The previous article in this series is Introduction To Using GEDmatch Part 4 (Alternate Way To Find People Who Match One Or Both Of 2 Kits).
This week, I happened to come across a fantastic blog post by Sue Griffith. In her post, she demonstrates an alternate method to the GEDmatch triangulation tool for finding overlapping segments. I really like her methodology, but the explanation of the steps seemed like it could possibly be a bit overwhelming for some people. With this post, I want to walk through Sue’s method step by step, providing additional information and screenshots, so others can also begin to take advantage of her great way to find segment overlaps.
Click here if you’d like to open the GEDmatch website in a new window/tab to make it easier to follow along with these steps.
1) After signing in to the GEDmatch website, click on the ‘One-to-many’ matches link.
2) Clicking on the ‘One-to-many’ matches link will bring up the One-to-many comparison page. Enter your kit number in the first box, leaving the other two boxes as they are, and then click the Display Results button at the bottom.
3) Clicking the Display Results button will take you to a page that looks similar to the following screenshot. It will take a bit of time (up to a few minutes in some cases) for this results screen to come up. If you have waited at least five minutes, however, and the results screen has still not come up, hit the back button on your browser, re-enter your kit number and click the Display Results button again.
4) Your results page will contain a list (in table form) of up to 1,500 people who have uploaded their DNA test results to GEDmatch, and match you on at least one portion of your DNA. These matches will be in descending order, from the person who shares the greatest amount of DNA in common with you to the person who shares the least amount of DNA in common with you. In the fourth column, select the matches you would like to check for common segments with by clicking in each of their selection boxes.
You can select as many as you want for this comparison, but keep in mind that the more you select the longer it will take for the results page to be generated. A practical limit of the number of people to select is about two hundred. Selecting more than this can cause the results page to take so long to generate that it will not return results at all. (Note: for an easy way to determine how many people you are selecting at a time, follow the first steps in my previous blog post to copy your data to a spreadsheet, where the rows will be numbered for you.)
5) After making all your selections, scroll back to the top of the page and click the Chr-Browse button.
6) Clicking the Chr-Browse button will take you to a page which will state “Your results have been generated. Click HERE to display Chromosome Browser.” It will take a bit of time (up to several minutes in some cases, if you have selected a large number of people) for this results screen to come up. Click the HERE link to continue.
If you have waited at least ten minutes, however, and the HERE link has still not come up, hit the back button on your browser and click the Chr-Browse button again.
7) Clicking the HERE link will bring up a very lengthy page of results which looks similar to the following two screenshots (I’ve broken it up into two screenshots because of how long these two images are). It will take a bit of time (up to several minutes in some cases, if you have selected a large number of people) for this results screen to come up. If you have waited at least ten minutes, however, and the results page has still not come up, hit the back button on your browser and click the HERE link again.
The results page is showing, in both table form and graph form, all the segments you have in common with each of the people you selected from your one to many matches comparison. These common segments are shown for each of your chromosomes. It is important to note that this is not the same as triangulation. The segments you share with multiple people will line up vertically. You need to run a one to one comparison between each of the people sharing a segment, to verify they match each other on the same chromosome and segment as they share with you, to have a properly triangulated segment. If they do not match each other on the same chromosome and segment they are matching you on, this is indicating that one of them is matching you on the maternal side of your family tree, and the other is matching you on the paternal side of your family tree.
Another important point to keep in mind is the general consensus among the genealogical DNA experts is that a DNA segment needs to be at least 7 cM in length to properly qualify as a match. The default thresholds GEDmatch has in place will display segments as small as 3 cM when using this method. The 7 cM length recommendation of the DNA experts is in general (as other factors are also involved), and is not an absolute, but it is a very good guideline to follow for those of us who are new to using DNA for genealogy. In future posts, I’ll cover the topic of segment length more in depth, including explanations of the additional factors that help determine whether a shared DNA segment is truly a match.
Yesterday, GEDmatch announced the introduction of a subscription service (Tier 1) providing additional tools, including the triangulation tool. This is wonderful news, and I will be showing how to use the triangulation tool in an upcoming post. Even with the return of the triangulation tool, however, Sue’s method above is still an extremely useful addition to the triangulation tool, for a couple reasons. The triangulation tool is limited to comparisons between only the first one hundred of your matches, while Sue’s method allows you to select from any of your first 1,500 matches. By working your way through your one to many match list (two hundred people at a time), you can easily find the overlapping segments with all of your first 1,500 one to many matches, if you wish. In addition, the selection of people using Sue’s method does not have to be contiguous. For example, on my one to many matches page, I could select the tests of known relatives, and compare them against matches 1,000 to 1,200 of my match list (or any combination of people thereof that is no greater than about two hundred at a time). Her method gives you some great flexibility in doing comparisons, and if you come up with additional ways to utilize this flexibility, I hope you will share it here for everyone’s benefit, as well as give a thank you to Sue.
In my next post, I will show how to take all the match data obtained from Sue’s method and import it into Genome Mate.
If you experience any problems while following these steps, or notice that a screen has changed from what I’ve shown in the screenshots, please let me know by commenting on this thread or sending me an email. Thanks for reading.
The next article in this series is Using GEDmatch Part 6 (The Phasing Tool).
This article last updated 19 Oct 2014