An identifiable region of DNA that is at a known location on a chromosome. Genetic markers are used to help scientists locate where a gene is on a chromosome. Scientists who are looking for genes that cause a specific disease or trait can look at the DNA of family members with the disease and compare it with the DNA of family members without the disease. If they find that certain genetic markers are present in the group with the disease, then the scientists can narrow their search to the area surrounding that marker.