Since Y-chromosome DNA is found only within the all-male patrilineal line and mtDNA only provides matches to the all-female matrilineal line, DNA testing is only applicable to lines going back through two of our eight great-grandparents - our father's paternal grandfather and our mother's maternal grandmother. If you want to use DNA to determine ancestry through any of your other six great-grandparents you will need to convince an aunt, uncle, or cousin who descends directly from that ancestor through an all-male or all-female line to provide a DNA sample. Additionally, since women don't carry the Y-chromosome, their paternal male line can only be traced through the DNA of a father or brother.
DNA tests can be used by genealogists to:
- Link specific individuals - e.g. test to see whether you and a person you think may be a cousin descend from a common ancestor
- Prove or disprove the ancestry of people sharing the same last name - e.g. test to see if males carrying the CRISP surname are related to each other
- Map the genetic orgins of large population groups - e.g. test to see whether you have European or African American ancestry
What is your goal?
To best use DNA testing to learn about your ancestry you should start by narrowing down a question you are trying to answer and then select the people to test based on the question. For example, you may wish to know if the Tennessee CRISP families are related to the North Carolina CRISP families. To answer this question with DNA testing, you would then need to select several male CRISP descendants from each of the lines and compare the results of their DNA tests. A match would prove that the two lines descend from a common ancestor, though would not be able to determine which ancestor. The common ancestor could be their father, or it could be a male from over a thousand years ago. This common ancestor can be further narrowed down by testing additional people and/or additional markers.
Most Recent Common Ancestor (MRCA)
When you submit a DNA sample for testing an exact match in the results between you and another individual indicates that you share a common ancestor somewhere back in your family tree. This ancestor is referred to as your Most Recent Common Ancestor or MRCA. The results on their own will not be able to indicate who this specific ancestor is, but may be able to help you narrow it down to within a few generations.
What can I learn from my results?
An individual's DNA test provides little information on its own. It is not possible to take these numbers, plug them into a formula, and find out who your ancestors are. The marker numbers provided in your DNA test results only begin to take on genealogical significance when you compare your results with other people and population studies. If you don't have a group of potential relatives interested in pursuing DNA testing with you, your only real option is to input your DNA test results into the many DNA databases starting to spring up on the Net, in the hopes of finding a match with someone who has already been tested. Many DNA testing companies will also let you know if your DNA markers are a match with other results in their database, provided that both you and the other individual have given written permission to release these results.