulative genetic distance.

So the rule

$$\text{map distance in cM}\approx100r$$

is most reliable for sufficiently short intervals. Genetic mapping turns recombination from a complication of Mendelian inheritance into a ruler for chromosome organization.