Intro to DNA
A bit about DNA testing:
There are two basic types of DNA tests available for genealogical testing:
Mitochondrial DNA (mtDNA) is passed down from the mother (not the father) to all of her children. Her female children will then pass it down to all of her children, and so on. The males do not pass on her mtDNA. Because the mitochondrial DNA is passed down from the mother along the female line, mtDNA testing can allow you to uncover the female line of your family tree.
mtDNA Tests - Mitochondrial DNA (mtDNA) is contained in the cytoplasm of the cell, rather than the nucleus. This type of DNA is passed by a mother to both male and female offspring without any mixing, so your mtDNA is the same as your mother's mtDNA, which is the same as her mother's mtDNA. mtDNA changes very slowly so it cannot determine close relationships as well as it can determine general relatedness. If two people have an exact match in their mtDNA, then there is a very good chance they share a common maternal ancestor, but it is hard to determine if this is a recent ancestor or one who lived hundreds of years ago. It is important to keep in mind with this test that a male's mtDNA comes only from his mother and is not passed on to his offspring.
Example: The DNA tests that identified the bodies of the Romanovs, the Russian imperial family, utilized mtDNA from a sample provided by Prince Philip, who shares the same maternal line from Queen Victoria.
Y-Chromosome DNA (yDNA) is passed down from father (not the mother) to all of his male children. The male children will then pass it down to all of his male children, and so on. yDNA testing can allow you to uncover the male line of your family tree.
Y DNA/Y Line Tests - More recently, the Y chromosome in the nuclear DNA is being used to establish family ties. The Y chromosomal DNA test (usually referred to as Y DNA or Y-Line DNA) is only available for males, since the Y chromosome is only passed down the male line from father to son. Tiny chemical markers on the Y chromosome create a distinctive pattern, known as a haplotype, that distinguishes one male lineage from another. Shared markers can indicate relatedness between two men, though not the exact degree of the relationship. Y chromosome testing is most often used by individuals with the same last name to learn if they share a common ancestor.
Example: The DNA tests supporting the probability that Thomas Jefferson fathered the last child of Sally Hemmings were based on Y-chromosome DNA samples from male descendants of Thomas Jefferson's paternal uncle, since there were no surviving male descendants from Jefferson's marriage.
Markers on both mtDNA and Y chromosome tests can also be used to determine an individual's haplogroup, a grouping of individuals with the same genetic characteristics. This test may provide you with interesting information about the deep ancestral lineage of your paternal and/or maternal lines.
What You Can and Can't Learn From DNA Testing
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 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 SYBALSKY surname are related to each other.
Map the genetic origins 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 Williams families are related to the North Virginia Williams families. To answer this question with DNA testing, you would then need to select several male Williams 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 internet, 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.
Note-Since writing this article an Autosomal DNA test has come out. This test can be done by both male and females. Autosomal DNA comes from both parents as well as smaller contributions from your grandparents, great grandparents, etc., going back through time. Think of it as a genetic record of your ancestors. What it means is that the test when compared to others who have done the Autosomal DNA test, it can determine better if you share a close or distant relative. The other thing this test can reveal is what ethic group is part of your ancestry. Note- not all ethic groups can be determined through this test. You need to really look at what you are wanting to know and then determine if this test will help you or not. Realize this test is pretty new so your test results can change as more people get tested.
Put together by Jill 2011