A Segment-ology TIDBIT
There are two ways of looking at small segments. But first please remember that ALL of your own DNA is true, even the very smallest part of your DNA came from a parent as a true segment. What we are talking about when we discuss “small segments” are small shared DNA segments with a Match – segments which are determined by a computer algorithm comparing your (true) DNA with a Match’s (true) DNA. Below about 15cM some of those comparisons report a false shared DNA segment. The smaller the segment, the more likely that it is false. The distribution curve starts at about 0% false reporting at about 15cM and drops down to about 50% false reporting at 6-7cM and drops down fairly dramatically below that.
In this post “segment” means a computer generated shared DNA segment.
1. Bad Segments: Small segments have a high probability of being false, and there is no easy way to tell if it’s a valid shared segment or not. And, perhaps, even if it’s a true segment, it’s probably from a very distant Ancestor – probably beyond your genealogy. These small segments are called names and referred to as POISON – DO NOT USE! However, in this derogatory sense we are talking about NOT using these small segments as evidence; NOT the basis of a hypothesis; NOT part of a “proof”. However, these segments are may not be worthless…
2. Good Segments. Shared segments are used by each company to identify DNA Matches, and report them to us. As noted above the small segments may be true or false. But what if they lead us to a person who is really related to us = a cousin? If the “Match” has a Tree we can check it out. We can look at the information presented. Finding a Common Ancestor is only part of the possibilities. Maybe this Match-cousin has more information about our Common Ancestor than we do. Maybe they’ve found records we don’t have, written an interesting story, uploaded pictures we didn’t have. Maybe we can establish a dialog (message, email, phone, in person…) I have made lasting friendships with some of my Matches – some of whom we still don’t know how we are related. The possibilities and opportunities are endless.
At AncestryDNA, ThruLines finds cousins with a Common Ancestor, down to 8cM (they used to go down to 6cM). I checked every one of them, and often found new information. With each DNA Match, keep your genealogy cap on. A small segment may in fact be false, but that doesn’t mean there isn’t a true relationship. Remember, about half of your true 4th cousins won’t share any DNA with you. My advice: don’t ignore a true cousin just because you share a small segment. Genetic Genealogists, myself included, have long stated that a Match with a Common Ancestor and a shared DNA segment does not necessarily mean the shared DNA segment came from the Common Ancestor. By the same logic, a relative with a Common Ancestor to you, may or may not have a true shared DNA segment from that Common Ancestor.
If you are trying to prove a bio-ancestor, or a brick wall Ancestor, or some other relationship using DNA, don’t use small segments. If they cannot be proved to be true segments, they must be ignored as part of a proof. But, on the other hand, don’t ignore a paper-trail relationship just because you share a small segment. Learn what you can from a genealogy perspective and ignore the DNA.
Just my perspective as a long-time genealogist…
[22BD] Segment-ology: Bad Segments – Good Segments TIDBIT by Jim Bartlett 20210930