Can DNA prove your hypothesis?
I love using DNA to help my genealogy! But let’s talk about how (autosomal) DNA works when I’m trying to find/prove the parents of a brick wall ancestor.
For example, my theory is that my 4x-great-grandfather Walter Griffith is the brother of John and Charles Griffith, two of the confirmed-by-probate-records sons of Hezekiah Griffith. See image 1.
If I have a matching DNA segment with John Belford Griffith’s descendant (let’s call him Cousin Bob), does that prove my theory that my ancestor Walter was John’s brother and Hezekiah’s son?
No. It is one piece of evidence that’s worth adding to my collection of evidence. Just like I consider the fact that my Walter named his oldest son Hezekiah one piece of evidence supporting my hypothesis. But that DNA match isn’t *proof*. Why not?
- How complete are our trees? Hezekiah Griffith is just one of Cousin Bob’s 128 5x-great-grandparents. Likewise, that unnamed fill-in-the-blank space in my pedigree chart is one of 128 slots in that generation. Bob doesn’t have all the other names filled in on his family tree back that far, and neither do I. So that DNA segment we match on might have come down to us from some other shared ancestor we don’t know yet; maybe not a Griffith at all. For a very insightful discussion of this, see Blaine Bettinger’s blog post How Much of Your Family Tree Do You Know and Why Does That Matter?
- How big is the match? Segments under 10cM are what Blaine Bettinger calls the “danger zone”, too likely to be a “false positive” to rely on, especially without other confirmed matches on the same segment. See his blog post The Danger of Distant Matches. What if the matching segment is 10 cM? Or 20? The ISOGG Wiki explains that “fewer than 40% of 10 mB segments are within the last 10 generations. Larger segments can still date back quite some time and it was found that around 40% of 20 mB segments date back beyond 10 generations.” (Check out that ISOGG Wiki article here, citing Speed and Balding’s study Relatedness in the post-genomic era, published in Nature Reviews Genetics, 16:33-34.) So a single segment match 20cM or less might come from a Most Recent Common Ancestor farther back than we hope.
- How accurate is my tree, and how accurate is my match’s? I know my tree has had mistakes in the past. A death certificate gave me the names of parents, so I put them in my pedigree chart, only to discover later that the death certificate was wrong. I may discover more glitches in my tree in the future. I may even learn that someone in my tree is wrong because of an unknown adoption. As for Cousin Bob? Is his line of descent back to Hezekiah Griffith solidly documented? Or did he copy from other people’s trees or unsourced family histories? Maybe I have to do more research on his line for him.
SO… if matching someone doesn’t prove who our common ancestor is, why bother with DNA? (And believe me, I definitely DO bother with DNA.)
Most of the time, I’m not counting on DNA to *prove* kinship when I don’t have strong documentary evidence. I’m hoping that DNA will connect me to new cousins who either have documents or artifacts I don’t, or their family trees will give me new leads to look for evidence.
I’ve matched someone with a family Bible that gave me the birthplace of my great-great-grandmother, which led to my identifying her parents and adding a new generation to my pedigree chart. Yay! On another line, a cousin had a marriage record I needed. I’ve matched kin whose ancestors came from a specific area in Ireland, helping me pinpoint where our common line originated. I’ve met cousins who were able to identify people in my old family photos. I’ve connected to distant relatives who shared pictures of my ancestors, photos I’d never seen.
DNA has revealed unknown family stories too. A young widow in the 1830s gave her little boy to her brother-in-law’s family to raise. The boy’s surname didn’t change, but DNA from his biological mother, passed down to today’s generation, suggests what really happened.
Adding DNA to my genealogy toolkit has definitely added value to my quest to discover more about my family history. But a single match doesn’t prove who the common ancestor is.
All this is not to say that we can’t use DNA to *prove* a question of kinship. It’s just not as simple as finding we match someone who is descended from the person we think we’re descended from. Experienced genetic genealogists have constructed convincing proof arguments with thoroughly exhaustive documentary research supported by DNA results with *multiple* match pairs and analysis that may include concepts like triangulation. Check out recent issues of the National Genealogical Society Quarterly for examples. It can be done.
But when we first discover a DNA match to someone whose family tree has the person we *think* we might be descended from? Don’t pop that champagne cork just yet. It is worth celebrating as a new piece of evidence. The way we’d rejoice if we just discovered in city directories or census records or tax files that our known ancestor was a neighbor of the man we think was his father. Similar to a find like that, a DNA match can make our kinship case a little stronger.
But it’s not case closed.
Ann Raymont © 2017