Three Things I Learned About DNA at WDYTYA Live

Last week, during Who Do You Think You Are? Live! in London, I attended a DNA workshop about Autosomal DNA. Autosomal DNA is the DNA on your non-sex chromomes (22 pairs). I have done autosomal testing with FamilyTreeDNA and 23andme and am always looking for information that helps me to interpret the results. Here are three things that I learned.

View of the floor

Who Do You Think You Are? Live! 2014

1. You need much larger segments than I thought

If your DNA overlaps with another person who tested, you can see how long your matching segments are. Very short segments are usually just a coincidence (called Identical By State, or IBS) while longer segments are indicative of a common ancestor (Identical By Descent, or IBD).  Chromosomes are measured in centimorgans (cM). I had previously heard that 7 cM was the length of shared DNA that indicated a common ancestor.

During the lecture I learned that at 7 cM, the chance that you inherited the DNA from a common ancestor (IBD) is only 30%; there is a 70% chance that the match is just a coincidence (IBS). At 8 cM the chances of having a common ancestor increase to 50/50. It is only at 10 cM that the chance that the match indicates a common ancestor gets over 99%.

Most of my matches are in the 7-10 cM range. Some of them made no sense to me, like a match that is 100% Polish while I’m 99% Dutch. With this new knowledge, I realize that my matching him was probably just a coincidence. I will focus my efforts on the few matches I have that are at 9 cM or above so there is an 80% or higher chance of us having a shared ancestor from whom we both inherited the same segment.

2. The relationship may be much more distant than predicted

When you receive your results, each match will not only show the length of DNA you share, but also have a predicted relationship. I knew that this was just an estimate, based on statistics. During the presentation I learned that a predicted 5th cousin could in reality be a 3rd or 4th cousin (15% chance) or a 6th to 8th cousin (50% chance). There’s even a 5% chance that the relationship is a 9th cousin or more distant! Only in 30% of the cases, will a predicted 5th cousin really be a 5th cousin.

Being 9th cousins means you share an ancestor in generation 11 if you count yourself as generation 1. At generation 11, I have found about 80% of my ancestors. Unfortunately, most of my matches have many more mystery lines. Most have hardly traced any lines back that far. If our common ancestor is in generation 11, there is only a small chance that this ancestor will already be in both our trees. This will make it hard to find the common ancestor although it may be possible to find a shared place of origin. At least that will give us some idea whether it is plausible that we share an ancestor further back.

fan chart

My pedigree chart (first 10 generations). Possible X-DNA lines in color.

3. How to interpret matches that don’t match your parent

My mother and I both took an autosomal DNA-test and naturally share many matches. There are also many people who match me but not my mother. The obvious conclusion would be that they would match my father, who has not been tested. Since my father’s ancestors all come from the same region (the eastern part of the Achterhoek in Gelderland), and I have no mystery lines on his side in the first ten generations, that would mean that these matches would have to have ancestors from the Achterhoek. This was a very closed and endogamous community where hardly any outsiders settled since the Middle Ages. Thousands of people emigrated to the US from this region in the 19th century, so it is not a surprise that I would have many matches from my father’s side.

However, I have found several people who match me and not my mom but who do not have any ancestors that could have come from the Achterhoek. Some of them will come from the British Isles, some will have early New Netherland roots but nothing more recent. I wondered if there could be a fluke, that these people did match my mother after all but did not show up because of some no-calls (missing data points in the results). She has some cousins emigrating to New Netherland in the 17th century. I asked a question about this, and the presenter explained that if the match shares a large segment (>15 cM) with me, than not matching my mother definitely means the match is on my father’s side.

Looking again at these unexplained matches, I noticed that most of them are nowhere near the 15 cM range, but in the 7-10 cM range. I now think they might be Identical By State, rather than actual cousins (see the first point).

I really enjoyed the presentation and believe it will save me from a lot of wild goose chases. Thank you FamilyTreeDNA, for sponsoring these workshops!

About Yvette Hoitink

Yvette Hoitink is a professional genealogist in the Netherlands. She has been doing genealogy for almost 25 years. Her expertise is helping people from across the world find their ancestors in the Netherlands. Read about Yvette's professional genealogy services.


  1. Thank you for this interesting and helpful report. With my three best matches, the ‘longest block’ is 32cM, 30cM and 27cM. The person who is the best match cannot grasp the concept of autosomal DNA. He said we ‘cannot be related because our Y-DNA haplogroups are different’, and he won’t discuss it (sigh). The second-best match was persuaded to do the test but is not interested and refuses to allow ancestral surnames to be revealed to other matches (sigh – again). The 27cM match, with a suggested relationship of 3rd cousin, turned out to be a 2nd cousin once removed, confirmed with documentary evidence. I have many matches with a longest block over 20cM, but they are in the USA and have not yet identified the homeland of their emigrant ancestor. I think DNA will be more useful to me when more family historians in Britain and Australia get tested!

    • Hi Judy,
      Great to hear that you’ve found this post helpful. Your matches are way better than mine! Apart from my mother, my best match is at 15 cM. I only have a handful above 10 cM. But isn’t it frustrating if people won’t share information? I’ve found that happens more on 23andMe, where most people joined for health purposes, than on FamilyTreeDNA. There, one of my best matches was a pioneer in DNA for genealogy. Unfortunately, he’s also dead and his account is not administered anymore. There are also very few people in the Netherlands who have tested which probably explains my lack of solid matches.

      • I have had so much fun with the matches to my DNA at I have met two fourth cousins with common Winterswijk ancestors. One woman descends from a Willink/Warnshuis couple and still lives in Clymer, NY, and the other man descends from a Grevink/Nijenhuis couple. I don’t know where he lives. He contacted me to tell me my Grevinks were wrong in my tree. 🙂

        • Ooh, the infamous Warnshuis family from Clymer 🙂 You can read more about them in two articles: Hendrik Jan Warnshuis (a.k.a. the ‘bad brother’) and Jan Hendrik Warnshuis (a.k.a. the ‘good brother’).
          There were several Grevink and dozens of Nijenhuis (‘new house’) farms near Winterswijk so it easy to get them mixed up.

          • Thank you for those articles! They are so interesting. My fourth cousin I mentioned and I descend from Warnshuis cousins of those two men.

  2. Guys..I hate this pre occupation our american cousins have with DNA testing.. the need for ancestors.. whilst it may be useful to prove who your parents are it is far to sweeping to prove your ancestry.. by its nature it covers hundreds of thousands of years wheres most people only want the past few hundred. It doe snot and cannot tell you who you ancestors are or were. Accept that this sudden demand came about because most people cannot trace there ancestry back beyond 1840.. which ofcourse puts the mockers on these companies financial models which are based on you paying them a monthly fee for ever…

    • I am not American but I am interested in DNA testing. Of course the analysis of remote ancestry is no more than suggestive but that is not the reason for interest. The matches tell you far more than who your parents are. In the generations of our grandparents and earlier many sisters, brothers and cousins emigrated from England Scotland and other parts of zUK and Europr to USA, Canada, Australia and other countries. It is interesting to make contact, locate the family link, and be able to send old photos and gravestone images. There also many branches in all parts of UK itself with which contact may have been lost but have photos of common ancestors.

  3. Kathy Lisowski says

    I did my DNA test through ancestry. I uploaded it to gedmatch. There are a lot of neat projects that they are involved with. There is one where eye color is predicted. It predicted my eye color pretty close. There is also one that matches your DNA to the archaic DNA samples found around the world. I have a lot of matches from Siberia. I know I have some ancestors from Winterswijk. Do you have your results on Ancestry? Thanks for sharing your insights.

  4. For the segment length are you talking about the longest segment or the total shared cm? I have a match I’m working with where the total is 31 cm and the longest is 12 cm. It sounds like there is a good chance we actually share an ancestor?

    • That was the length of an individual segment. A segment of 7 cM has a 30% chance of being identical by descent. A 12 cM segment would definitely indicate shared ancestry, but segments that short have an approximately 88% chance of inheriting intact, so the shared ancestor could easily be ten generations back or more. Unless you both have a very complete tree, it is unlikely that you will find the common ancestor.

  5. Conundrum

    In 10 generations theoretically we have over 1000 ancestors! turn it on its head and ask how many descendants are there roaming the earth of these thousand ancestors. But are they family?

    Then we have the problem of Endagamy, which would show thousands of non existant ancestors..

    Concluding I still believe the whole DNA business has been created by the genealogy companys to compensate for the fact that few people can go back beyond 4 generations (ask WDYTYA)

    Reality is how relevant is the fact that you share a dna segment with 1000s of other people.. family they are not

    • I don’t understand your remark that few people can go back beyond four generations. I have traced most of my lines back ten to twelve generations. For the tenth generation, I have found about 800 of the 1,024 ancestors. I share DNA with many of their descendants. Endogamy does not show thousands of non-existant ancestors, it just makes cousins seem closer than they really are. I share DNA with some seventh through tenth cousins that appear as third to fifth cousins because of endogamy. We should always educate ourselves and use our own judgement to interpret the results.

      • You are a very lucky person.. if you can go back 10 generations and get the details of 800 ancestors that is min blowing you should very poud. The harsh reality most of us is 4 generations if we are lucky, in my case one line I could go back simply because of the Bevis Marks Records.

        As to my comment about thousands of living descendants I was not Joking it does raise the question’what do we mean by familty’

        On a final note the DutchRecords are amazing but I did find errors on some connections.

        • Every record can have mistakes. But many Dutch records were created near the time of the event, with eye witness information, and have been well preserved. BUt any record in isolation does not prove anything, since even if the error rate is very low, you can only know that the record is not one of the erroneous ones by comparing and contrasting it with other records. Because we have so many good records, it is usually not difficult to find records to prove the case.

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