How To Use The GEDmatch Shared Matches Tool

The shared matches tool is part of the free tier on the GEDmatch site. GEDmatch refers to shared matches as “people who match both kits”.

You probably use a similar feature on the website which tested your DNA. The GEDmatch version has some great additional features.

What Are Shared Matches on GEDmatch?

When two kits both share DNA with a third kit, this is known as a “shared match”. It’s also known as a match “in common”.

Although two kits may have a match in common, it’s important to remember that this may not be due to all three kits having the same ancestors.

Let’s say Jim and Joe both have Mary as a match in common. Mary may share DNA with Jim through her paternal line, and with Joe through her mother.

Why Are “People Who Match Both Kits” Useful To Your Research?

I’ll give two examples of how useful this report can be. These are from my own experience of using the tool.

Finding shared matches with family trees

Let’s say you have a DNA match of 300 cM, but you can’t tell how you’re related. The kit owner hasn’t uploaded a tree to GEDmatch and hasn’t replied to your polite message.

So, you run the “people who match both kits” report and work your way down the list. You find a shared match who has a family tree going back six generations.

When you use other GEDmatch tools (e.g. the One To One Report) to examine the relationship between this match and the mystery kit, you realize they are probably first cousins (to each other, not to you).

If you can find the intersection between the trees, you will be able to identify how both these matches are related to you.

Finding shared matches who reply to messages

The report may show you a little cluster of closely related DNA matches.

The first match that grabbed your interest may not reply to your message. But now you have several more options to try communicating with.

How Do You Launch The Tool?

The tool can be launched from the Home Page.

The link is under the free tools section, and is named “People who match both or 1 of 2 kits”.

Report Parameters

The mandatory parameters to run the report are the two GEDmatch kit numbers.

Genealogy Quotes By Writers x
Genealogy Quotes By Writers

You’ll often want to look at common matches while you’re reviewing the One To Many list. I like to keep the One To Many report page open, and use a second browser tab to launch “People who match both or 1 or 2 kits.” This makes it easy to copy and paste kit numbers from one report to the other.

All the other parameters have defaults. Let’s look at when you should alter them.

cM threshold of largest segment

This parameter sets the threshold of shared DNA for a match to appear in the report.

The default is 10 which may suit you if you have thousands of DNA matches above this threshold.

Personally, I only have about five hundred GEDmatch relatives above at or above 10 cM. So, I often drop this threshold parameter to 7 cM.

If you also lower the threshold, always bear in mind that low cM matches could be false positives (i.e. the shared DNA is due to chance and you are not related).

cM threshold of total matching segments

Two kits may have shared DNA on different chromosomes or different areas of the same chromosome.

This parameter may be a little confusing, so I’ll explain it with an example.

Suppose you drop the largest segment threshold to 7 cM and leave this total threshold at 10 cM. This means that if two kits share 7 cM in one area, they will only appear on the report if they have a second common segment of at least 3 cM.

Setting parameters like that doesn’t make much sense and may eliminate plenty of useful matches.

I suggest that you use the same value for the largest and total thresholds unless you have an advanced scenario in mind.

Difference in generations

GEDmatch reports usually include an estimate of the number of generations between you and your DNA match.

This parameter looks at the difference between the estimates for you and both kits. The default sets the threshold to a very high value.

You should leave this parameter unchanged unless you’re dealing with a high level of endogamy with the matches.

How To Read The “People Who Match Both Kits Or 1 Of 2 Kits” Report

This report is in three parts:

  • Kits that match the two input kits
  • Kits that match the first input kit but not the second
  • Kits that match the second input kit but not the first

I’ll give you tips on using each part of the report. But first I’ll go through the information in the display.

Each match in the display shows their kit number, name, and email with further info about the amount of shared DNA:

  • Shared: the amount of shared DNA in centimorgans.
  • Largest: the number of centimorgans in the largest segment
  • Gen: an estimate of the number of generations between the kits

Don’t pay too much attention to the generations estimate. It’s not particularly accurate.

Now that I’ve looked at the columns, I’ll take a closer look at the three different displays.

Using The “Matches Both Kits” Report

The first part of the display shows the kit numbers that match the two kits you provided. This is what we’re usually looking for when reviewing shared matches between two kits.

Finding shared matches with pedigree information

The main strategy is to find shared matches that have a public family tree on GEDmatch or another site.

Then you use traditional genealogy research to identify the common ancestors. You may need to stitch together branches from several different trees to find the common ancestral line.

You may need to hunt for the DNA matches on sites like Ancestry.com, MyHeritage, or FamilyTreeDNA. The “Matches Both Kits” report doesn’t show you the source site for the kit. Instead, you’ll find that information in the “One To Many” report.

Our article on using the One To Many tool on GEDmatch will show you how to identify the source.

Unfortunately, it’s not always easy to identify the same users on different sites. When you’re in luck, your DNA relative will use the same name in different places.

Separating maternal and paternal matches

You may not have a parent available for DNA testing. But you may have identified a handful of matches that you know fall on either your mother or father’s side.

Run the “Matches Both Kits” report on these known matches (with your own kit, of course).

You can examine the shared matches with the extra knowledge of which side of your tree you are dealing with.

Just watch out for situations where a match shares DNA with you on both sides.

Using The “Matches 1 Of 2 Kits” Report

When you scroll past the “Matches Both Kits” display, you’ll find displays for kits that only match one of the kits.

This can be very helpful when a parent and child both have kits on GEDmatch. If you match the child and not a parent, then you know that you are related to the child through their other parent.

You can spot this when you run the “One To Many” report for the child kit. Of course, the parent kit will be top of their list.

Warning: Confirm Matches With The One To One Report

You may find people that show up on the “Match Both Kits” but don’t show up on your One To Many Report.

Even worse, they may show up on both these reports, but they are false positives (i.e. not related to you).

The most accurate GEDmatch report is the “One To One” Report. When you’re dealing with DNA matches with lower centimorgans, you should always run this report before researching them in depth.

Our article on how to use the One To One Report on GEDmatch gives more explanation as to the discrepancies between the different GEDmatch reports.

Differences Between Shared Matches On GEDmatch And Ancestry.com

If you tested with Ancestry, you will notice differences in the shared match list on Ancestry.com and the “Match Both Kits” report.

GEDmatch will usually show you a lot more shared matches for every kit. This is because Ancestry’s threshold for shared matches is 20 cM.

This is one of the great benefits of GEDmatch for Ancestry users. But don’t get confused when you recognize kits on both sites that are “shared” on GEDmatch but not on Ancestry.

Triangulation

The drawback of shared matches is that you don’t know if you’re all descended on the same line.

GEDmatch has other tools that let you look at groups of matches who are likely to have inherited the same piece of DNA from a common ancestor. This research method is known as triangulation.

It can seem to be a little complicated, but if you’re used to reviewing your shared matches – you’ll find a lot of similarities with the process.

We have a separate article that gives examples on using triangulation on GEDmatch.

More Articles And Tutorials?

Our articles and tutorials cover the many different features of the GEDmatch platform.

Check out our guide on how to use GEDmatch which gives you an overview of the site with links to the tutorials on specific tools.

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