genetic linkage

Two passengers sitting in the same train car tend to arrive at the same station together, while passengers in different cars can end up anywhere. Genetic linkage is the same idea for genes: genes that sit close together on the same chromosome tend to be inherited as a unit, because crossing over rarely splits them apart.

Linkage is the exception to Mendel's law of independent assortment, which assumes genes are passed on independently. That law holds well for genes on different chromosomes, but genes physically near each other travel together unless a crossover happens between them. The closer two genes are, the more tightly they are linked.

The strength of linkage is measured by how often crossing over separates the two genes, giving a recombination frequency. Low recombination means tight linkage and close physical spacing; high recombination means the genes behave as if unlinked. This relationship is what makes it possible to build genetic maps from breeding data.