About those small segments…
For years, The Genetic Genealogist Blaine T. Bettinger has been warning about the dangers of relying on small shared DNA segments — those chunks of seemingly shared DNA that come in at 5-6-7cM or thereabouts — to draw genealogical conclusions about a match.
In December 2014, for example, based on a study from 23andMe and his own analysis of his DNA matches against those of his parents, he warned that “small segments are prone to be false positives” and said “it is the responsibility of the genealogist to place a VERY high burden on any argument that utilizes these small segments, if only because of the known high rate of false positives.”1
That provoked a firestorm from those who wanted to use small segments to help support genealogical theories, and he followed it up with a response addressing the most common of the arguments in favor of small segments.2
Three years later, Blaine added a round-up post reviewing the issue, noting again that “Many to most small segments (at least 7 cM and smaller) are FALSE, meaning they are NOT actually shared by the two matches, and therefore do NOT indicate shared ancestry” and concluding: “Since we can’t tell the difference between false small segments and valid small segments, we must avoid these small segments to avoid poisoning our genealogical conclusions with false data.”3
In lectures and blog posts, he uses a powerful visual tool to bring the point home: a jar of M&Ms. And, he tells us, one of every two or three is poison but there’s no way to know which is which. Do we want to eat one of those M&Ms — to add that DNA to our tree — without knowing if the one we’re adding is safe… or poison?4
The Legal Genealogist‘s own review of the data and literature has fully supported Blaine’s caution. Sure, some small segments may indicate shared ancestry. But many don’t. And we have no way to tell the good from the bad.
But admittedly they don’t teach this stuff in law school. Blaine’s got a Ph.D. in biochemistry, I’ve got a J.D. in law.
So… has the science behind his caution against relying on small segments changed?
In fact, we just got more evidence that Blaine’s cautionary tone is exactly correct. A new white paper from Family Tree DNA has come to exactly the same conclusion as earlier research: using current technology, small DNA segments are very likely to be false segments, that is, not actual segments inherited from one parent or the other and therefore we can’t use them to try to prove shared ancestry with a match on that segment.5
Take a look at the numbers here, from Table 2 in that White Paper:
At 7cM, roughly one in 10 segments is a false positive. That’s a 10% chance of poisoning a family tree if we try to draw a genealogical conclusion from that segment. At 6cM, two in 10 are false. That’s a 20% chance of poisoning our trees. At 5cM, it’s just about half — a 50-50 chance of poisoning our results, not supporting them.
Bottom line: “Since we can’t tell the difference between false small segments and valid small segments, we must avoid these small segments to avoid poisoning our genealogical conclusions with false data.”6
Don’t rely on small DNA segments in trying to prove a genealogical conclusion.
Don’t poison that tree.
Cite/link to this post: Judy G. Russell, “Don’t poison that tree,” The Legal Genealogist (https://www.legalgenealogist.com/blog : posted 5 Sep 2021).
- Blaine T. Bettinger, “Small Matching Segments – Friend or Foe?,” The Genetic Genealogist, posted 2 Dec 2014 (https://thegeneticgenealogist.com/ : accessed 5 Sep 2021). ↩
- Ibid., “Small Matching Segments – Examining Hypotheses,” The Genetic Genealogist, posted 8 Dec 2014. ↩
- Ibid., “A Small Segment Round-Up,” The Genetic Genealogist, posted 29 Dec 2017. ↩
- Ibid., “Losing Distant Matches at AncestryDNA,” The Genetic Genealogist, posted 29 July 2020. ↩
- Family Finder Matching 5.0, Matching Algorithm and Relationship Estimation White Paper, 2021-08-18, Family Tree DNA blog, posted 31 Aug 2021 (https://blog.familytreedna.com/ : accessed 5 Sep 2021). ↩
- Bettinger, “A Small Segment Round-Up,” The Genetic Genealogist, posted 29 Dec 2017. ↩
An important warning. Thank you for helping to spread the word, Judy. I wonder if the technology will advance and grow sophisticated enough to disaggregate the true from false matches.
It would be wonderful if we ever get to that point, but we’re sure not there yet.
Thanks for that Judy
I was wondering if the unreliability of small segments was solely caused by the testing companies only using one side of each matches chromosomes. And following from that if 2 matches had their whole genome tested could small segments down to 1cm be used. If I’m out to lunch on this please ignore me.
We may need a lunch date. Testing companies compare your DNA — both chromosomes — to the DNA of everyone else in their database. You can be a match on one chromosome in any given pair, both chromosomes in any given pair or neither chromosome in any given pair. Whole genomic sequencing may well tell us more and open up the potential for more and more accurate matches but it isn’t at all clear if it will make these small matches more useful as genealogical evidence.
My father, his sister and I all share around 5 cm with 1 particular person -could all 3 of us be mistakes?
If you can see that this is the same segment to share with your father, it’s less likely to be a false segment. Of course it’s still possible, but it’s certainly a hint to consider.
Totally agree. But phasing can help greatly and the news on that has been out for years too. So why not point to help with that?
Well, I am already back to 8C in a few lines without it and I am running into tree problems. Not just problems with my own or with my match, but this is getting back to late 1600s when I find some records are not as readily available or are actually missing.
Especially around 1650s Commonwealth period in England.
And in my parts of Eastern Europe, records thin out beyond 1800.
But I can see that if the suspected match is around 3C/4C then a small match could be extra helpful and phasing would be well worthwhile.