What is Linkage and recombination frequency?

Two genes on the same chromosome are linked: their alleles tend to travel together into the gametes, and a dihybrid $AB/ab$ makes more parental gametes ($AB$, $ab$) than recombinant ones ($Ab$, $aB$). Recombinants arise only when a crossover falls between the two loci, so their frequency $r$ — counted directly in a test cross — measures the distance: $r$ ranges from 0 (complete linkage) to $\tfrac12$ (genes so far apart, or on different chromosomes, that they assort independently). The map distance unit is the centimorgan (cM): $1\,\mathrm{cM}$ is $1\,\%$ recombination for closely linked genes; in humans $1\,\mathrm{cM}$ is about a million base pairs. Linkage was the first proof that genes lie in a line on the chromosome.