We know that small segments shared between two individuals can be problematic (see Small Matching Segments – Friend or Foe?), whether the two individuals are closely related or distantly related (or not related at all, as we’ll see). I call small segments (which I usually classify as 5 cM or less) as POISON because it is currently impossible to decipher between which are real segments and which are not.
In the following analysis, I use the wonderful new Match-O-Match tool at DNAGedcom to compare my and my parents’ match lists from AncestryDNA. The Match-O-Match tool is a powerful spreadsheet analysis tool developed by Don Worth. It is available to DNAGedcom subscribers as part of the DNAGedcom Client. For more, see page 10 of the PDF HERE. Thank you Don for this great new tool!
What the following analysis shows is that 32% of my matches are not shared with either parent. For my matches, there is a SAFE ZONE above 15 cM in which a match has a 99.3% probability of being shared by either or both parents.
Below 10 cM, there is only a 59% probability of being shared by either or both parents. And below 7 cM, there is only a 40% probability of being shared by either or both parents. This numbers will vary for others, and I’d love to see others joining in this analysis.
I conclude from this data that matching below 10 cM is poisoned by false matches. Yes, without a doubt a significant percentage of these matches are real, but how do you decipher between them? In other words, if the poison is colorless and odorless, how do you know what has been poisoned? Having both parents tested may help, but it is not a guarantee as false segments might be inherited.
Do I think that all segments below 10 cM are forever poisoned? No, I believe that this an area where whole genome sequencing will have the strongest impact. And other methodologies and technologies may help alleviate this issue in the future. In the meantime, however, genealogists must be aware of the issue of false matches and how that might impact their research.
NOTE: I’m not the first genetic genealogist to do this type of analysis, by far, but I’m just adding my voice to the mix.
My Matches at AncestryDNA
Right now, I have a total of 16,193 matches. The matches breakdown like this:
- 39 share 50 cM or more (of these, I target-tested 15)
- 169 share 25 cM or more
- 411 share 20 cM or more
- 1,130 share 15 cM or more
- 4,190 share 10 cM or more
- 12,003 share fewer than 10 cM (75%)
- 5224 share between 6-7 cM (32%)
Sharing With My Parents
Of these 16,193 matches, I share 3977 (25%) with my mother and 7144 (44%) with my father. There’s some overlap there, my parents share 93 of my matches in common (eliminating me and my children).
A total of 5135 (32%) are not shared with either my mother or my father. These matches – not shared by either parent – breakdown like this:
- The largest match is 19.1 cM
- 8 share 15 cM or more
- 261 share 10 cM or more (5%)
- 1784 share between 7-10 cM (35%)
- 3090 share between 6-7 cM (60%)
Welcome to the Danger Zone
So the safe zone – meaning a match is almost guaranteed (99.3%) to match my mother and/or my father – is 15 cM or more, which is 1,130 of my matches. You might be able to lower this safe zone to 10 cM (4,190 of my matches), as there is a 94% probability that these matches will match my mother and/or my father.
The danger zone – meaning a match is NOT guaranteed to match my mother and/or my father – is below 10 cM, as 41% of my matches below 10 cM were not shared with either parent.
The really dangerous zone – meaning a match is not likely to match my mother and/or my father – is below 7 cM, as 60% of my matches below 7 cM were not shared with either parent.
Why do I have matches that neither parent has? These matches could be false positives, meaning that I have the match when I shouldn’t. These matches might also be false negatives, meaning that my parent fails to have the match when they should.
Having my parents tested may help me decipher which of my matches might be real, although of course there is no guarantee that a distant match I share with a parent is a real match. A false positive match that I have could be a parent’s false positive match as well.
But, if you don’t have both parents tested, how do you determine which of your matches below 15 cM are solid matches?