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Devon FHS's DNA WWWBoard
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What constitutes a DNA Match?

Posted by Roger Donne on 18/11/2020, 9:31:14

I have been using the chromosome browsers on several websites to try to find hints to family connections, plotting the results on an on-line tool called DNA Painter. I have an instance where the autosomal DNA results from two individuals clearly overlap, but the host website doesn't flag them as a match. What information is the website using to reject the match, and if it could be useful, why can't it be made available?

Re: What constitutes a DNA Match?

Posted by Barry PYCROFT on 13/9/2025, 4:18:46, in reply to "What constitutes a DNA Match?"

You do not say with which company you are seeing or not see this one particular match. Are the two individuals related to you? Are they also related to each other? Does the chromosome browsers show paternal AND Maternal chromosome? Do you know which ONE of a couple is the MRCA or MDKA?
These are simply thoughts I have and might not be relevant!! ;-)

Re: What constitutes a DNA Match?

Posted by Debbie Kennett on 30/11/2020, 13:25:45, in reply to "What constitutes a DNA Match?"

Determining what is a "match" is not necessarily that easy as there can be false positives and false negatives. All the companies set a minimum threshold for declaring a match so in theory there will be some false negatives that don't meet the companies' thresholds. However, there is a bigger problem with false positives. This is especially the case when the "match" is only on a small segment, and particularly on segments that are less than 15 cM.

There are now additional problems because of the different SNP microarrays used by the testing companies which have a low overlap. Companies have special techniques such as phasing and imputation to try and overcome the low overlap but there is inevitably a high false positive rate on the smaller matches.

Here are a few articles for further reading:

https://thegeneticgenealogist.com/2017/12/29/a-small-segment-round-up/

https://isogg.org/wiki/Identical_by_descent

https://isogg.org/wiki/Autosomal_SNP_comparison_chart

In general the matches at AncestryDNA are the most reliable as they have the most sophisticated algorithms backed up by solid science.

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