Thank you to Certified Genealogist Melissa Johnson for her deep dive into genetic genealogy. With the commercialization of DNA testing for health and genealogical purposes, more and more people are sharing their DNA, which can be a vital tool when trying to break through genealogical roadblocks. While there is much we can learn from DNA testing, it is not the golden egg; research and documentation is often required in order to make sense of matches found through DNA testing. So let’s take a look at the world of genetic genealogy and how it can help us in our quest to unlock our ancestors.
Why DNA?
DNA testing is a new and fast growing way to help trace hereditary lineages through the use of a variety of DNA tests. DNA can help confirm or deny pedigrees, hypothesized relationships, or other genealogical theories or conclusions. DNA testing has revolutionized how individuals can address issues of adoption where records are missing or sealed. Furthermore, DNA testing can uncover living relatives for us to connect with.
However, even with all of the information that DNA testing can provide, it does not serve as proof in determining ancestors. It can help support and uncover new avenues as well as fill in gaps, but it cannot with 100% certainty identify who is related to you in a specific capacity; that still requires good, old fashioned genealogical research through documents.
Types of Testing
There are 3 main types of DNA testing that are used for genealogical purposes, each with their own advantages and uses.
Y-Chromosome
The Y-chromosome test only examines the Y-chromosome, genetic material that is passed down from father to son. In general, the DNA in the Y-chromosome changes very little from generation to generation, but changes that do occur can often indicate mutations that then can be traced to a specific family group. The Y-chromosome tests create match lists that use a genetic distance value to determine how close other matches are to you. In general, a close match has a genetic distance of 0-3, but there can be outliers both within and outside that range; a reason why documentation is still very necessary. Y-chromosome testing can provide clues to a male line ancestor’s identity, but cannot easily distinguish men from the same family, such as brothers, or a father or son. Y-chromosome testing is best when used to help confirm or support an idea rather than “fishing” for matches.
Mitochondrial
Mitochondrial testing examines the X-chromosome, genetic material that is passed down from a mother to the child, male or female; however, males will not pass mitochondrial DNA to their children. Mitochondrial tests have the same practical elements and benefits of Y-chromosome testing and can also provide clues to a female line ancestry, but cannot distinguish women within the same family, such as sisters or a mother and daughter.
Autosomal
Autosomal DNA testing is the most common and what we generally think about when we do a DNA test – look at these people I’m related to! Autosomal DNA is passed down from both sets of parents and contains elements of DNA passed down through previous generations, making it a great resource in trying to find extended family and members of different family lines. It is those patterns in our ancestral DNA that the testing companies utilize to generate our match lists, often telling us how many centimorgans we have in common with others. The following chart from DNA Painter provides a nice visual to showcase how those centimorgans translate into potential relatives:
Autosomal DNA testing allows us to look at the bigger picture by giving us access to all of our family lines, generally up to 6 generations.
Methodologies for Problem Solving
Now that we have our DNA results and all of those “matches,” we have to follow up on it. First, try to think about research problems or roadblocks. DNA tests are great when used trying to focus on a specific research question and not every question needs a large DNA component. Several key matches can help add to an existing strong body of documentary evidence. The amount of evidence needed depends on how close the DNA matches are and what the other non-genetic evidence suggests. The following methodologies can help you navigate the complexities of DNA test results and help guide your journey to finding the correct individuals and family lines.
Shared Matches or “In-Common-With”
When you receive your DNA test results, they are often accompanied by “shared matches” or “in-common-with” results, showing possible DNA connections with others who have taken the test. Generally, these lists can show maternal or paternal side matches, moving back each generation. However, caution is advised because you may be related to a DNA match in more than one way and attributing shared matches to a certain family line should be done with care and proper documentary evidence.
Centimorgans and Context Evaluation
As referenced above, shared centimorgans can help predict relationships, even multiple times removed. Best practice would be to use this data in context with other information you may already know, such as location (is this the right family line?) and age, especially for removed relationships. Other considerations when looking at shared centimorgans include endogamy (people sharing DNA from the same area, possibly due to intermarrying) and pedigree collapses, where family lines die out.
Targeted Testing
While the databases of DNA testing companies are growing every day, its possible that there may not be good matches for you because not enough people in your family lines have been tested. It can be advantageous to undertake targeting testing by asking known individuals of a family line to take a DNA test to help confirm those lines, rule out relationships, and unlock other potential matches.
Triangulation
A more dated technique when using DNA data, triangulation occurs when 3 or more DNA matches share an overlapping segment of DNA, which strongly indicates that DNA segment was inherited from a common ancestor. Triangulation does not mean that if Person A overlaps with Person B, and Person B overlaps with Person C, that Person A and C must have some overlapping DNA; far from it. In reality, more data can be obtained by using shared matches than from individuals who triangulate.
X-DNA
This methodology specifically looks at analyzing data from the X-chromosome. Since the X-chromosome can only be passed on through the maternal line, it can help you eliminate family lines associated with some male ancestors. When using X-DNA, the match must be of a significant size to eliminate specific lines of ancestry from consideration. Inversely, the absence of X-DNA, with the exception of close relationships, cannot be used as evidence to prove relationships on a specific ancestral line.
More Information
Melissa provides case studies that help illustrate how to apply different types of testing and methodologies so please view the recording on our YouTube channel at https://youtu.be/koQ-Q5W_nJI or download a copy of the handout at https://www.njstatelib.org/wp-content/uploads/2023/08/Diving-Deeper-into-Genetic-Genealogy-Handout.pdf. If you have any questions, please contact Melissa at mjohnson@johnsongenealogyservices.com.
