AncestryDNA, Longest Shared Segment and Timber

Diahan Southard

Share with a friend: 

AncestryDNA now reports longest shared segment with DNA matches—yay! There’s some confusion about longest shared segment v. total shared DNA. Here’s how their Timber algorithm affects these, and why it’s good for your family tree.

Most of the time, I leave the difficult task of naming things to others. (I didn’t even name my own children or pets.) Naming is just not one of my talents. So I appreciate it in others. And let me tell you, AncestryDNA nailed the name of the tool they use to filter their matching DNA segments. It’s called Timber. It is so appropriate because essentially they are chopping down all the DNA segments that don’t help you find a common ancestor with your DNA match.

AncestryDNA and the Timber algorithm

Think of your shared DNA with your cousin Jim as a forest, in which each tree represents a segment of DNA. Some segments are giant sequoias and others are just little saplings. But Ancestry doesn’t just look at the segments you are sharing with Jim. It can see these segments across the entire database. So if 892 other people have that exact same segment or “tree in their forest,” it is not likely to have come from a recent common ancestor between you and Jim. It could just mean that you both have Irish ancestry, for example. Ancestry uses Timber to chop down that all-too-common Irish segment/tree so we can get a better view of your familial relationships within recent generations.

Most of the segments they remove are small saplings, but according to this table, the Timber algorithm filters out pieces of DNA up to 50 cM:

This table also helps us better understand why we shouldn’t trust small segments of DNA, as, according to the table, the proportion of segments under 10 cM that are filtered out is very, very high. For example, 95% of the time, segments that are 5-6 cM long are shared by hundreds of other people, and therefore do not represent a connection to a single recent common ancestor. (Ancestry’s recent white paper explores this more, if you’re interested.)

Timber and Longest Shared Segment Update

While I have always appreciated the role of Timber in ancestry’s DNA matching algorithm, it is causing some confusion in the newest AncestryDNA match update. In this recent update, we finally get some long-anticipated information: the size of the biggest piece of shared DNA. This is SO HELPFUL when trying to determine if a match is worth your time or not, especially if you have any endogamous relationships in your family.

However, many people are finding that they have a largest shared segment that is actually bigger than the total amount of shared DNA.

(My adorable daughter Eleanor, whom I didn’t name.)

(My adorable daughter Eleanor, whom I didn’t name.)

How does this make any sense?

Well, it all comes down to Timber. Here’s the very general process your DNA sample goes through when it arrives at the lab:

  1. Your DNA is processed (they figure out what values you have)
  2. Phasing (they separate your DNA jumble into two camps- one from mom and one from dad)
  3. DNA matching with the entire database
  4. Identify the longest segment with each DNA match
  5. Timber to remove segments that are not likely to be from a recent common ancestor
  6. Estimate your relationship with your DNA match

So, you see, if your total amount of shared DNA is lower than your longest segment, part of that longest piece must have been shared by hundreds of other AncestryDNA members, and thus, was chopped off by Timber.

Now I get it Eleanor 11 (1).png

Don’t let the learning end here!

We know the world of DNA testing and genetic genealogy can be a lot to step into, that’s why we’ve put together this guide on finding birth relatives using DNA. Check it out and see what you can learn about your family!

Show me that free guide!

Get More DNA Inspiration

Our free monthly newsletter delivers more great articles right to you.

<a href="https://www.yourdnaguide.com/author/guideyourdnaguide-com" target="_self">Diahan Southard</a>

Diahan Southard

As founder and CEO of Your DNA Guide, Diahan Southard has been teaching people how to find family history answers in their DNA for several years, and she's been in the genetic genealogy field since its infancy. Diahan teaches internationally, writes for popular magazines, consults with leading testing companies, is author of Your DNA Guide–The Book, and producer of Your DNA Guide–the Academy, an online learning experience.

52 Comments

  1. Leslie Sawyer

    My closest match is a half-aunt that I didn’t know I had. We shared 997cM / 41 segments. Ancestry have now reduced the segments to 32, with the longest being 84cM. I wish they had a chromosome browser. This is now even more nesessary than before.

    Reply
    • Diahan Southard

      Leslie, remember that before they were over-estimating the number of segments due to the way their process worked. So it is natural that the total segment number is reduced. But you still have all of the important things, like the total amount of shared DNA.

      Reply
  2. Christopher Schuetz

    The above process ends up cutting out some of my 3C matches of people who live just up the road. While leaving a very large number of people all of whose ancestors arrived in Colonial USA before 1680, with Common Ancestors born before 1600. These Early Colonial matches are 20-30cM and around 9-12C. With Shared Matches cutting out at 20cM, and there being far, far more descendants of people from back then compared with someone born in the mid-1800s, this is probably just a natural statistical quirk. But other early groups are excluded by Timber.
    Some people who come from those groups would love to be able to toggle Timber off so they could explore those origins.
    And I would love my Early Colonial matches to research which county/town in Britain their ancestors came from. Maybe how to do that could be the topic of a future blog? Just some links would be a help.

    Reply
    • Diahan Southard

      Hey Chris. Yes, Timber isn’t perfect. Do you think it is possible that you have a closer connection with some of these matches than the Colonial times? Even just back at the 5th cousin level you have 32 4X great grandparent couples. So perhaps it is possible that you share one of those couples, as well as a colonial couple (which most people have on their tree just because those lines are often better researched).

      Reply
  3. Tiffany

    I have a DNA match who shares 138cM with me, the longest segment being 85cM. However, this match only shares 9cM with my mother and doesn’t match my father at all. We all have tested with Ancestry.
    How does this happen?

    Reply
    • Diahan Southard

      Tiffany, YIKES! This is very interesting. Part of the matching process at Ancestry is called Phasing. It is where they try to determine which pieces of DNA came from the same side of your family – like which came from your dad and which from your mom. So what may have happened is an error in phasing which created a new 85cM piece of DNA that is a cobbled together mix of your dad’s DNA and your mom’s DNA. But, that is a pretty big error. Let me contact my friend on the science team at Ancestry and see what he comes up with.

      Reply
      • Tiffany

        Hi Diahan

        No need to contact anyone. It was a communication error between my parents and I. This match actually shares 155cM with my mother, which makes much more sense.
        This match also appears in Thru Lines via my potential 5th great grandparents, along with 9 other matches who I share 6-19cM. Only one of these matches is a shared match with my 138cM match. Does this mean I’m related to this 138cM match in another way?
        Thanks

        Reply
  4. Judith Broadribb

    Hi Diahan
    My Dad has an unknown 1st cousin match on Ancestry at 1051 cmgs, 29 segments, the largest segment 143 cmgs. I match this person at 418 cmgs, 10 segments, largest 96cmgs.My father’s 1st once removed cousin matches the person 471 cmgs, 20 segments, largest 66 cmgs. At first I attributed this person as an MPE of my fathers uncle, but now that Ancestry has reported that the largest segment is 143 cmgs, is it possible this person is a half sibling of my Dad? My Dad and this NPE are very close in age.
    Attended your talk at RootsTech recently boought your amazing book
    Many thanks
    Judith Broadribb
    Judith

    Reply
    • Diahan

      Judith, this is more complicated that it looks (maybe I don’t need to tell you that!). Please email me this same information, and include how much DNA your dad is sharing with his first cousin once removed that you mentioned. My email is guide@yourDNAguide.com.

      Reply
  5. Jay Martin

    I share 1249 cms across 29 segments with the largest being 117cm. Ancestry has her listed as a first cousin but I believe she could be a half sister . Is that possible with such low CMs and is it possible that dna info on GEDmatch could give other I do that would help answer this ?

    Reply
    • Diahan Southard

      Jay, yes, with that amount of DNA a first cousin relationship is more likely, but a half-sibling is still absolutely possible. If you click on the total amount of shared DNA at Ancestry, it will bring up a table of relationships and their likelihood based on the shared DNA. There are ways we may be able to think about this. Shoot me an email and let’s discuss. guide@yourDNAguide.com.

      Reply
  6. Carolyn

    I share 489 cM 17 across 69 longest segment. Ancestry shows possible relationship 1stc 1Xr or ½1st C. I know this is from my father side (deceased no DNA).
    My father’s sister daughter is only my half first cousin. Found out form DNA Her and I match 437 cM across 20 43 longest segment. She is not related to the above person. We are related by our grandmother. My Grandfather is not my Dad’s bio father. Some how the first person I listed is from my Dad’s bio side. Person 1 niece and I share only 8cM across 1 longest 8. Large family and rumor not all person 1 siblings are from the same father. Also this is Endogamy – Acadian. I have so many matches. Confusing because of endogamy. still lots of fun….LOL!

    Reply
    • Diahan

      Interesting! Yes, it does sound like you have found a connection to your biological grandfather. I think a key would be to try to figure out how old this match is compared to you. I they are about your age, the half 1C seems more likely, but if they are much younger or older than you, the 1C1R would be more likely. You have a great match there holding lots of potential answers!

      Reply
  7. Lorraine

    Hello i share 1,723 CM across 32 segments with the longest segment being 217CM. We are 6 years apart in age. My next closest match is his cousin with 886 CM’s across 31 segments with the longest segment 78CM’s. My match of 1,723 i believe is my half brother but his father is one of a twin. Could he be my first cousin or is the match too high.

    Reply
    • Diahan

      Hi Lorraine. At 1723 you are very likely half siblings. However, if the father is an identical twin, there isn’t a way to genetically determine if your father was him or his twin brother.

      Reply
  8. Marv

    Hello-

    I am getting this w/ someone who I do not know. She was adopted. I am thinking it is either my cousins kid, my dads secret kids kid or my Grandpa’s secret kids kid? I don’t really see any other options.

    Thoughts?

    Shared DNA: 699 cM across 23 segments
    Unweighted shared DNA: 699 cM
    Longest segment: 124 cM

    Reply
    • Diahan

      Hi Marv. There are ALWAYS multiple explanations for any relationship. Some of the factors are their age vs your age (you can sometimes guesstimate based on their name or if they have a family tree posted). But yes, the relationship is likely close, and likely one of the relationships you listed. To find out which side of the family they are on, you can use Shared Matches and look at the resulting list to see who you do recognize, and that will help.

      Reply
  9. neil

    Can a DNA match of 713cMs over 20 segments longest segment 136cM be my half sister I know the numbers are low but everything turns to half sister every way you look at it except the numbers I am male and the match would be my father if that makes a different. Thanks in advance for any help Neil

    Reply
    • Diahan

      Hi Neil. This is something I would need to review personally in order to better advise you. We do offer mentoring services you can check out here http://www.yourDNAguide.com/mentoring. But in general, no, this person cannot be a half sibling when they share that amount of DNA. So there must be another explanation for your relationship.

      Reply
  10. Jennifer M Kline

    My dad is an identical twin. I someone show as a half niece her and I have closer cMs but my cousin from my dads twin has closer longest segments. My matches mother has passed and so has my father and his brother. We were trying to figure out which twin is the grandfather of my match. Is there anyway of knowing with an identical twin? My cousin has shared DNA 723cM and longest segment 209cM with her and I have shared DNA 936cM and longest segment 155cM.

    Reply
    • Diahan

      Nope, unfortunately, there is no way to genetically tell the difference between identical twins.

      Reply
  11. Lise Oikle

    can you please help .me and another lady on ancestry have
    24% shared DNA | 1,693 cM across 38 segments
    Unweighted shared DNA: 1,693 cM
    Longest segment: 153 cM
    is this a half sister?
    my full sister and me result on ancestry 45% – 52% shared DNA | 2,553 cM across 45 segments
    Unweighted shared DNA: 2,553 cM
    Longest segment: 156 cM
    what could you tell me about this .

    Reply
    • Diahan

      Half sibling is one answer. Another would be an aunt/niece relationship. So that would depend on how old you are compared to her.

      Reply
  12. Randy

    I am using the relationship chart on DNA painter as a research tool. The database used to develop the chart was built over several years. To compare apples to apples – should I use Ancestry total DNA or "weighted DNA"? I assume that data contributed years ago has not been filtered by Timber.

    Reply
    • Diahan

      Good question. In most cases it won’t make that much difference. And there have been updates to the shared cM project since Timber has been implemented, so I don’t think there will be much of a bias there.
      If you use the Beta version of interactive table of values (click on the small beta on the left of the page to use the Beta) those numbers are actually not generated by the user-entered data, but by a computer algorithm.

      Reply
  13. craig

    Hi Diahan.

    My DNA match with a gentleman is listed below. There is an outside chance he may be my nephew, it seems much more likely based on our respective ages and the DNA profile that he is my half brother. Is my analysis correct? Is there any other explanation? Thanks. Craig
    Shared DNA: 1,967 cM across 37 segments
    Unweighted shared DNA: 1,967 cM
    Longest segment: 137 cM

    Reply
    • Diahan

      Craig,
      There are always other explanations.
      Like, you could be double first cousins.

      But, as you said given what you know about your own family, and what you know about this other person, half sibling is the best estimate.

      Looking at this person’s relationship to other people you know who have tested would help. For example, if you know your first cousin on your mom’s side has tested, but what you use shared matches with this new match, that cousin does NOT show up, then you know that match is related on your dad’s side.

      Reply
  14. Stacy

    Good Evening, I have two DNA matches that are really high and I am trying to identify their relationship because I am not sure who my father is but they do not come up on shared mataches with relatives I know that is on my mother’s side.
    1). 13% shared DNA/ 924 cM across 20 segments, Longest Segment: 140 cM. Is it more likely she is a 1st cousin or Half Niece? because the other relations are far fetched.
    2) the other mystery person is 29% shared DNA/ 2,055 cM across 33 segments, Longest Segment: 158 cM. This person is also showing a shared match with the other relative #(1) above. Is it likely that this person is my half sibling or what options?

    Thank you

    Reply
    • Diahan

      Best option for #2 is half sibling.
      To tell if #1 is a half niece or a 1C, it is tricky. It really comes down to age. If she is about your age, she is likely a 1C. If she is younger than you, well, then niece.

      Reply
  15. Allison Knightly

    I have a problem with Ancestry’s DNA ethnicity estimate and I believe it is due to the algorithm. My ethnicity is currently reported as 58% total Eng/NW Eur, Ireland, Scot; 40% Italy, 2% Germanic. My son’s is 99% Eng, Ire, Scot and 1% No. Africa(?!).

    My mother is 94% Eng, Ir, S, 5% Scandinavian, 1% Portugal. My father [deceased and no DNA sample exists] was Italian on both his mother’s and father’s side, 100% Italian, I assume. Each of my daughter’s tested 7% Italian. My children’s father is Irish, presumably 100% since he was born there.

    Yet so far I have identified 12 people that my son DNA matches to that have major Italian ethnicity, 10 of them 86%-100% Italian. 9 at 20 cm minimum. Most of these DNA matches are Southern Italian. I match to 11 of the 12. One of the 12 is someone I know – my father’s half 1st cousin. Her father was my Italian grandmother’s 1/2 brother. They share the same mother but different fathers. But both fathers were born in Southern Italy.

    My match to her, my ½ cousin, once removed: 315 cm, 13 segments, 61 cm longest segment.
    My son’s match to her, his ½ cousin, 2x removed: 218 cm, 11 segments, 34 longest segment.

    So how can my son have no Italian ethnicity? I brought this to Ancestry’s attention last week and the representative said she had not seen this ever. It escalated to a supervisor. How can I be certain of any of Ancestry’s DNA reporting?

    Reply
    • Diahan Southard

      Allison, this is interesting.
      The important thing is that the DNA matching is working just as it should, as you demonstrated with that half cousin of yours. You are correct that your son’s ethnicity should reflect his connection, but ethnicity estimates are tricky, and when it’s all said and done, the ethnicity is just an estimate. But it is still frustrating when it seems they are not predicting correctly!

      Reply
      • Allison Knightly

        Thanks for answering. I’ll let you know if Ancestry does anything.

        Reply
        • Allison Knightly

          Hello again Diahan, With the Ancestry update for April 2022, my son who had no Italian Ancestry before now has 11%. Mine also went up to 48%. This is concurrent with Ancestry rolling out their “SideView” method of attributing the DNA to each parent. From what I read it sounds like this is driven by analyzing DNA matches in their system. Do you think the work of Ancestry members setting up connections to their shared matches has influenced their ethnicity estimates?

          Reply
          • Diahan Southard

            Hi Allison. The update to your ethnicity estimates is based on the new SideView technology. This doesn’t have anything to do with members assigning themselves as connected to various DNA matches in different ways. The SideView technology just makes the assignment of DNA segments to one parent or another much much better, allowing them to get much better ethnicity estimates for you.

  16. Wes burkhalter

    Hi Tiffany, would love your feed back on a couple of questions! – Do you think that its normal if i share around 200cm with 3 different 2nd cousins/ Twice removed? Also if my Great Grand father was a son to First cousins then will it inflate my dna with other people from those lines?

    Reply
    • Diahan Southard

      Sharing 200 cM with 2C2R to me suggests you have multiple relationships with those people. So, if your great grandfather was the son of first cousins, and these matches are also descendants of this couple, it makes you 2C2R two times, which would account for how much DNA you see shared.

      Reply
  17. Wes burkhalter

    My apologies, One last question! If I share 117cm with a match, but have a largest segment at 79cm… what does that suggest to you? is it possible for this relationship to still be a 3rd cousin Once removed?

    Reply
    • Diahan Southard

      That is a large longest piece. If this is part of the same family you mentioned in your last question, then I can see the multiple relationships showing up here as well. But technically, yes, a 3C1R is still possible.

      Reply
  18. Barbara Helen Bradshaw

    Hello, I have finally found my deceased mothers birth family. She was left on a doorstep and died never knowing where she came from, but I have tracked them down and building relations, though all her possible parents are deceased too. One of my matches is with 2 of my mums either cousins or sibling. One is 25% DNA, 1766cm across 52 segments, unweighted 1766 cm and longest segment 138cm. Her sister is 23% DNA, 1603 across 44 segments, unweighted 1603 cm and longest segment 130 cm.

    This womans daughter is also a match, simliar to a cousin to me. 9% DNA with 41 cm as the longest segment. The above women have a cousin who has been matched to me but not as closely. They all look like my mum, as we have met just the once, but all think it was another sibling who fathered / mothered my mum. Very complicated as you can see, but I really would appreciate any insight from someone who knows, as to if these women are actually my full aunts. That would mean my mum was the first child, maybe her mother was very young. Or I have thought of the possibility of incest, and wondered how this would show up, regarding my aunts and my cousin. Any help would be amazing, thank you.

    Reply
    • Diahan Southard

      Barbara, great job getting this far. For these kinds of close relationships, I really like to look at the data myself. Those do sound like half sisters or aunts that you found, but I would want to review the data to be sure. This would be a good subject for a mentoring session. http://www.yourDNAguide.com/thementor.

      Reply
      • Barbara Helen Bradshaw

        Thanks for getting back to me, I will look into maybe doing a session. At first I felt satisfied at getting this far and finding them, but as time goes on, and people get older, I feel a need to know as much as I can before its too late. I will be in touch, thank you.

        Reply
  19. Julie

    Hello
    I have a real dilemma with my ancestry.
    My mother, and two known first cousins, only have three lines of dna matches until we get to 4th-6th and 29 cms. They are missing closer matches on my mothers paternal grandfathers line. So my mother’s, father’s, father’s line Before 29cms on my mothers matches, there are only two people who we believe should be second cousins on that ancestry line, and one matches mum as 91 shared over 4 segments, largest 40, and the other one is 60 shared over 5 segments, largest is 21.
    To complicate even further, My mothers parents both had the Irish surname of grady, although she says they weren’t related, and there is a lot of Irish ancestry on all lines,
    We have been looking at this on ancestry and gedmatch. We just don’t understand why mum, does not have any 2nd cousins ( except the two mentioned earlier), or any third,4th-6th cousins on this line.
    For reference, I have access to two first cousins of mine, my own sister, one first cousin of Mums, and one of the second cousins (mentioned earlier) results. I know that ancestry shared matches doesn’t go beyond 4th .. and I have manually checked everybody I can with each other’s matches. There is no commonality on this line that I can find.
    So desperate to solve this for my mum, who is 87.
    Are you able to tell me if it is possible to only have 3 lines of ancestry dna matches??
    Julie.

    Reply
    • Diahan Southard

      Hi Julie. It sounds like this might be a good case for a mentoring session. http://www.yourDNAguide.com/thementor.
      In short, if you are missing some cousins, it is often because your ancestor was born outside the US, and so people just haven’t tested. It is also common if your ancestor had a small family. The best way to know what is going on is to find a second cousin yourself from his line and have them tested.

      Reply
  20. Rick W

    I was curious if 1613 cM across 29 segments was strong enough to be half siblings? He’s 39 and I’m 37 so he can’t be many of the things that it said he could. He’s definitely not a grandparent! I’m having a hard time with a stranger that popped up at the top of my list on ancestry with that much DNA shared! We’ve been in contact but it’s so hard to believe.

    Reply
    • Diahan Southard

      Yes, it is, but there are always other possibilities, like a double first cousin, or something. One way to check would be to see if he shares any DNA with any of your known matches. That can help you see if he is on your mom’s side or your dad’s side. It can be difficult to come to terms with a new close relative. We have a great blog post to help. https://www.yourdnaguide.com/ydgblog/dna-test-surprise-unexpected-news

      Reply
  21. emma bird

    Shared DNA: 604 cM across 20 segments
    Unweighted shared DNA: 604 cM
    Longest segment: 73 cM

    what family relation could this be possibly ???

    Reply
  22. White

    Hi there, I am trying to determine if my fathers match on ancestry is with his half brother or his uncle. They are only 4 years apart, however, the father or this person had 18 kids over a 45 year span. This would mean he would have had my father when he was 60 years old, not far fetched as he had other children around the same age. But this could also be his uncle. Their match says:
    DNA 25% shared DNA: 1,763 cM across 44 segments Longest segment: 116 cM Unweighted shared DNA: 1,763 cM. And when I did my ancestry DNA it says I matched with this same person as:
    13% Shared DNA: 939 cM across 31 segments
    Unweighted shared DNA: 939 cM
    Longest segment: 115 cM
    Would you think half sibling or Uncle?

    Reply
    • Danielle Francis

      Thanks for your comment! This is one of those situations where shared cM can’t tell us the whole story. We would need to look over your shared matches and other factors to be able to determine the exact relationship. I would recommend checking out our mentoring sessions: https://www.yourdnaguide.com/thementor

      Reply
  23. Allison Knightly

    Hello Diahan.

    If I match to with woman on Ancestry with 166cm, 5 segments, the longest 54 cm and her daughter by 131 cm, 5 segments, 51 longest and my mother does not match to either can I definitively say this match has to be on my father’s side? My children match to them, one very strongly. I and 2 children also match to a first cousin of theirs.

    I have shared matches that point to my father’s side, but not matching surnames, and very little ethnic overlap. The only thing in common may be location.

    Reply
  24. sophistry

    Hmm it loоks like your website ate my first comment (it was extremeⅼy long) so I gսess І’ll juѕt sum it up what I wrоte
    and sɑy, I’m thoroughly enjoying your blog. I as well am an aspiring blog writer but I’m
    still new to everything. Do you have any poіnts for inexperienced blog writers?
    I’d definitely appreciate it.

    Reply

Submit a Comment

Your email address will not be published.

Send this to a friend