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General => Ancestral Family Tree DNA Testing => Topic started by: dsquared on Tuesday 02 January 24 13:19 GMT (UK)
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Does anyone know how reliable DNA tests are when the cm connection is low e.g sub 15cm?
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What a question? There is no Yes or No answer as there are so many aspects that can affect the cM value.
Feed 15cM into DNA Painter’s Shared Tool and there is a vast range of Probabilities presented with a 6C being statistically within the 60% probability but a 6C would mean a MRCA of 5xGGP so a long way back and one where documentation may not be available or even reliable.
The process of DNA inheritance means you could have a close “Cousin” who you share very little DNA with and by the time you get to a 3C and beyond you could then get a set of results where you share no DNA with them.
Each DNA testing company undertakes their test different than their competitors and the likes of Ancestry applies a routine to filter the results of DNA tests to give a more reliable match.
When you get below 20cM the technical limitations of the testing processes can lead to errors and below 10cM there can be “False Positive” matches actually presented.
15cM though is very possible to find where the match sits within your tree, it depends upon where the MRCA is eg a 5C shares 4xGGP’s conversely at the 15cM they could be a 7C2R and the extra generational levers would add to the difficulty.
Yet 15cM could mean a MRCA at 2xGGP level and so much easier to find and substantiate via documentation.
The image below is from a well know testing company that shows the probability of DNA being present and detected which may answer your question.
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Thanks you've answered my question which (although I didn't articulate it clearly) is whether you can get false positives as I wondering about that. I'm tending not to take matches below about 18-20cm too seriously unless I can see an obvious link.
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I have a number of confirmed 5 - 7 Cousins with matches as low as 6 cM
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I to do have some low cM matches that I have included in my tree.
Most have been via using Ancestry’ Common Ancestors filter and triple checking each suggested person in the link pathway. Do note that not all the hints work.
Those who do not have a tree can be very problematic and may or may not be worth the effort.
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Whilst it is perfectly possible that some 6cM matches will be correct, and can be corroborated by alternative methods, I would never rely on a 6cM match without additional corroboration as proof of a relationship.
According to my textbooks, at a match length of 7 to 8 cM, around 50% of matches can be expected to be false (identical by chance, by population, by state etc.) So the probability of a 6cM match being genuine is slightly lower than that, i.e. it is fractionally more likely to be false.
Think of it this way: Ask 1000 people to write down a random string consisting of four numbers. Then ask another 1000 people to write down a random string consisting of 100 numbers. The chances that two or more of the random 4-digit strings may be identical by chance are much greater than for the 100 digit sequences.
Because lower DNA match lengths are shorter, the same probability applies.
As the length of a match increases, the possibility of false matches decreases. It is generally accepted that at and above lengths of 16 to 20cM, matches can be considered reliable.
Bear in mind however that many shorter matches on Ancestry are given those match lengths after application of Ancestrys' Timber algorithm, which aims to strip what the process considers to be false match lengths from matching segments. That must decrease the possibility of some 6cM matches being false, but to what extent I don't know, and it is also possible that Timber may in some circumstances strip out genuine segments. You can check the unweighted match length for Ancestry matches at lower lengths, and if it it longer than the headline match length, it indicates that Timber has removed part of the original match. In those circumstances, the probability of the match being genuine may be slightly higher than it appears.
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Thanks everyone for your thoughts. It's all been very helpful. :)
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Does anyone know how reliable DNA tests are when the cm connection is low e.g sub 15cm?
Can I ask which company we are talking about? It makes a big difference as to whether they are likely to be reliable or not. On Ancestry, you can also check for the unweighted figure by clicking the cM number.
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Ancestry. :)
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Ancestry. :)
OK. I think most 15cM matches will be 'real'. But when you get down to 7cM or 8, a fairly high percentage are false matches. When I say false, it can be two things really, one it is basically an algorithm 'hiccup', or two, it is a segment that is inherited from an ancestor beyond the common genealogical timescale - i.e. for most English people that is the late 16th century (Scottish later due to scarcity of records and much later for Irish ancestry due to fact Catholic baptisms weren't really recorded until 1750s at the earliest and much later for some places). Family History Fanatics has several videos on the subject:
https://www.youtube.com/watch?v=OF5-PPMQvCg
https://www.youtube.com/watch?v=aFSNXlYFnVI
https://www.youtube.com/watch?v=ZqK9b2Vt6gQ
If we are talking about sites that take uploads, things get a lot more complicated and lot less reliable as imputation is used to cover the fact different chips have very low overlap i.e. MyHeritage and GEDMATCH, for GSA chipset uploads i.e. 23andme 2017-, MyHeritage/FTDNA 2019, LivingDNA -2019 the majority of matches up to about 50cM are false for this reason.
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Thank you melba_schmelba. That's very helpful.
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I presently have 42 DNA matches that I have included in my Family Tree.
These 42 have cM values in the range from 6 to 10 cM.
All have supportive BMD, census records and any other corroborative information that I could find.
So yes you can get there with low cM matches and whilst 42 may seem a high number when it is compared to the thousands of DNA matches that Ancestry says that I have then the 42 is a pretty small number or looking at it another way a 1 in 500 chance of finding a link.
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See also my post #6 above.
I have several 6cM or slightly higher matches in my tree, but most are marked as tentative. I would be wary of accepting a single 6cM match as proof of a specific relationship, even with paper or other records to corroborate it, unless you have investigated all subsequent intervening relationships in your wider tree. If you find several similar matches to a single MRCA or couple though, the chances are correspondingly higher that there must be some weight to the evidence.
But as an example, I was delighted last year to discover a new 6cM match, which on the face of it corroborated my research back to my Gx5 GF, as both I and my match were direct descendants of different children of that man, but my match by his first wife, and myself by his second wife.
Quite simple you might think. But one of the benefits of researching a tree widely and deeply is that I was able to see that there were at least three other local families whose members had at various times married into and become direct ancestors of either my match or myself. So in fact it is impossible for me to say with any certainty whether we share a match to my Gx5 grandfather, or one of the other families whose siblings appear in both our lines. One for example being my Gx3 grandmother, whose brother it transpires was also one of matches' intervening grandparents.
I have another 6cM match who descends from my Gx6 grandmother and her second husband on a different line. I am descended from the same woman and her first husband. But a direct descendant of the second husband's brother married my Gx2 GF and is therefore my Gx2 GM. So which line do we match on, either or both? It is impossible to say, or could the match just be IBC?