The Value of Comparing Two DNA Kits on GEDmatch Genesis

What can you learn while comparing two DNA kits on GEDmatch Genesis to assist in your genetic genealogy research? Discover the changes that occur when you match cousins of a known common ancestor, a parent-child, and a family with endogamy.

As genealogists, one of the things we’re research is how people are related to us or related to other people. With DNA, we can actually see how people might be related to us as far as which families or common ancestors without even knowing anything about that other person’s family tree. To do this, we need to have some matches that we already know how they’re related.

GEDmatch Genesis Has A Cool Tool for Comparing Two Kits

Previously, I’ve invited you to check out the tool on GEDmatch Genesis that can reveal if your parents are related. I’ve also explained the One-to-Many tool. This post talks about a tool that can help you filter if you know the common ancestor that you share with a close relative.

For instance, if you have close family members like first cousins or second cousins and you have identified which grandparents or great-grandparents the descend from, then you can start to classify your matches into groups of people that are related through your common ancestors. To do this, we’re going to use a tool on GEDmatch Genesis.

GEDmatch two kit comparison tool screen shot

In the DNA applications, there is a tool called “People who match both, or 1 of 2 kits.” Follow these steps to utilize the feature.

Step 1: Have Two Kit Numbers Ready

You need two kit numbers, such as yourself and one other known relative. I used my cousin. We’re related through dad’s side.

Step 2: Enter the Kit Numbers

On the “People who match both, or 1 of 2 kits” tool, you enter in the two kit numbers next.

Step 3: Set Thresholds

GEDmatch will ask you to define your threshold. Initially, I’m not gonna change those basic thresholds.

Step 4: Press “Display the results”

Click on the words “Display the results” to activate the “People who match both, or 1 of 2 kits” tool.

People who match both, or 1 of 2 kits” In Action

GEDmatch two kit comparison tool results screen shot
Notice how the kit numbers segment the data for shared matches

What Genesis has done is it searched through my DNA matches and compared my match list with my cousin’s match list. Then the program compiled the information into one listing.

Section One: DNA Kits That Match Both People

You will see the first set of columns is going to be the information about me, how much shared DNA, and what is the largest segment.

The next set of columns details the amount for my cousin. You will see different numbers in these columns that the first columns, which belong to.

I recognized my dad’s kit (the fourth row in the table). In the two columns, you will see that I match him at 3,500 centimorgans. My cousin matches him at about 1,800 centimorgans. That makes sense because my dad would be my cousin’s uncle.

On this page, this first list identifies DNA kits we both match. Since I know we’re related through my paternal grandparents, then all of DNA matches will be related through that line. Therefore, I don’t need to search my family tree on my mother’s side.

Section One: DNA Kits That Match Only the First Kit

As you scroll down the results page of “People who match both, or 1 of 2 kits,” the next section identifies people that only match me, and not my cousin.

BE CAREFUL!

One of the mistakes that people make here is that they assume that if a kit only matches you, not both of you, then these kits are exclusively related through my mother’s line. That is NOT the case in every instance. Many kits that only match me will be relatives through my mother’s side.

At the same time, some of these matches will be matches through my father’s line. The only distinction is that I match DNA with them while my cousin doesn’t.

Section One: DNA Kits That Match Only the Second Kit

Scroll further, and you will see the third section.

This section identifies kits that match my cousin but not me. Again, this doesn’t mean that these people will not match my paternal grandparents. I could be missing the segment of DNA that my cousin shares with this match even though we’re all related through the same common ancestor.

A Quick Review of DNA Inheritance

When you are matching third cousins to more distant cousins, you don’t share DNA with all of those cousins. You only share DNA with about

  • 90% of your third cousins
  • 45% of your fourth cousins
  • 15% of your fifth cousins

There may be a lot of these people on the list of matches only to my cousin with whom I’m still related. I just don’t share DNA them.

How Does Comparing Two Kits on GEDmatch Genesis Help Your Research?

This “People who match both, or 1 of 2 kits” tool can be handy, particularly for people who don’t have Tier 1 access. If you have tested one of your parents and transferred their DNA to GEDmatch, you can phase the matches on your DNA.

When a change the kits compared from my cousin to my mom, I get different results. When comparing myself to my cousin, I was focused really on the matches both kits because all of those matches will be through my father’s line.

When comparing my mother’s DNA and mine, I will focus on the list of matches that match me but don’t match my mom. These kits have to match through my father’s line. This list will be the most useful, particularly if I don’t have the DNA of one of your parents.

Experiment with the Thresholds Comparing Two Kits on GEDmatch

After you have started to sort your matches based off of other known relatives, you can start playing with those thresholds and see what additional information you can pick up. There are two thresholds to explore.

Largest Segment Threshold

The first focuses on the largest segment. This is how big of a segment does somebody have to have to call a match. You may share a lot of DNA with someone, but if it’s all in tiny segments, the tool might not call the match.

Typically, I leave this threshold at 10. Occasionally, I will decrease the threshold to about seven if I have a large family. For smaller families, you might want to increase this number a little bit.

Total Matching Segments Threshold

The threshold I adjust the most frequently is the total matching segments. This threshold examines all the segments that you match with and how much cm do you have to share in order to call this a match.

For instance, I may have a large segment of ten, but maybe I just want to look at matches that share at least 30 percent.

This threshold is valuable for families that have endogamy. In such cases, you have so much shared DNA, and a lot of that may be in just little segments. The higher you set your total matching segment threshold, the more you will eliminate out some of those matches who may discover you are very, very distantly related. For a further explanation of this tool and how it applies to endogamous families, watch the video.

Learn how comparing two kits on GEDmatch can help you determine which ancestors three or more kits share in common. #geneticgenealogy #dnamatches #gedmatch
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Comparing Two DNA Kits on GEDmatch Genesis Helps You Classify Your Match Results

The comparison of two kits tool on GEDmatch Genesis is a great way to help narrow down your matches and start to classify people into different family groups. If any of your relatives have already tested, use their kits combined with yours to see how your other matches might be related to you.

If you have any questions about how to use this tool, put them in the comments for this post.

Andy Lee

Andy Lee

Author, Engineer, Educator. Andy Lee is the DNA guru of Family History Fanatics. He is also your knowledgeable but casual genealogy researcher.

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