There are many reasons that you might want to use the Gedmatch One-to-One Comparison tool. Whether you know you should use it to double-check the results from the One-to-Many tool, or you want to compare your DNA to a specific person, the One-to-One tool provides a wealth of valuable information. In this post, I will discuss what you can learn from using this tool.
Before you contact any of your DNA matches on the One-to-Many list, you should always run this One-to-One tool first. This way, you will have as much knowledge as possible as to how you might be related. Knowledge is power.
In order to help me explain the results, I have a partial screenshot of a comparison that I ran of my DNA and my dad’s first cousin’s kit. He is a known first cousin once-removed to me, and he was the first person from my known family to take a DNA test. When I first got my Ancestry DNA matches back and he showed up in the 1st-2nd cousin category, I knew that this “whole DNA thing” was accurate.
To use Gedmatch, you have to first take a DNA test
You might already know this, but I thought I would first mention that Gedmatch is not a DNA testing company. Instead, you must first take a DNA test with another company. Then, you can download your raw DNA data file and upload it to Gedmatch.
If you need to get started on a DNA test in order to use Gedmatch (now Gedmatch Genesis), you can use the following links to my recommended DNA testing companies. Any of the following companies will allow you to download your raw data file once your results are complete.
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Largest segment shared
In the image above, you can see that me and my match share a segment of 62.6 cMs (centimorgans) in length. This is actually a large segment, and if you find that you share a segment this large with your match, they are not distantly related to you. Most likely no further than 2nd cousins.
The larger the segment, the closer the relationship. The smaller the segment, the more distant. When people have children, their children get the DNA passed down in nice, long solid segments. For example, I share 281.5 cM segments with both of my parents. When I had kids, they each got a randomly selected portion of my DNA, and they got nice, long segments, too. There is a very high probability that my children and my sister’s children will share large “chunks” of these segments, as well. Each generation, however, the DNA gets broken up, and some of it doesn’t get passed down. That’s why the big chunks are important – the bigger they are, the closer the relationship.
Total of segments
You can see in the example that I share 610 cMs with my dad’s cousin. This amount of DNA shared with a first cousin once-removed is actually on the high end of the average range. This only means that I randomly got a bigger chunk of those segments from my dad than I could have. In theory, I could have inherited a larger portion of my dad’s DNA that didn’t match this cousin – but I didn’t.
The total number of cMs shared is important because it also gives you a clue as to how distantly (or closely) you are related. In general, the larger the number of cMs, the closer the relationship. If you share lots of small segments with someone, however, it is possible that you are related in more than one way. It’s a crazy thought, but it is possible to be 4th cousins once-removed AND 6th cousins, twice-removed with the same person, just on different lines. DNA can really help you figure out things like this.
The tool is set to only pick up on segments that are larger than 7 cMs, and it will provide you the total number of segments that match this criteria. This is important because some segments smaller than 7 cMs are “coincidentally” identical to others, and other smaller segments are valid but too far back to reliably trace, even under the best of circumstances.
Number of matching segments
The number of matching segments that I share with my dad’s first cousin is 18. I share even more than that with my sister and my parents. The takeaway from this, just like in the paragraphs above is the more segments that you share, the closer the relationship. Especially if the total amount of segments is high, and the size of the largest segment is high. There is definitely a pattern here.
So in general:
- Large number of matching segments, and a few large segments = close relationship
- Small number of matching segments, but at least one large segment = it is still possible to be a relatively close relationship
- Large number of matching small segments = possibly related in more than one way, perhaps distantly
Estimated number of generations to MRCA
MRCA= Most Recent Common Ancestor
The last number you see in the example that I shared is “Estimated number of generations to MRCA”, and in my comparison with my cousin, it is 2.3 This is actually really very accurate in this particular case, since I know that my great-grandparents are his grandparents. This tool is usually very accurate in estimating relationships when there are no “half” relationships involved and the relationships are close. Full siblings, parents, full cousins and second cousins, for example, will usually fall within the correct estimated number of generations on this tool.
The problem lies when the tool is trying to estimate generations between more distant cousins. It’s possible that 4th cousins share higher amounts of DNA than usual, for example, and in this case, the tool could throw you off and lead you to believe that you are actually more closely related than you are. It’s also very tempting to want to believe that the tool is correct, too, since it’s much easier to search through 8 great-grandparents searching for a common relative than trying to sift through 64 great-great-great-great grandparents (assuming that they are all known, which isn’t always the case).
The MRCA estimate is a very useful tool, but the software is only basing its estimate on the number, size, and total of shared segments and making its best guess. When we are understanding the results, we need to take that into consideration.
I always like to include a word of caution along with discussion about shared DNA and relationships. I’ve mentioned this in a few other posts, but it’s important to talk about again here. I have a personal example of how a not-so-distant relationship can share almost no DNA. My daughter and her 1/2 2nd cousin twice-removed share only a 6 cM segment. If she didn’t know that this was her grandmother’s 2nd cousin, she might be tempted to ignore a matching segment so small. It turns out that my mother’s great-grandfather remarried and had a son with his second wife, meaning the children from those two marriages shared 50% less DNA right from the start. There was much less DNA to be inherited each generation, and now my daughter shares almost no DNA with that cousin.
I hope that this article helped shed some light on what you can learn from the Gedmatch One-to-One comparison tool. If you have any questions about what you have seen in your own comparisons, I would love to hear from you in the comments.
Thank you so much for stopping by!