However, these maps are still somewhat incomplete. Thus, looking for a gene is a difficult task and often takes years to accomplish. Searching for genes that cause a specific disorder is somewhat like trying to find a street on a city map that lists the city's major landmarks, but not the streets. Just as gas stations or restaurants can be used as landmarks when locating a friend's house, scientists use markers to find a gene. These bases are the critical chemicals from which DNA is made.
Scientists can track which marker came from a person's mother and which came from a person's father.
Scientists have maps of the markers on each chromosome, just like people have maps that tell them where streets are. These maps have been developed by scientists all over the world. Each year this map becomes more dense, providing researchers with more markers to test when looking for genes that cause disorders. In fact, there are so many markers on the genetic map that the scientists' ability to find genes responsible for disorders has progressed rapidly. Scientists test many different markers on all the chromosomes, trying to find markers that are consistently found in family members who have a particular disorder, but not in family members without the disorder.