When they are on the same chromosome, and are close enough together that they do not segregate independently, they are said to be linked.
Linkage is a powerful tool in modern genetic counseling.
In many families it allows predictions of an outcome rather than probabilities of an outcome. The human genome project now occupies a major effort of the human genetics community. This goal is supported by a vast majority of human geneticists. The second, and more controversial goal of the human genome project is to establish a complete DNA sequence for each chromosome. Linkage studies in experimental organisms quickly led to the accumulation of rather complete maps of loci on individual chromosomes.