The previous article in this series is Using Genome Mate (Importing/Updating the email addresses of your matches from GEDmatch).
In following the steps that have been previously outlined, we now have imported a sizable amount of data from GEDmatch into Genome Mate. This post will show how to begin working with this data to identify groups of triangulated segments. The process of identifying triangulated groups can be very time consuming, and like genealogy itself, is something that is never ending. The concepts of identifying triangulated groups are fairly simple to grasp, but there is no shortcut to taking the necessary time to work through and analyze your DNA data. As one leading genetic genealogist, Dr. Blaine Bettinger (http://www.thegeneticgenealogist.com), summed it up so accurately, “Finding genetic matches is easy, but finding the common ancestor from whom we inherited a segment [of] DNA is very hard.”
There can be many factors that go into determining whether or not a DNA segment is indeed a valid match with another person. My goal with this post is to demonstrate the basic principals of triangulating DNA segments, and provide a general overview of the process, rather than overwhelm the reader with all the many small details that can quickly overwhelm a genetic genealogy newcomer. To this end, knowledgeable readers will recognize that I’ve simplified some of the concepts of DNA matching shown in this post, and that I’m not showing the GEDmatch triangulation tool. This is by design. We need to learn the basics before we can move beyond them into all the nuances of triangulation. In future posts, I will cover the more detailed concepts of DNA matching, or provide links to where these concepts are covered more in depth, for those wishing to continue with learning about all the fine points of genetic genealogy. I will also show how to use the GEDmatch triangulation tool in an upcoming post. First, let’s get a bit more acquainted with how Genome Mate displays our matches.
After having imported some GEDmatch data, the main Genome Mate screen will look similar to this screenshot. Unless you’ve changed the default, the segment data will be displayed as a graph, which facilitates quick visual identification of where overlapping segments occur. While we are viewing this screen, let’s look at three items that will be helpful as we move forward with identifying triangulated groups and confirming segments.
The first item to take note of is the chromosome number dropdown box, as I’ve highlighted by the red rectangle. This dropdown box is used to step through all the chromosomes to see the matches you have on each chromosome. The chromosome selections are numbered 1 through 22, plus there is also an additional X chromosome selection.
The next item to take note of is the Start and End columns for the segments you share. The start and end points listed on the graphical display of the Genome Mate screen are rounded to millions. For example, the segment I share with PA340556P1 Lawrence Stone actually has a start point of 11,244 and this is displayed as 0 (aero) in Genome Mate since the actual number is less than one million. The segment I share with PA340556P1 Lawrence Stone actually has an end point of 63,374,274 (displayed as 63). The segment I share with A242029 R. E. H. (administered…) actually has a start point of 12,171,406 (12) and an end point of 24,425,401 (24). The column to the right of the End column is the cMs column, which is commonly referred to as the length of the segment, as expressed in centimorgans (cMs).
To see the full start point and end point for a segment, move your mouse cursor over the relative name or the segment bar, and a box will appear showing the specifics of the segment.
The last item to make note of is the graphic display of your chromosome map for this chromosome. The upper bar is the paternal side of your ancestral line, and the lower bar is the maternal side of your maternal line. Both bars start out as gray, and as you confirm your matching segments, colored portions will be added to the bars (as shown in this example). How to confirm segments, and fill in your chromosome map, will be demonstrated in the next post following this one. Now, let’s get started with the process of identifying triangulated groups.
As you step through each of your chromosomes in Genome Mate, using the chromosome number dropdown box, you will be looking for groups of segments that overlap vertically, as I’ve highlighted by the red rectangles.
We need to check each of the segments that line up vertically, to make sure they match each other, in order for them to be considered a triangulated group. An important concept to remember is that each of us inherits a set of chromosomes from both our mother and our father. Therefore, it is not possible to identify whether a segment you share with a match is from your mother’s side of the family, or from your father’s side of the family, by just looking at the segment itself. The determination of which side of your ancestral line the match falls on is done by comparisons with other matches.
For each group of overlapping segments you find, you need to identify which of them also match each other. When they match each other it is referred to as in common with (ICW). You can verify whether two people are ICW by running a GEDmatch ‘One-to-one’ comparison between these two people. Check the ‘One-to-one’ comparison result to make sure the same chromosome and segment these two people are sharing with you are also shared between them. If so, there is a common ancestor, or set of ancestors, shared by the three of you. When three or more people share the same chromosome and segment, this is called a triangulated group (sometimes referred to as TG). Anyone else who also shares the same chromosome and segment with a member of the triangulated group will also share the same common ancestor, or set of ancestors, and can be added to the group.
Note that although we’ve identified a triangulated group, we still do not know yet whether the common ancestor, or set of ancestors, for this group falls on the maternal side or paternal side of the ancestral line. If one or both of your parents are alive, and agreeable to taking a DNA test, it will greatly assist you with determining which side of your ancestral line your matches fall on, and is something I would highly encourage you to do. Even without DNA tests of one or both of your parents, however, it is still possible to determine which side of your ancestral line a match falls on, although the process can be a bit more involved.
After you’ve identified a triangulated group, even though we have not yet identified which side of your ancestral line they match on, if someone else matches with you on the same chromosome and segment as the members of the triangulated group, and does not match another member of the group on the same chromosome and segment, we will know that they are matching with you on the opposite side of your ancestral line from the side the triangulated group matches on. Let’s work through an example.
In this example, we will be working with the three overlapping segments at the far right, which I’ve highlighted in red, and labeled A, B, and C for reference (since I’ve blurred out the GEDmatch kit numbers of all the matches shown here).
I start by doing a GEDmatch ‘One-to-one’ comparison between my kit and kit A, making note of the Start Location and End Location. You’ll notice that the GEDmatch ‘One-to-one’ comparison tool displays the full start point and end point of matching segments, rather than rounding to millions as Genome Mate does in the main Genome Mate screen.
I next do a GEDmatch ‘One-to-one’ comparison between kit A and kit B, and check the chromosome number, and Start Location and End Location, of any matching segments. In this case, kit A and kit B match each other on the same segment of chromosome 2 that I match kit A on. This is an indication of shared ancestry between the three of us, and forms a triangulation group. [Note: Keep in mind that, due to several factors, the start locations and stop locations reported by GEDmatch are approximate, and not absolute. As long as the start locations between two segments are reasonably close (within about four of each other when rounded to millions), and the stop locations for the two segments segments are reasonably close (within about four of each other when rounded to millions), you can be comfortable in saying the segments are a match.] With this example I’m using, the Start Location is 181 when comparing my kit to kit A and 180 when comparing kit A to kit B (a difference of three million when rounded), while the End Location is 202 when comparing my kit to kit A and 199 when comparing kit A to kit B (a difference of one million when rounded), so I would consider the segments to be a match since the difference in start locations between the two segments is less than four million when rounded and the difference in end locations between the two segments is less than four million when rounded.
I then do a GEDmatch ‘One-to-one’ comparison between kit A and kit C. They do not match each other on any segments at all. This is an indication that kit C shares common ancestry with me on the opposite side of my family tree from the side I share common ancestry with kit A and kit B. Therefore, kit C is not part of the same triangulation group that kit A and kit B are a part of.
As you work through your matching segments checking for triangulation groups, when you do a GEDmatch ‘One-to-one’ comparison between two kits, be sure to verify that any matching segments between them are the same segment and chromosome you match match each of them on. If I got the result shown below when I did the above GEDmatch ‘One-to-one’ comparison between kit A and kit C, this would be an indication that kit C shares common ancestry with me on the opposite side of my family tree from the side I share common ancestry with kit A and kit B. The segment that kit A and kit C share is on a different chromosome (chromosome 1) from the segment I share with each of them (chromosome 2). Also, in this case, even if the matching segment happened to be on chromosome 2, the segment has a dramatically different start location and stop location than the segment I share with each of them on chromosome 2, and this would again be an indication that kit C shares common ancestry with me on the opposite side of my family tree from the side I share common ancestry with kit A and kit B. Therefore, since kit C does not match kit A, I know that kit C is not part of the same triangulation group that kit A and kit B are a part of.
Once you’ve identified a triangulated group, the next step is to contact each of the testers in the triangulated group (in this case, kit A and kit B) to ask them about their ancestral lines. We are trying to determine which ancestor(s) of ours is also an ancestor of each of them. Sometimes this is rather easy to determine, but the majority of time it is not immediately obvious. Contacting your matches, and working with them to determine who you share as common ancestors, is where the greatest amount of time is spent in genetic genealogy research. Keeping track of your contacts with matches, using good notes, is essential. Genome Mate provides several tools to help with this process, and these will be covered in depth in future posts, but for now know that you can enter notes for a match by clicking on their name or segment bar, which brings up a details screen with a box for research comments at the lower left. In the next post, we will use this details screen to confirm our triangulated segments and add them to our chromosome map.
As I mentioned earlier in this post, there are many factors which can go into making sure a segment is a valid match. I will be covering these factors in upcoming posts. For the time being, in order to maximize your success rate in identifying the common ancestor(s) of a triangulated group, my suggestion is to limit yourself to working with segments that are 10 cMs or greater in length. My reason for suggesting this is that statistics indicate segments at least 10 cMs in length are 99% certain of being a valid match. Even by limiting yourself to these larger segments, you should still have plenty of segments to work through, and once you’ve identified the MRCA for these larger segments, they will be very helpful for identifying the MRCA of the smaller segments after we’ve covered the factors that help determine a smaller segment is truly a valid match.
In addition to limiting yourself to segments 10 cMs or greater in length, I would also suggest you skip working with any segments for which the ICW appears in the main Genome Mate screen as orange or yellowish/brownish in color (as shown in the screenshot below), unless it is a known close relative (second cousin or closer in relationship). The significance of these orange and yellowish/brownish indications will also be covered in an upcoming post.
Congratulations, we now know how to work through our matching segments and identify triangulated groups. In the next post in this series, we will learn how to confirm our triangulated segments and add them to our chromosome map. [Note: When you have finished a Genome Mate session, it is a very good idea to get in the habit of always backing up your database file before moving on to something else and/or exiting the program. You can never have too many backups of your data to fall back on, if needed.]
If you experience any problems while following these steps, notice that a screen has changed dramatically from what I’ve shown in the screenshots, or have an easier/better way to do the steps shown, please let me know by commenting on this thread or sending me an email. Thank you.
The next article in this series is Using Genome Mate (Add confirmed segments to chromosome map).
This article last updated 31 Dec 2014