Posted by Anna Swayne on January 6, 2016 in DNA, Science

AncestryDNA recently released a feature to your DNA match results that we call Amount of Shared DNA. It allows you to see some scientific details behind your relationship with each of your matches. You now have access to this for each of your matches.  See an example below.

Ammout Shared DNA

New DNA Matching Details

In this example, the user shares 71 centimorgans (cM) across 5 DNA segments with this particular DNA match. The box containing these details appears when you click on the info icon (the “i”) located to the right of the confidence level. From here, you can also click the What does this mean? link to learn more about centimorgans and how they are used to calculate relationships and confidence scores for each match.

How Shared Centimorgans Are Calculated

The total number of shared centimorgans represents the amount of DNA (across 22 pairs of autosomal chromosomes) that you and your DNA match likely have in common because you both inherited it from a recent common ancestor.

As relationships get more distant, it is important to distinguish between DNA that is identical because it was inherited from a recent common ancestor (6-10 generations ago) and DNA that is identical for other reasons (i.e., due to common ethnicity or older shared history); read more on this here. The number of centimorgans we report is our estimate of the amount of shared DNA that we can reliably attribute to a recent shared ancestor rather than more distant shared history. While it is possible that you share more DNA with your match, it may be shared in very short segments that are difficult to attribute to a recent shared ancestor.

In order to identify DNA shared because of a recent common ancestor, we use cutting-edge technology called Timber, which weights matching segments across the genome accordingly. Because close matches (parent/child through 2nd cousins) share so much DNA, it isn’t necessary to weight their shared segments – so, we only use Timber for matches estimated to be 3rd cousins or greater. As a result, while reported centimorgans for close matches reflects our unweighted estimate of the amount of shared DNA, for more distant DNA matches, the reported centimorgans reflects the weighted estimate using Timber. Dr. Julie Granka provides more details about Timber in this blog post.

How the Number of Shared Segments Are Calculated

This new feature also reports the number of DNA segments you share with any particular match.  Although it provides some useful information obtained from our matching algorithm, there are a few practical details that affect this number.

First, for accuracy and speed, our algorithm chunks the genome up into 44 sections across your 22 pairs of autosomes. This chunking does not always correspond with the chromosomes themselves – Centimorgans imagefor example, one chromosome may be left intact, while another may be broken up into three or more pieces. In the example above where 71 centimorgans (cMs) were shared over 5 segments, you might have a 12-cM segment and a 14-cM segment shared on section 1 of chromosome 2, (see an example to the right), a 15-cM and a 9-cM segment on sections 2 and 3 of chromosome 8, and a 21-cM segment on section 1 of chromosome 18 for 71 total cMs on 5 different segments.

Second, a single segment of DNA inherited by two people from a recent common ancestor could actually show up in our algorithm as multiple shared segments within that originally inherited segment. This is due to some inherent difficulties in chromosome phasing, genotyping, and IBD estimation methods. IBD means “identical by descent” and refers to segments of DNA inherited from a recent ancestor, as we’ve discussed earlier. Even the best available phasing pipelines have some issues with obtaining perfect phase. Because of this, an IBD segment may switch between inferred chromosome copies, and be split across multiple identified segments. GERMLINE (our IBD identification algorithm) has a process for mitigating this effect in which it allows matches to jump back and forth between chromosome copies – but it is not perfect at stitching these back together. As a result, DNA matching algorithms will frequently identify two distinct matches near one another that in fact represent one long segment of IBD. As a final note, we do not use the number of shared segments in our relationship predictions -we only use the total amount of shared centimorgans, as it has proven to provide accurate relationship estimates. You can find more details about this in our DNA Matching white paper.

So, while you might expect to have no more than 22 shared segments with your mother, for example, you will actually see many more—both because of the way that the chromosomes are sectioned in our algorithm and because smaller identified shared segments are not always stitched together after they are identified. Although the number of shared segments can be a helpful guide in your research, because of the caveats we mention above, we recommend focusing your attention on the number of shared centimorgans.

We hope that you will find this information useful as you explore your relationship with other AncestryDNA members.

Anna Swayne

Anna Swayne has 9 years of experience in the DNA genealogy world. At Ancestry, she leads efforts in developing education to help our community maximize their experience with AncestryDNA. She believes there is real power behind DNA and the story it can unlock for each of us. When she is not talking DNA you can find her hiking or cycling in the mountains or cooking at home.

49 Comments

  1. Jason Lee

    Ancestry has bastardized and oversimplified the matching segment information. Countless research opportunities are being wasted needlessly.

  2. Can you add a way for us to show you the correct cousinship for our matches? I come from an endogamous population and all the people you show as 2nd cousins on my page are actually 3rd cousins or greater on multiple lines. My only real 2nd cousin you show as a 3rd cousin. I’d like to get that corrected.

    Thanks for this blog post.

  3. Lauren Snellgrove

    Great addition to AncestryDNA ! I’m looking forward to AncestryDNA adding more DNA tools, like triangulation.

  4. Diana Nelson

    I would like to know why an Ancestry user is a match to me and my brother on GEDmatch, but not Ancestry. We share 17.6 cM over 1 segment, so it should not be eliminated because it’s a bunch of small bits. I have many other matches in this range over 2-3 segments.

    • pumpkinsnoopy

      I’ve asked ancestry DNA this same question over and over and the reps cant answer it. my aunt and my mom are blood related but on ancestry DNA they do not pull each other as close match but on gedmatch they are very close related….My dad’s uncle doesnt pull his 3rd cousins and their grandparents are siblings… something… i’ve talked to other members on ancestry about this same issue where first cousins dont pull each other but on gedmatch and 23 and me they do.. I think something is very fishy about ancestryDNA testing….

  5. Nancy Mullins

    Wish I could get a better mental grasp on the whole DNA technology. Looking at the new DNA circle info… I have the results for myself, my husband and our daughter, our only child. When I look at her circles, she has DNA matches that do not appear on either of our profiles. I know there is probably some scientific explanation but on the surface, it is confusing. HOWEVER, with that being said, I love, love, love that new feature!!!

  6. Margaret Jordan

    I think AncestryDNA is just defending its lack of transparency. It will eventually have to provide details of the locations of the matching DNA segments on the chromosomes, in the form of a chromosome browser. It is frustrating that Ancestry is dragging along with making this basic analysis tool available. Ancestry now tantalizes us with the “total DNA shared and how many segments are involved” information, without details on where these segments are and how long they are. It provides a limited “shared DNA” tool which could be extended to all matches. Using the excuse that the matches beyond a certain cut-off point are uncertain, it not a valid excuse.

    • Jeani

      If we were given all the information suggested here, then there would be no need to keep re-subscribing to Ancestry. At least, that has always been my suspicion. I have grown weary of seeing close relatives suddenly having ‘no relationship to you’ as a comment. Now am using WiKiTree more extensively, that seems to be much more reliable. WikiTree does use the DNA results from Ancestry, 23andMe and Family Tree, etc.

  7. I would like a way to tell you what the correct level of cousinship is for our matches. I come from an endogamous population and the people you list as my “seconf cousins” are all third cousins with multiple connections. The only true second cousin I have is listed as a third cousin by your system. How can we get these mistakes corrected?

  8. Charles Fisher

    This is all very interesting but I hope you plan to release a Chromosme browser with the option to view unedited matches and Timber matches. Based on standard genealogical research, a lot of people discover multiple connections (i.e., double cousins) to some relatives. Knowing the exact location of these matches helps triangulate with other matches and gives us a clue as to which segment of the family tree to research. As it stands, we only know there are common matches but we don’t know where the common matches are so we waste time looking through irrelevant branches.

    Please stop making us beg for such a basic tool.

  9. Giddyupcentaur@aol.com

    U.S.A.
    interesting how something Soo micro can help us fined our biological beings.
    From Lorraine Leonard Macry

  10. Laraine

    When will you provide a chromosome browser? Other than uploading my DNA to GEDmatch.com (or DNA.land–less DNA results there but it does specifically identify near term vs long ago DNA). Both of those sites are free; we paid for Ancestry! Thank you–please let all of know as soon as possible.

  11. Sam leonard

    Why don’t you show Y DNA matches where you have y DNA from SMGH and Genetree? I used to have matches at SMGH that no longer match me with Autosomal DNA

  12. Leigh

    Please provide a chromosome browser or invest in GEDMATCH to enable its tools to become more improved. It is crazy that Ancestry users are going to another web site to analyse their data in the most basic of ways.

  13. Stephanie

    FRUSTRATING, FRUSTRATING, FRUSTRATING…. Adding total shared cM’s in addition to the confidence levels that were already there gives us nothing more to work with. Those who are seriously investing in genetic genealogy to further document their family lines encounter the biggest brick wall ever which is Ancestry DNA. My family is very colonial american so I have many matches where I’ll have 3 to 7 shakey leaf hints. Ok, great. Who contributed the DNA to us? The process of endless messaging begins — begging people to upload to Gedmatch so that we can confirm which segments we actually share DNA on. Want to make your customers happy?? CHROMOSOME BROWSER !!

  14. Gary

    This is step in the right direction, but only a small one. A bigger step — as many others have said — would be the additon of a chromosome browser. It would be so much more helpful to being to actually *see* where shared segments are located, rather than just being *told* how much total sharing exists. One reason not to do so: greater transparency = actually having to get it right.

  15. Gary

    Being able to make edits to one’s own comments would be nice, too … in case of misspelling “addition” as “additon”, or something.

  16. CathyD

    Chromosome browser, chromosome browser… please. It’s also ridiculous my dad and i have 113 matching segments over 22 chromosomes when, by definition, we are half-identical on each of those chromosomes.

  17. Alan Rabe

    When I click on the i, I get a box discussing relationship distance–but not the centimorgan feature.
    After I find out my papertrail and establish a known relationship, it would be nice to note it rather than just with a gold star.
    Lastly, I apprecaite what youi have accomplished. The DNA results are not overtechnical for a novice user and that has resulted in many more usable hits on Ancestry than I got on other other test sites.

  18. April W

    Please add a chromosome browser to make life simpler for AncestryDNA users. Most users are not uploaded to Gedmatch and, in order to solve some of these match mysteries, I need to see the location of the matching segments. Thank you!

  19. Diana

    Please give us a Chromosome browser…your new shared segment tool is like a GPS that only gets you a short distance down the road and then leaves you stranded! What good is that?

  20. Tony

    My mom and I share 3,079 centimorgans over 153 DNA segments and it has up as immediate family and states that we are siblings or she is my grandparent instead of parent/child relationship and I want to know how accurate is this? Where are the same other 321 centimorgans if you’re suppose to have 3400 from each parent?

  21. Linda

    Here’s what people are saying above: Ancestry is estimating that we are related to someone else. They can tell us how many DNA segments we share, but they’re not going to show them to us. It would be like me sourcing my family tree and then hiding all the sources: “I have evidence, but I’m not showing it to you!” PLEASE add a chromosome browser. With Ancestry’s huge database and its ability to find common ancestors, the addition of a chromosome browser would make Ancestry DNA the most amazing product EVER for family history.

  22. Jane

    Parent matches are very useful. But, if one parent is deceased, it would be nice to have phasing to estimate what would match the other parent, so we could just click it as we do with parent whose DNA was tested.

  23. Burks

    I don’t see this feature. Not when I hover over the little i, not when I click on the little i. I’ve tried Firefox and Edge on a Windows 10 computer. Any suggestions?

  24. Gerald Chase

    Please add my name to those who wish to see a Chromosome Browser. Will dropping Ancestry membership be the only effective means to effect the addition of that tool?

    • Jeani

      Yes, if enough people drop Ancestry, then they will most likely give the people what they want, what they paid for?

  25. Robin Layne

    My great grandparents were first cousins, the parents of my grand dad, my dad’s dad. The relationships have shown up in the dna circles as dna matches, etc. However, I saw a ‘4th’ cousin from that union… The math doesn’t add up…

  26. Martha L Reinhart

    Still waiting for that chromosome browser. As a DAR member and adoptee I could really use the matches to connect the pertinent information I need for an analysis. I have to tell prospective members that the option DAR has chosen is family tree dna even though I dislike their attached tree feature.

  27. Kathryn Jardine

    I don’t have the “i” to show the centimorgans and stuff. Is there a way I can get that?

  28. Ronald Douglas

    Why doesn’t the outcome of my DNA test drill down to location by area within a given country? I have Scots heritage on both my paternal and maternal sides, yet your test only reflects Great Britain (70% in my case). Isn’t the test capable to making this distinction? Your TV advertisement clearly states, by the man/actor talking, that he found part of his heritage was Scottish! I would appreciated your reply. Thanks.

  29. Rodney Sam

    I really wish AncestryDNA would had a chromosome browser or triangulation database instead of being so secretive about their process. I’m starting to become skeptical or how they determine my matches. and the above explanation because all human populations share common ancestry at one point in history. That is why you will have people with shared DNA markers. AncestryDNA has been useless to my research for the most part. I would highly recommend 23andme and even FamilytreeDNA more so because they explain their process more and at least provide you details.

  30. Patricia Leonard

    I was adopted. My DNA results showed one first cousin. I clicked on the tiny (i) and they had 1,800 centimorgans & 65 DNA segments. I’m told that it’s very high for a first cousin. Could this be a long, lost sibling?

    • bryan calfee

      no that’s a grandparent/grandchild, aunt-or-uncle/niece-or-nephew, half-sibling relationship.Congrats on the excellent match

  31. Eydie Jackson

    I am very frustrated that I am unable to compare chromosome with relatives to better determine our connection, my ancestors all came from the same isolated community and I am finding your lack of a chromosome browser difficult considering the hedgemony.

  32. kari wartena

    I took your dna test but when I get on relatives that I know for sure are related to me it says I am not related. Why? Am a I a non person now

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