Like HowStuffWorks on Facebook!

How DNA Profiling Works


Use of DNA Profiles in Genealogy
Jefferson's and Sally Hemings' great great great granddaughter, Julia Jefferson Westerinen
Kimberly Butler/Time Life Pictures/Getty Images
DNA profiles used to match ancestry linked Julia Jefferson Westerinen to the third president of the United States. Ms. Westerinen's great-great grandfather was Eston Jefferson, the son of Thomas Jefferson and Sally Hemings.

Aside from criminal trials and appeals, DNA profiling has become an important tool in genealogy. Many companies provide DNA profiling for this purpose. One of the largest ones, Family Tree DNA, uses Y-SRT testing to determine paternal lineage and mtDNA (mitochondrial DNA testing) to determine maternal lineage. When an embryo is conceived, its mitochondria -- structures within cells that convert energy from food -- come from the mother's egg cell, whereas the father's sperm contributes only nuclear DNA [source: Human Genome Project, U.S. National Library of Medicine]. For more information on mtDNA, see How DNA Evidence Works.

National DNA Database

The creation and storage of DNA profiles are also very controversial. As the databases searched by CODIS have expanded to include profiles of more than just convicted criminals, some people have begun to worry about what law enforcement, the government or even private companies may be able to do with the information. Once your profile is in a database, it can be removed only via court order. If you're using a private database for the purposes of genealogy, however, you can request the removal of your profile.

­In April 2008, the Genetic Information Discrimination Act was signed into law. It's designed to keep insurance companies and employers from discriminating against people who may be genetically predisposed to a disease. To learn more about what expansions of DNA databases might mean for the future, see How Future Crime Databases Will Work.

The profiles vary in the amount of detail they can provide and in how far back in your ancestry they can determine a match. A Y-DNA67, for example, can show an extremely close connection between ancestors. It tests the Y chromosome for genetic matches between males. A perfect match of 67 markers on each person's DNA strand means they have a common ancestor in recent history [source: Family Tree DNA]. Family Tree DNA maintains databases of people looking for ancestors, and when a match is found, both parties are notified.

Although DNA profiling can reveal ancestry, companies that specialize in them don't perform­ any kind of testing specifically to trace hereditary defects or disease. However, genetic testing, which involves more than just DNA profiling, helps reveal hereditary predispositions to some diseases and birth defects. During genetic testing, DNA is profiled and analyzed along with RNA, proteins and other factors.

­So DNA profiling can be very useful, but how accurate is it in determining a match? Family Tree DNA claims that it can determine within a "99.99 percent probability of y­es or a 100 percent probability that no relationship existed" in the case of matching with an ancestor [source: Family Tree DNA]. That seems pretty irrefutable, but DNA profiling, especially in criminal cases, isn't infallible. In the next section, we'll look at some of the controversy associated with DNA profiling.