Imagine that your mother makes pumpkin bread. Warm, soft, delicious pumpkin bread with semi-sweet chocolate chips sprinkled liberally throughout. You have lovely memories of toasting a slice for breakfast and sitting down in front of Saturday morning cartoons. This nostalgia drives you to make some pumpkin bread for your own family. Of course, in your version, you’re going to church it up a bit. Maybe you replace the chocolate chips with hand-chopped chunks of dark chocolate. And coconut works with pumpkin, right? While you’re at it, maybe you toss some turbinado sugar on top. Perhaps you’re so proud of your little loaf that you make more and take one to your mother.
She eats a slice and smiles politely, but you can tell she prefers her version. “You know, Grandma’s pumpkin bread had no chocolate in it. I’m the one who started that,” your mother, Pumpkin Bread Pioneer, says with no uncertain amount of pride. “And you should have had Great-Grandma Johnson’s. It was dense and dry and nutty. Just like she was...” she mumbles, staring through the kitchen window at nothing in particular.
Think of your DNA like that recipe for pumpkin bread, one that’s been handed down in the family for generations. The major components are the same as the original but it's been tweaked. New bits have been added, old bits removed. The further back you go, the more likely the pumpkin bread your grandmother or great-grandmother made tasted like the original version.
Just like the recipe represents the instructions for making the pumpkin bread, so does your genetic material make up the instructions for you. Our parents pass to us half of their DNA, and thus a quarter of their DNA makes its way to our children. As DNA is inherited by each generation, the amount received from each individual ancestor is halved, and the inherited DNA is composed of smaller and smaller shared stretches or "segments." These segments of half-identical DNA shared between parents and children are the longest you share with anyone on the planet. Your children will share smaller half-identical segments with your parents—their grandparents—than you do, and so on down the line until, eventually, an individual shared segment of DNA will be small and common enough to not provide credible evidence of relation between two people.
And this is where an autosomal DNA (atDNA) test can help you. You may or may not have any immediate or direct-line relatives or ancestors who have test results at either 23andMe or AncestryDNA, but you undoubtedly have many cousins who do. If you can find or build a family tree for these matches, you will be able to either biologically confirm or newly identify ancestors. These cousins will share DNA with you stated in percentage shared and/or in centimorgans (cM), a unit of measure of the genetic distance between you and another individual. The more centimorgans you share with a match, the more closely related you are to them.¹
Degree | Expected shared DNA (cM) | Average # of cousins | Shared ancestors |
1st Cousin | 900cM | 7.5 | Grandparents |
2nd Cousin | 225cM | 38 | 1st great-grandparents |
3rd Cousin | 56cM | 170.4 | 2nd great-grandparents |
4th Cousin | 14cM | 431.5 | 3rd great-grandparents |
The number of cousins you have, will of course, vary greatly depending on family size and how recent your ancestors came to the United States. If most of your ancestors are recent immigrants (1800s or later), you will likely have fewer matches. It will also vary based on which populations are and are not taking these tests. Germans, for example, test less often due to privacy laws, and atDNA test takers with strong German backgrounds—especially those whose ancestors came to the U.S. more recently—often have fewer matches than testers with longer familial footprints in the U.S. or more varied ethnic backgrounds.
Looking at my own results, the number of individuals with whom I share a minimum of about 0.5% DNA (or about 34cM) who have tested just at 23andMe is about 456 people. I have about 600 who are 34cM+ matches at AncestryDNA, few of whom are duplicates of the 23andMe matches.² It's important to note that none of these sites know how you are related to these more distant matches. Or if they do, they aren't telling. That's where the hard work comes in. They will label these matches broadly, as 2nd/3rd/4th/etc. cousins, and their guess is just that. There are many factors that influence how much DNA you will actually share with a cousin, including the sex of each person in the line of inheritance, the specific population involved, the possibility of multiple points of relation, and randomness.
Of my 1k matches, a few I know. My mother, my aunt, a few first and second cousins. Even if I discount these super close relatives who are known to me, I still have over a thousand people to whom I am related closely enough that I can probably identify how (with a bit of research). So where do you start?
There are many different methods for sorting through your DNA matches at 23andMe or AncestryDNA, and developing a sound approach and getting started can feel overwhelming at best. Rootsbound Genealogy can help!
To sum up, an autosomal DNA test can help you with:
Automatic identification of DNA Matches who are very close relations. Parents and full siblings are more or less unmistakable given the amount of DNA they share with a tester, and atDNA test results will label DNA Matches who are parents or full siblings as such.
Identifying DNA Matches who are 1st-4th cousins. Again, these sites are only going to approximate what degree of cousin they are, and they aren't going to tell you how you're related to them. That's up to you to research and determine.
Using your DNA Matches to identify your ancestors, up through about 5-6 generations (e.g., up to the tester’s 3rd great-grandparents).
General approximations of your ethnic background. Remember, this is based solely on what DNA you happened to inherit from your ancestors. Your results can vary from your siblings, and just because your brother has, say, more Ashkenazi Jewish ancestry evident in his DNA than you (according to the test site), this doesn't make him "more Jewish" than you. "Heritage" and "ethnicity" do not mean the same thing. Further, these ethnicity approximations are just that—approximations. For example, none of the major sites are able to make a clear delineation between German and French ethnicity. They are constantly updating their algorithms and shifting ethnic categorizations and percentages for testers.
An autosomal DNA test is unlikely to help you:
Identify unknown living siblings if they haven't tested, particularly if you are young. Meaning, if you know your biological parents, but you're looking for an unknown sibling/half-sibling, and they haven't tested at any of the major sites, your matches aren't going to help you. The vast majority of your matches are going to be cousins, and mostly cousins of the 2nd degree or further beyond. The only way to find unknown living relatives via an autosomal DNA test is for those individuals (or their children/grandchildren) to have tested.
Identify ancestors further back than 3rd- or 4th great-grandparents. We can pretty reliably use 4th cousins or closer to confirm biological relationships to 3rd great-grandparents or closer. We may be able to use several 5th cousins to confirm 4th great-grandparents. Maybe. But any further back, atDNA is not going to cut it. Y-DNA or Mitochondrial DNA (mtDNA) may help you, but that's a topic for another day.
¹ More or less. There are other factors that can influence this, particularly in populations where families frequently intermarry.
² In contrast, a client with strong and fairly recent German roots has only 100 or so matches at AncestryDNA who share 34cM+ with them.
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