National Human Genome Research Institute
National Institutes of Health
U.S. Department of Health and Human Services
- What is genetic mapping?
- How do researchers create a genetic map?
- What are genetic markers?
What is genetic mapping?
Developing new and better tools to make gene hunts faster, cheaper and practical for any scientist was a primary goal of the Human Genome Project (HGP).
One of these tools is genetic mapping, the first step in isolating a gene. Genetic mapping - also called linkage mapping - can offer firm evidence that a disease transmitted from parent to child is linked to one or more genes. It also provides clues about which chromosome contains the gene and precisely where it lies on that chromosome.
Genetic maps have been used successfully to find the single gene responsible for relatively rare inherited disorders, like cystic fibrosis and muscular dystrophy. Maps have also become useful in guiding scientists to the many genes that are believed to interact to bring about more common disorders, such as asthma, heart disease, diabetes, cancer and psychiatric conditions.
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.
What are genetic markers?
Markers themselves usually consist of DNA that does not contain a gene, however they can tell a researcher the identity of the person a DNA sample came from. This makes markers extremely valuable for tracking inheritance of traits through generations of a family, and markers have also proven useful in criminal investigations and other forensic applications.
Although there are several different types of genetic markers, the type most used on genetic maps today is known as a microsatellite map. However, maps of even higher resolution are being constructed using single-nucleotide polymorphisms, or SNPs (pronounced "snips"). Both types of markers are easy to use with automated laboratory equipment, so researchers can rapidly map a disease or trait in a large number of family members.
The development of high-resolution, easy-to-use genetic maps, coupled with the HGP's successful sequencing and physical mapping of the entire human genome, has revolutionized Genetics research. The improved quality of genetic data has reduced the time required to identify a gene from a period of years to, in many cases, a matter of months or even weeks. Genetic mapping data generated by the HGP's laboratories is freely accessible to scientists through databases maintained by the National Institutes of Health and the National Library of Medicine's National Center for Biotechnology Information (NCBI) [ncbi.nlm.nih.gov].