Bad Segments – Good Segments

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

What Do You Get from a DNA Test?

A Segment-ology TIDBIT

You get a LOT with a DNA test at 23andMe, FamilyTreeDNA, AncestryDNA and/or MyHeritage. Each of these companies is different, and has its own tools and programs, but some of the features are fairly common among all of them. It’s actually quite amazing how much you get, no matter where you test.

1. Genetic testing of over 500,000 Markers (SNPs – pronounced: snips). Your DNA has two of each marker so actually you are getting over a million data points.

2. This is your data – you may download and save/use this file (about 10MB), but by itself it’s not much use to you (exceptions noted later).

3. A password protected/secured web site for your profile and information about your test results. This is your hub, or base, for your interactions with your results. Each company is different and has different tools and configurations for your personal page. See #16 below.

4. Ethnicity (aka admixture, deep ancestry, origins, geographic makeup, etc.) report. This is interesting in it’s own right (and many take a DNA test solely for this report). It’s important to remember that these are estimates! But there are many other features of your DNA test that are much more valuable – read on.

5. A list of people who share DNA with you – called Matches. These Matches are usually related to you – both of you sharing some DNA that you got from the same Common Ancestor.

6. A profile for each Match – some info about themselves that they may post.

7. Access to any genealogy Tree that your Matches may post

8. The ability for you to post/upload your own Tree and build on it – highly recommended.

9. Tools – these vary by company and include DNA tools and genealogy tools. See #16 below for some of them.

10. A list of Shared Matches (aka InCommonWith or Relatives in Common) that both you and each Match share – these lists are very important, particularly in grouping Matches.

11. Notes – each company offers a way to enter and save notes on each Match Profile – you type these in, and they are available to you when you return to that Match.

12. Communicate – each company has a way for you to contact your Matches. Yes, probably only 10% of the Matches will reply, but it’s often worth a try.

13. Updates – each company adds new Matches frequently as new people test; they also update/improve their ethnicity program every few years; they add new tools; etc.

14. Bargain prices – all of this for an up-front, one-time, $50-100 price. (Try to get any genetic test for under $100). Yes, each company will try to get you to buy more/other tests and reports, and some offer subscriptions to use their tools.  You decide – in any case, you get an ethnicity report and list of DNA Matches which are updated… forever!

15. NB: If you want, you can upload your DNA data file to other companies – lets you compare tests between companies; other companies offer analysis and services for health or wellness, based on your DNA, etc. Research carefully before you upload your DNA data file anywhere.

16. The International Society of Genetic Genealogy (ISOGG) has a page that compares the features of the major DNA testing sites: This table covers a lot, including links to various blog posts about each company.

Readers are invited to post comments on other insights that all/most of the major companies offer. Please do NOT tout your opinions about each company and/or their extra features – that is not the objective of this post. The target audience for this post is someone deciding to take a DNA test, or not. Feel free to pass this blog-post to others.

[22BC] Segment-ology: What Do You Get from a DNA Test TIDBIT by Jim Bartlett 20210929

Do You Have a Suspicious Branch in Your Tree?

A Segment-ology TIDBIT

Although this Segment-ology blog is focused mainly on understanding and using DNA segments, I’ve also tried to look at the genealogy part of the genetic genealogy equation. We need both genealogy and DNA tools. AncestryDNA has some good genealogy tools that help us with our DNA Matches.

One of the powerful tools is ThruLines. Ancestry uses this tool to analyze your Tree, each Match’s Tree, and every other Tree in its inventory to try to build links to a Common Ancestor (CA) for you and each Match. This includes finding CAs with private, but searchable Trees, and with small Trees that may have only a Match’s parent or grandparent. In all cases Ancestry will try to fill in any gaps between the CA and you and/or your Match. The result is a diagram showing how you and the Match are related through a CA, along with reference material to indicate how they determined any generations they used to fill in a gap. This is a powerful tool, which can be used in a variety of ways.

I’ve written about:

  • How ThruLines Works, here.
  • Helping ThruLines help you, here.
  • ThruLines Xray vision (into private Trees), here.
  • Adding ThruLines info into Match Notes, here.
  • Using ThruLines and Shared Matches to form Clusters, here.
  • Using ThruLines to Extend the MRCA of a group, here

All of these posts use Matches who share DNA segments with you, and ThruLines adds the added dimension of genealogy – using the power of Ancestry’s huge database of Trees. ThruLines usually uses multiple Trees which are in agreement. This is a good, easy, place to start – a good hint – but like all such “hints” you should validate the result. Yes, some of the Trees are flawed, but most are not. Based on my 45 years of genealogy research, I’ve found ThruLines to be correct about 95% of the time.

This post is about another way to use the power of ThruLines – checking on a suspicious branch of your Tree.  Suppose you have a Tree and have been documenting Matches who have Common Ancestors with you. And you notice that one branch of your Tree isn’t getting as many Matches as you expected. It may be because the branch is one that recently immigrated to the US; or because the Ancestors in the branch had relatively few children. Both reasons would tend to reduce the number of Matches from that branch. But if you’ve ruled those reasons out, what’s left? Well, the elephants in the room are a non-biological parental relationship in your Tree (an NPE or MPE) or faulty genealogy research.

One way to check a suspicious branch is to use ThruLines, as follows:

1. Determine the Ancestor who is the base of the suspicious branch – use your judgment.

2. Click on the child who is your Ancestor

3. Open that (child) Ancestor’s profile page

4. Open the Edit tab (top right)

5. Select Edit relationships

6. Click on the X next to the suspicious parent(s) – one or both

This will remove the suspicious branch from your Tree. It will also preserve all the work you’ve done on that branch, and at any time you can easily go back to the child (#2 above) and add the parents back in using the Select someone in your tree option (just type in the names you had before and select them).

After you’ve removed the suspicious branch,  just wait a few days. ThruLines will try to find Matches who are cousins from this line and will identify Potential Ancestors the fill in any gaps. This works out to the 6th cousin (6C) level – your 5xGreat grandparents. If ThruLines identifies Potential Ancestors who were the Ancestors you originally had – well nothing lost (but be sure to use the Select someone in your tree option to get back the branch as you originally had it). If ThruLines identifies alternative Ancestors – well then, you’ve got some work to do to understand more about those Ancestors and decide which Ancestors to use. Remember the ThruLines version is just a “hint” – it’s still up to you…

What I would do is accept the ThruLines Potential Ancestors (later, they can always be deleted or removed from the Tree with Steps 1-6 above) and see if I got a more ThruLines Matches than I had before. If so, these ThruLines Matches would have Trees that should be reviewed for additional evidence. My go-to evidence is the census records if they are available for these new Ancestors – are the times and locations appropriate? At this point, this is mainly a genealogy exercise, although a review of relationships and Shared cMs should also be done.

This is yet another way to use the power of ThruLines. It’s not guaranteed to work, but it does give you a quick and easy look into the huge Ancestry inventory of Trees for potential alternatives.

[22BB] Segment-ology: Do You Have a Suspicious Branch in Your Tree? TIDBIT by Jim Bartlett 20210928