7
Oct

What is Overlap on Gedmatch?

Do you want to know what overlap means on Gedmatch results? In this post, learn all about overlap, including low overlap and overlap cutoff.

Understanding Gedmatch tools can seem complicated at first glance, especially if you are new to the Gedmatch site. However, if you learn about each of the terms and concepts, you will find that understanding all of the DNA details is not difficult at all.

What is Overlap on Gedmatch

The word “overlap” appears all over Gedmatch tools and results. For example, you might have noticed that there is an entire column for overlap in the One-to-Many results.

What do all of these large numbers mean? Is higher better? What does it mean if there is low overlap?

Let’s find out.

What does overlap mean on Gedmatch results?

Every DNA testing company tests hundreds of thousands of positions in your DNA, called SNPs. SNP is short for single nucleotide polymorphism.

These SNPs (pronounced “snip”), sometimes also referred to as “copying errors”, are the locations in our DNA where we (humans) are different. Companies choose specific SNPS to test because they are known locations where SNPs vary from person to person.

Some of the SNPs tested by the various testing companies will overlap with those positions tested by others. This is one reason why we see slightly different numbers for shared centimorgans between DNA matches that have tested with multiple companies.

All of the major DNA testing companies test approximately 700,000 SNPS of the 4-5 million SNPs that the human genome might contain. According to one chart, the exact number of SNPs tested between companies currently ranges from about 612,000 to 702,000.

The exact number of your SNPs that were tested will depend on the company that you chose to test with and the version of test they used. As technology improves, testing more SNPs becomes more feasible.

The Gedmatch tools can only analyze regions with testing “overlap”. The number of SNPs reported in the overlap column is the total number of SNPs within the region where the shared DNA is found, though it doesn’t mean that every single SNP is identical.

We like overlap. The more overlap, the higher quality the DNA match.

We will also notice that higher overlap can coincide with a higher total amount of shared DNA. In addition, people who tested with the same DNA testing company will find higher overlap of SNPs.

How high can overlap be? Pretty high.

For example, take a look at the image below. It’s from my Gedmatch One-to-Many results.

Two of my matches are highlighted by red rectangles. One is my mother and the other is a 2nd-4th cousin, approximately.

The top match in my Gedmatch One-to-Many results is my mother. We share more than 3500 cMs and the overlap is more than 320,000 SNPs.

We would expect a very close match to share lots of cMs and have a lot of overlap. It’s important to note that we can see high overlap in more distant matches, however.

The other match that I highlighted in the image above is a more distant cousin. We both tested with Ancestry DNA (important, since SNP locations are largely the same), and share only 30.5 cMs with an overlap of more than 300,000.

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What does “overlap cutoff” mean on One-to-Many tool?

When we begin using the One-to-Many tool on Gedmatch, we see that we have several options, including the option to adjust the overlap cutoff. Adjusting the default settings for the tool will provide different results.

The default cutoff number for SNPs is 45,000. You can choose 56250, 72000, and 90000.

To adjust the overlap cutoff, just click the down arrow to expand the dropdown menu and choose the cutoff level you desire
45,000 is the default cutoff number of SNPs

If you choose a higher number of SNPs for the overlap cutoff, you will receive a smaller number of matches. The quality of the match (i.e. the probability that you are descended from a common ancestor in the genealogical time frame) will conceivably be higher, however.

What does it mean when the overlap column is pink on Gedmatch?

When the cell in the overlap column on Gedmatch is pink, it means that there is “low overlap” shared between you and your DNA match on the site. This simply means that there is a low number of SNPs in the region where you and your match share identical DNA.

Low overlap between DNA matches on Gedmatch can often signal a low quality match – meaning that the shared DNA reported may not be significant. Do not automatically ignore low overlap matches, however.

For example, if you tested with Ancestry DNA and your match tested with 23andMe, you might notice that the overlap column is either pink or has a lower number in it. These are both high quality tests and as long as the shared DNA is significant, you could research the connection.

This image shows the low overlap column on my Gedmatch One to Many results, and some of the matches have low overlap (shown in pink)
The pink overlap column shows low overlap for two matches who tested with 23andMe

The matches shown in the image above have low overlap. I noticed that they tested on 23andMe, so I looked them up on my DNA match list on 23andMe (the test I uploaded to Gedmatch was from Ancestry DNA).

These matches were on 23andMe, though the number of shared centimorgans was slightly different than what showed up on Gedmatch. The point is that you shouldn’t always ignore low overlap matches.

What is Overlap on Gedmatch pinterest image

Conclusion

Did this article help you understand more about “overlap” on Gedmatch? I hope you learned enough about it to be able to understand your results more completely.

If you have any questions about something that you read in this post, or if you would like to add your own insight about overlap, I would love for you to join us in the discussion below.

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