The science of DNA analysis is an amazing tool that many genealogists can benefit from in their efforts to link families together. However, like any tool, it is important to understand what a DNA analysis can and cannot reveal. The most important aspect of DNA testing, or any research project for that matter, is having an idea of what you want to determine, and defining the evidence that will prove or disprove a conclusion. With DNA testing this means selecting the right person (or people) to be tested. In some cases, testing the DNA of more than one person is the only way to solve genealogy roadblocks. In order to know whom to test, you must understand who has inherited the DNA in question.
How it Helps
1. DNA Matching—Comparing your results (your ancestral signature) against other people's DNA signatures can help you find new relatives and ancestors, or ancestors for whom there are no written records. The two primary tests for this purpose are the Y-chromosome test, and the Mitochondrial DNA sequencing. The Y-chromosome test relies on the male line while the Mitochondrial DNA sequencing needs a female line to test.
Both of these tests can be very helpful, but the Y-chromosome test has more scientific studies to support its use for genealogical purposes. While the Mitochondrial DNA sequencing is thought to be just as useful in proving relatedness, there are fewer tools and studies available to help interpret the results.
2. Proving Relationships—Proving relationships is one of the most useful aspects of DNA testing for genealogists. If you have a theory that two people might be related in your pedigree, but you have been unable to locate any documentation to prove your theory, DNA testing might be able to help. If the correct two (or more) people have their DNA analyzed, a signature match will indicate that they are closely related. A mismatch will confirm the two people are not closely related.
Exact matches of Y-chromosome tests will indicate that the two people are related within about five generations. If one or two markers on the signature are different, the two people are still related but the relationship is likely too distant to be genealogically relevant. A statistical model is used to determine the most likely number of generations between the two people.
Exact matches of Mitochondrial DNA sequencing also indicate relatedness, but there are not sufficient studies to determine the number of generations separating the two people.
3. Surname/Clan Reconstruction & Regional Migration—This is another common use of Y-chromosome tests (Y-chromosome Haplotype). Groups of families with a common or similar surname origin might want to know if (and how) they are related to one another. Individuals and groups can also discover genetic connection to others living in a country where the family is thought to have lived within a migration path. For example, a group of possibly related families with the surname of "Thomas" wants to find out if they all have a common male ancestor from whom they descended, who lived in a particular country. If a male surname representative from each family submits his DNA for the Y-chromosome test, the signatures can be compared to essentially reconstruct the tree back to a common ancestor.
Some individuals or families may have completely different signatures indicating they did not descend from the same common ancestor as the others. With a large enough sample of signatures, it is possible to tell which branch of the Thomas tree each family came from and how close that branch was to the common ancestor.
Public Access Databases
When you get your results, you can enter them at any or all of the following to see if any additional matches or other information are revealed:
YHRD: A database for the scientific community that furnishes no genealogical data, but can be used for indications of geographic origin.
YSearch: Sponsored by Family Tree DNA, but open to all regardless of which company you tested with. Can also be searched by haplotype and surname and includes useful features such as the ability to attach a GEDCOM to your results.