Entry courtesy of
The National Human Genome Research Institute (NHGRI)

Genetic marker

A segment of DNA with an identifiable physical location on a chromosome and whose inheritance can be followed. A marker can be a gene, or it can be some 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 indirect ways of tracking the inheritance pattern of a gene that has not yet been identified, but whose approximate location is known.

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Dr. Robert Nussbaum, of the National Human Genome Research Institute's Laboratory of Genetic Disease Research, defines genetic marker.