Ancestry and the Longest Segment

All the DNA testing companies, except Ancestry, show you a chromosome map of the segments you share with a DNA match. For the casual user those maps, called chromosome browsers, may not be of interest. However those of us with intermarried families among our ancestors (endogamy) need to see the segments in common in order to know if a match is a findable relative who shares large chunks of DNA with us or just someone who shares multiple distant ancestors. Third cousins and closer family will always share some large segments, at least 20 or 30 centimorgans and even larger for closer relatives (see Blaine’s chart below).

Ancestry now shows the longest segment on a DNA match’s profile page which could be very useful to help decide which matches to pursue and which ones to ignore.

A known fourth cousin on my Norwegian line with some endogamy, notice how much larger his largest segment is than the size of the match

A word of warning, the size of the largest segment that they show is uncut, that is it is listed before they remove matching DNA that is expected to be population specific. For example, many of my one segment matches show a longest segment that is larger than the match size as shown above! If you click on the longest segment number there is a very informative popup about relationships and segments that includes this statement:

“In some cases, the length of the longest shared segment is greater than the total length of shared DNA. This is because we adjust the length of shared DNA to reflect DNA that is most likely shared from a recent ancestor. Sometimes, DNA can be shared for reasons other than recent ancestry, such as when two people share the same ethnicity or are from the same regions.”

A Jewish match not to pursue, likely related many times since the largest segment is 15

My often requested advice for Ashkenazi Jewish researchers is to look for one segment greater than 20cM and another greater than 10cM plus several others in order for a DNA match to be recently related enough to find the common ancestor(s). Therefore it would be even better if Ancestry showed the two largest segments. Subtracting the largest segment from the total to figure out the sizes of the other segments is not very accurate since the total is adjusted by removing population specific sections (Timber algorithm) while the largest is not.

Blaine Bettinger includes the longest segment in his DNA statistics collection form and below is a chart of those 2015 results by cousin level; click here for his blog post.

Chart of longest segment statistics collected by Blaine Bettinger. Click it for the full article

I spent some time looking at the cases where matches are tested at both Ancestry and 23andme in my family’s results.

My recently discovered South African 3rd cousin (click here for that story) who shares Jewish gg-grandparents with me is also tested at both places. At Ancestry she shares 38 cM over 4 segments with a longest segment of 25cM while at 23andme she shares 56cM over 4 segments with a largest of 25.22. With my brother at Ancestry she shares 60 cM over 4 segments with a longest segment of 40cM while at 23andme she shares 90cM over 4 segments with a largest of 57.35. This gives me a feel for how much the Ancestry algorithm removes; in this case of endogamous DNA, about one third was removed.

Next I looked at my 100% Jewish husband’s matches, first C, a “3rd cousin” match. At Ancestry she shares 126 cm with a longest of 43 while at 23andme she shares 137 cM with a longest segment of 42.18. Not very different. definitely a match to keep working on.

Looking at EV’s match to my husband, at Ancestry he shares 190 cm with a longest of 50 while at 23andme he shares 171 cM with a longest segment of 48.82. Hmm that is backwards from what I expected. Perhaps different chips? I will ask EV when he tested with each company and recheck at GEDmatch after making a combined kit for my husband.

Finally I started sorting through my husband’s 35 “third cousin” matches, 33 unknown, to see how many I could eliminate for not having a large enough longest segment. So far only four. Thus this new feature is not as helpful as I had hoped. On the other hand, looking through my own many Jewish 4th cousin matches, a few of whom are known 3rd cousins, I found I could eliminate almost all of them.

My friend Kalani Mondoy has done a detailed analysis of his very endogamous Polynesian family’s DNA and this new feature on his blog at https://hawaiiandna.wordpress.com/2020/08/19/ancestry-is-finally-showing-longest-segment-size/

Those of us with intermarried families among our ancestors (endogamy) can have DNA matches that are related to us multiple times so they share a lot of centimorgans but are not as closely related as they appear. A good way to spot this is to look at the size of the segments. This new feature at Ancestry is a step in the right direction.

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12 thoughts on “Ancestry and the Longest Segment

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  1. It’s amazing to see how varied these longest segments can be. I realized that 2 matches that I’ve seen for a year are actually close relatives, since I saw the size of the longest segment. A now confirmed 2C2R, 64cM was the longest segment size. I have a 4C with 52cM. He showed up high on my list even before several 2C of mine including a 1C1R.

    Then I have these relatives where their longest segment isn’t long at all.

    2C 23cM
    2C1R 18cM
    2C1R 7cM

    But it has definitely been helpful, especially with the 2 near the top of my list that are actually close.

  2. Ah! Endogamy!
    I’ve been exploring my cousin’s matches. Her grandfather was Ashkenazi Jewish. She has one paper trail verified match to the Rothschild side of his family and another one on his Mayer side. Just about all her matches from her grandfather’s side are 100% Ashkenazi Jewish and they all turn up as shared matches and come in at the 4th thru 6th cousin range! I guess I’ve seen endogamy in action. The rest of her tree is Ulster Scots, Colonial Stew, and Spanish. Those branches are easy to sort out!

    • Sharon, Do you know about the Mennonite DNA Project from a couple of guys in Canada. Wonderful stuff and sort by surname and also has some of the MT DNA too. Bill

  3. Kalani

    Are those three you mention, where the longest segment isn’t long, related to you in only the way you have listed? If so, I am more confused than ever as to how the longest segment can be useful. You’ve confirmed the 2C2R, which is great–but couldn’t a long segment have come down for more generations than expected? Which leaves me with no clear direction on a long segment. In the one case, it suggested the person was close but in the three you mentioned, you’d normally assume they were much more distant, right?

    In my case, a half 1C is looking for his grandfather, and shares a 120-cM segment (almost the entire chromosome) with a match. But is the match a half 1C or half 1C1R?

    I hope this makes sense.

    • Robin –
      You cannot use the longest segment to decide on which relationship. Only if it is small to eliminate from being close and even that is not for sure

      Kalani –
      Have you checked those 2nd cousins on both sides that there is no break in the line? Was the testing on the same chip at the same company? Those look very dubious to me

      • Kitty,

        I made a mistake, the last 2C1R, that person’s mother & I are at that relationship while that person longest segment 7cM is actually my 3rd cousin. My mistake.

        So the other 2C1R did not upload anywhere.

        All of us tested at Ancestry of course. The 2C however I did upload to GEDmatch and MyHeritage. At GEDmatch it was 28.7cM and MyHeritage is 30.4cM for the longest segment size.

        Robin – the longest segment size won’t be able to distinguish a 1/2 vs. full 1C – 1C1R.

  4. Also Robin there is something called an inversion which can cause a segment not to be able to be recombined anymore. Rare but happens.
    Look at the other matches to the half IC1R?

    • Kitty
      All other matches are very distant. Two men were in the right place (NY) at the right time–father and son. One of them was the grandfather of my two 1/2 1Cs. Other than this match, the closest matches are similar in overall cM size on 23&Me but live in a highly endogamous region (Waldensian Valley, Italy). The father and son were French and thanks to Rocco, we now know names several generations back–but none match the Waldensians. The sister matches the NY-based match at 72 cM on this chromosome where her brother matches at 108 (or maybe 120, depending on the source). Father disappears, son stays in the area for a couple more decades and then leaves the country. No other offspring. So I really want the longest segment to talk to me!

  5. Kitty,
    I have a mystery DNA match of 1,679 cM across 39 segments with the Longest Segment: 171 cM.
    My sons who are 1/2 siblings share 1,403 cM across 59 segments with the Longest Segment: 62 cM.
    Son A and I share 3,471 cM across 28 segments Longest Segment: 277 cM.
    Son B and I share 3,435 cM across 33 segments Longest Segment: 279 cM.

    Son A and mystery DNA match Share 743 cM across 25 segments
    Longest Segment: 77 cM.
    Son B and mystery DNA match share 847 cM across 28 segments
    Longest Segment: 99 cM.

    Can you please help figure out how our mystery match is really related to us?

    • Hi F –
      Your mystery match is your half sibling and a half aunt/uncle to your sons. So the match is the child of one but not both of your parents. Look and see whether your match shares any maternal or paternal cousins …

      Or use the X. If you are both female and share a Dad, you will share the full X chromsome. If you share no X, and your match is male and you are female it might have be via your father. If you share some X but not a full X then likely you share your mother.
      To look at X you need to have both tested at 23andme or FamilyTreeDNA. Else you need to both upload to Family Tree DNA or GEDmatch.

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