How do researchers create a genetic map?

To produce a genetic map, researchers collect blood or tissue samples from family members where a certain disease or trait is prevalent. Using various laboratory techniques, the scientists isolate DNA from these samples and examine it for the unique patterns of bases seen only in family members who have the disease or trait. These characteristic molecular patterns are referred to as polymorphisms, or markers.

Before researchers identify the gene responsible for the disease or trait, DNA markers can tell them roughly where the gene is on the chromosome. This is possible because of a genetic process known as recombination. As eggs or sperm develop within a person's body, the 23 pairs of chromosomes within those cells exchange - or recombine - genetic material. If a particular gene is close to a DNA marker, the gene and marker will likely stay together during the recombination process, and be passed on together from parent to child. So, if each family member with a particular disease or trait also inherits a particular DNA marker, chances are high that the gene responsible for the disease lies near that marker.

The more DNA markers there are on a genetic map, the more likely it is that one will be closely linked to a disease gene - and the easier it will be for researchers to zero-in on that gene. One of the first major achievements of the HGP was to develop dense maps of markers spaced evenly across the entire collection of human DNA.

http://www.genome.gov/10000715