The term "genetic marker" is broader than a gene. A genetic marker is simply a segment of DNA with an identifiable physical location on a chromosome whose inheritance can be followed. A genetic marker can have a function and thus be a gene. Or a marker can be a section of DNA with no known function.
Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as tools for tracking the inheritance pattern of a gene that has not yet been identified but whose approximate location is known.
The statistical estimate of whether two loci are likely to lie near each other on a chromosome and are therefore likely to be inherited together is called a LOD score. A LOD score of 3 or more is generally taken to indicate that the two loci are linked and are close to one another.