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General => Ancestral Family Tree DNA Testing => Topic started by: ConfusedMason on Monday 27 May 19 04:35 BST (UK)
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There is a girl I work with who comes out to a DNA match of 8cm with myself. I am also a DNA match of her mothers as well.
My Parents have taken the DNA test too but are not related to her.
Are 8cm just statistical anomalies that you are probably not related to because the CM is too small?
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The only way to establish this is to work out family trees of both of you, to see if there is an obvious connection.
It is a very small match, so may not be significant, however, it may be significant.
Even if you work out a common surname, that may not be where the match lies.
Look at matches you share with your colleague at lower levels (at Ancestry you usually only get shared matches at 20 cMs and above).
Since matches this small could relate to mutual 7G grandparents or beyond, the likelihood of you finding the connection isn't very strong, but you might strike lucky.
Regards Margaret
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You would have to put a lot of work in to figure out where the connection might be with a match of 7cM. Are either of you able to narrow it down by looking at matches you have in common or eliminate some possible lines by looking at geographical locations?
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Oh, well I'm not too interested in finding out a Family Tree way of seeing how we match, it doesn't interest me too much since I know it would be far too much work.
It was mostly querying the accuracy of such low CM matches. Could it match you with someone you aren't related to at all?
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At that distance it could either be a real match or a false positive.
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The simple answer is yes.
There is IBD, identical by descent, i.e. you are related to your colleague with common ancestors, possibly as far back as 7G grandparents or further.
Also IBS, identical by state, i.e. you match because you are from the same region, DNA handed down through the generations in perhaps the same cM block.
As Craclyn says, a lot of work. Unless you both trace all your 7G grandparents, or find a match in common, you are unlikely to find the connection.
Regards Margaret
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Oh, well I'm not too interested in finding out a Family Tree way of seeing how we match, it doesn't interest me too much since I know it would be far too much work.
It was mostly querying the accuracy of such low CM matches. Could it match you with someone you aren't related to at all?
Yes some low CM matches could point you to people you are not related to.
I expect the chance of this happening will reduce as the science of DNA develops and the tests get more refined, but it will be a long process.
In a similar vein some DNA from direct ancestors whose DNA you could have inherited will not be present in your DNA. This is due to the random way DNA is passed from parents to their children.
Cheers
Guy
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Interesting, thanks everyone.
I'll let her know we probably aren't related, just statistical noise.
She thought it was a sure thing since it said I was related to her mother as well.
DNA is weird.
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Ancestry Thrulines have found me a number of matches at 8cM which are accurate and I wouldn't have picked up on. One set of three, cousins to each other, descended from a Marguerite Howland whose ancestor was halfbrother (different mothers) of my 4xggfather and (slightly higher cMs) a set of 4, cousins to each other but not previously known to each other, descended from my 2xggmother's nephew. cb
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Interesting, thanks everyone.
I'll let her know we probably aren't related, just statistical noise.
She thought it was a sure thing since it said I was related to her mother as well.
DNA is weird.
I don't think anyone said you probably weren't related, more that it could take time and effort on both your parts.
You might need to identify 1024 ancestors between you!
Regards Margaret
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For context, my most distant confirmed match is a 3rd cousin, who shares 6 cMs with me.
This is not a fluke, or IBS, we both have several shared matches to the same couple.
We all appeared in DNA circles when I last looked, this match shares higher cMs with the other members of the circle.
Regards Margaret
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I've found some oddities with close-ish cousins, but on MH rather than Ancestry.
My great grandparents are great great grandparents to two matches. They share 105cM between them. I share 120cM with one of them, but 9cM with the other.
My 2x great grandparents are 3x great grandparents to two matches. They share 120cM between them. I share 75cM with one of them, but 18cM with the other.
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Margaret and others I would be interested in your opinion on the following. A couple of days ago I watched a YouTube video suggesting that even if you have one or more very good segments of DNA matching with someone and some smaller ones under 7 centiMorgans, that you should actually ignore those small segments as they are probably still background noise, Even though you have one or more other good segments.
I had never considered it before, but it now seems a logical argument.
Martin
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I haven't counted my 7 cM matches recently - I know there were many when I last counted (before the new -style DNA pages). However, I have managed to identify 6 of my 7cM matches with particular lines by looking at surnames/trees/shared matches and geographical locations.
Gadget
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Margaret and others I would be interested in your opinion on the following. A couple of days ago I watched a YouTube video suggesting that even if you have one or more very good segments of DNA matching with someone and some smaller ones under 7 centiMorgans, that you should actually ignore those small segments as they are probably still background noise, Even though you have one or more other good segments.
I had never considered it before, but it now seems a logical argument.
Martin
Martin, For those of us who tested at Ancestry we get many thousands of '5th to 8th cousins', ranging down from 19.99 cMs to 6 cMs. I currently have 28, 410 such cousins.
Also, you only get given 2 figures - amount shared and number of segments shared, not size of largest segment.
My closest is 723 cMs across 32 DNA segments, my most distant are all 6cMs across 1 segment.
If the latter has a very well researched tree going back as far as 7G grandparents on all lines, I might be able to make an educated guess at the relationship. I haven't however achieved that on many of my lines, so there are plenty of gaps, similarly in their tree.
If I can persuade them to upload elsewhere, I can see more information, can check them against other matches, etc, etc.
A 10 cMs match of mine, relatively small, is it genuine, is it not? Descended from the same family as me from Chitterne, Wiltshire, our mutual 6G grandparents, paternal side.
A 7.8 cMs/1 shared segment match at myheritageDNA, descended from this same family, same line as my Ancestry match but split off at their mutual 4G grandparent level.
Makes it more likely.
Can I persuade my Ancestry match to upload elsewhere - no, she can't work out how to do it, doesn't particularly want to do it.
Does my cousin share both these matches? - yes, lilely to be a paternal match.
Can I persuade anybody else to upload elsewhere? Yes, this family well represented in my Ancestry matches.
Does this person match anyone at myheritageDNA or elsewhere? Yes Eureka. But at only 6 cMs/1segment.
Do I ignore this because of only a 6cMs match?
No, I add it to my list of mounting evidence that we are all descended from Stephen Leversidge Leverstretch Leversuch from Mere and Chitterne, Wiltshire, and that he was indeed my 6G grandfather, something which had been only a (fairly strong) assumption before DNA testing came along.
To me, it is about building a case, regardless of size of match, and I rarely know what size segments I share with my Ancestry matches. Other than that below 12 cMs, by definition there can only be 1 segment, as they ignore segments under 6cMs.
Regards Margaret
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It would be a mistake to ignore these smaller Ancestry matches down to 6 cMs. They don't necessarily lead back as far as 7ggp and can be much closer. What is really necessary is spending time doing tree research based upon whatever info you can glean from the match. Believe it or not, thru-lines if interrogated and interpreted correctly and with some luck/research hard graft can bring results. Sharing those connections with the match may or may not bring confirmation but is likely to be more rewarding than sitting on your hands...
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I use my low matches. They can often lead to interesting clues that can help to develop your tree further. However, you need to be careful about presenting them as evidence of common ancestors. There is no guarantee that the particular segment you are matching on comes from the common ancestors you identify and choose to add to your tree.
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I just found this chart, dated 2016.
https://thegeneticgenealogist.com/2016/05/24/family-tree-dna-updates-matching-thresholds/
I'm not supporting its content, nor refuting it. I am just adding it to the discussion.
Martin
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Martin, It is a statement regarding ftDNA's matching algorithm, i.e, how they determine a match. They may or may not have changed that since 2016.
At ftDNA my matches appear to show a much bigger match than at Ancestry, because they include small segments in their total figure.
Ancestry has a different algorithm, whereby matches of as low as 6 cMs are considered a match. They reckon that 'Timber' enables them to match at this level with accuracy. Ancestry ignore completely matches less than 6 cMs.
Thus a 25 cMs match at ftDNA, 4 segments, largest 8 cMs, would appear as only 8 cMs at Ancestry.
Which approach is better is perhaps open to question, but the statement as made in the article is not open to question as such, it is a fact.
Regards Margaret
(Modified slightly 16:15)
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Ancestry Thrulines have found me a number of matches at 8cM which are accurate and I wouldn't have picked up on. One set of three, cousins to each other, descended from a Marguerite Howland whose ancestor was halfbrother (different mothers) of my 4xggfather and (slightly higher cMs) a set of 4, cousins to each other but not previously known to each other, descended from my 2xggmother's nephew. cb
Most of my Ancestry Thrulines DNA matches are 8cM, on one segment. On one of them, I share 8cM on one segment, with two generations, so that tiny bit of DNA got passed down intact.
I don't think Ancestry use anything less than 8cM for Thrulines matches, or if that 8cM is split on more than one segment?
I was looking at one of my Thrulines, where I have two separate matches, where I share 8cM on one segment with one, 4th cousin 1x removed, and 43cM on three segments with the other, 5th cousin. This relates to two different siblings, born in 1815 and 1833, of my ancestor born in 1828, with the Most Recent Common Ancestors, their father born in 1792, and their mother born in 1797.
So I actually share a lot more DNA with the more distant cousin.
On a different Thruline I can see an 8cM match with a 4th cousin. I would imagine that is a small amount for a relatively close match? This relates to my ancestor born in 1817 and their ancestor, his sibling born in 1830, with the MRCA's, their parents, born in 1797.