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 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.”
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.
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.
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.
But it has definitely been helpful, especially with the 2 near the top of my list that are actually close.
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!
Yeah, this should help with my mother-in-law’s Mennonite matches as d well!!
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
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.
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
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
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.
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?
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!
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.
Thank You so much.
Hi can you help me I’ve been searching for the name of my father for alot if years I did a test on ancestry and I have a match unweighted dna 516cm across 22 segments the longest being 72. What is this person to me as I’m finding it confusing and is my father related to him
If you look up that number at https://dnapainter.com/tools/sharedcmv4 you will see all the options. Most likely is first cousin once removed (1C1R) among many. The half relationships would only apply if your unknown father had half siblings. Use the methodology for adoptees to find him. See https://blog.kittycooper.com/more-dna/help-for-adoptees/
Thank you kitty so his dad sons could be my dad would that be right x
No, then you would be his great niece, possible at 7% but the most likely scenario if he is much older than you is that your dad is his dad’s brother’s son. Great great neice is in range where his brother is your dad’s grandad … Build his tree.
Thank you so much so x
Hi Kitty. My top endogamous match (known 3rd cousin in 4th cousins list) shares 79cM split into 6 segments, top segment is 21cM. I think Ancestry added a new feature during the last few days – Unweighted Shared DNA. It shows 85 cM for my 3rd cousin. Any idea what this is?
Yes the unweighted DNA is the amount of shared DNA before they apply their algorithms to remove the DNA that is population specific. Their algorithms improve the results for those from very intermarried populations but it is useful to see the before number as well.
Got it, appreciate your feedback =)
Hi Kitty had my DNA done on ancestry . When my dad was conceived back in 1939 his father was unknown (war baby) . is there a way to work out who’s from this side of my dads tree 🙂
Yes people figure this out all the time. As long as enough of his relatives are tested which is not always so for foreign nationals. The techniques used by adoptees work well for this. Read this:
30% shared DNA | 2,065 cM across 55 segments
Unweighted shared DNA: 2,065 cM
Longest segment: 123 cM
You can use the DNApainter calculator to see the likely releationships:
with that large a largest segment I would suspect a paterna half sibling see this post of mine
Their age and who else they match can help figure this out
I have a match with 323 cM , across 8 segments. The longest being 117. I know what side he matches, but no family tree matching. Families from same area.
There are many possibilities Cheryl. One likely one is that someone’s grandad is not the person of record making you two half first cousins. Look at the matches you have in common to figure out which line that might be…
I have come across a somewhat unusual match on Ancestry.
Shared DNA: 102 cM across 1 segments
Unweighted shared DNA: 102 cM
Longest segment: 102 cM
The longest segment is quite long, but we only seem to match on a single segment. Most other matches of mine with more than 100 cM shared DNA – including 2nd/3rd cousins where I am aware of how we are related – match on several segments but have a shorter “longest segment”.
Do you have any thoughts on what my relationship might be with this particular match?
Single segment matches can be from very far back in time, even large ones. You might both upload to GEDmatch where you can take a closer look at this match and see who else matches you on this segment.
Thank you so much for your reply, Kitty. That’s very helpful!
I came across an unexpected DNA match on myHeritage.
Shared DNA: 216.3 cM across 6 segments
Longest DNA segment: 85.5 cM
My son also matched this individual
Shared DNA: 67.2 across 3 segments
Longest DNA segment: 38.8 cM
I would be grateful if you could offer some insights on our possible relationship. Age wise he is a few years older than me.
I’ve proceeded to ask my sister and my mother to take the DNA test. We believe he is a relative from my father’s (deceased) side based on the surname. Would this be helpful to unravel the mystery of how we are both related?
Your mother’s test should let you figure out which side this match is on. It is in tbe range of a 2nd cousin. Can you get any 1st or 2nd cousins on your father’s side to test,?
The calculator at https://dnapainter.com/tools/sharedcmv4
Can show you the possible relationships
Hi. I came across this useful site while trying to figure out a match of my sister’s. She my half sibling and we did DNA tests because neither of us know who are father’s are. She has a 1st cousin match from her father’s side, but the numbers are a tad confusing. They share 1076 cm of DNA (15%) with the longest strand being 170cm across 32 strands. That 170 is confusing because she and I only share a strand of 100 cm across 54 segments.
The longest segment can vary greatly and they are usually larger on the paternal side where there is less recombination so no worries. Those numbers jut mean a very close relationhip. Look up the total cM at https://dnapainter.com/tools/sharedcmv4
You will see that this match is likely either the child of a half sibling or a first cousin
My match (closest match, actually) in question is this:
Shared DNA: 78 cM across 1 segments
Unweighted shared DNA: 85 cM
Longest segment: 85 cM
By the chart above, he would be a first cousin?
No by the chart at https://dnapainter.com/tools/sharedcmv4
that is the 3rd=4th cousin range BUT a single segment match can be much further back on your tree even though it is a large segment
Good Afternoon Kitty Cooper,
You have been a great help in my past searches.
I have a match on Ancestry, (paternal side). 1695 cMs across 43 segments. Unweighted: 1695. Longest segment: 167.
Family thinks it’s a paternal grandmother but I’m leaning towards Aunt? Thank you.
Family is probably right, that is a huge longest segment… can you compare at GEDmatch? If it is a paternal grandmother the entire X will match. Also read this https://blog.kittycooper.com/2017/09/the-25-relationship-a-first-look-at-the-data/