Chromosome Mapping And GEDmatch: An Overview Of What They Are And What The Benefits Are

As I’ve been contacting the Ancestry DNA matches of the tests I oversee, I’ve found many people are interested in chromosome mapping and/or GEDmatch, but have questions about what they are, what the benefits are, and what the privacy implications are. My goal in writing this post is to address these questions and offer some simple, easy to understand answers.

What is chromosome mapping? Chromosome mapping is the identification of which specific portions of your DNA were contributed by which ancestors. I will cover how to do the chromosome mapping in an upcoming blog post. For an example of what a chromosome map look like, please see the one for myself that is posted on my website (

How does chromosome mapping help me in my genealogy research? There are two key things chromosome mapping helps with. First, when known relatives share the same ancestors, and the same parts of their DNA, this acts as a confirmation that each of their lineages back to the most recent of their common ancestors is quite likely to be correct. The DNA evidence will offer further support for the traditional sources and documentation you’ve found.

Secondly, once portions of your DNA are identified with specific ancestors, you can then look for additional people who also share this same portion of DNA with you. These additional people will share the same ancestors with you somewhere in their family tree (although they may not yet have identified them in their family tree, which I will talk further about in an upcoming blog post). These newfound relatives oftentimes have additional information which can be used to expand your tree further.

What is GEDmatch ( GEDmatch is a website which allows detailed comparisons of DNA to be done. It is the tool which identifies which portions of your DNA you share in common with other people and also allows you to search for other people who share the same portion of DNA with you. It is a free website run by volunteers, but donations are encouraged if you find the site helpful in your own research efforts.

Doesn’t the Ancestry DNA test already tell me who the ancestors are that my DNA match and I have in common? When you have a shared ancestor hint (indicated by the leaf) with a DNA match on Ancestry, this is simply an indication that you and the other person have at least one ancestor the same in each of your direct ancestral lines. While it can be an indicator of which ancestor contributed the portion of DNA you share with your match, it is not a certainty that the ancestor(s) Ancestry suggests is the same one associated with the portion of DNA you share with your match (see an example of this). You must use a tool where you can see the specific portions of your DNA that you share in common with your DNA matches to properly be able to determine which portion of your DNA came from which ancestors. Unfortunately, such a tool (commonly called a chromosome browser) is not currently available from Ancestry DNA. GEDmatch has such a tool. Family Tree DNA and 23andMe also have chromosome browsers.

What are the benefits of using GEDmatch? GEDmatch not only allows the proper comparisons to be done between two Ancestry DNA testers, but allows you to do comparisons with people who have tested at Family Tree DNA and 23andMe. This helps to expand the amount of people with whom you can identify shared portions of DNA. Since Ancestry DNA currently only allows testing in the United States, being able to do comparisons with Family Tree DNA and 23andMe testers can help you make connections in other parts of the world where our ancestors originally immigrated from.

GEDmatch also has several more detailed methods of searching for DNA matches than the rather basic searches that Ancestry DNA currently has. An example is searching for people who match you on a specific portion of your DNA, which is extremely helpful for locating previously unknown cousins along a specific line when you’ve already identified which ancestor contributed that specific portion of your DNA.

What about privacy? Should I be revealing personal information like this on the Internet? While privacy issues are certainly a very valid concern regarding anything on the Internet, there are some things to be aware of which I feel help to keep the privacy of your DNA test results under your control:

1) You are assigned a kit number when your test results are uploaded to the GEDmatch website. While anyone can see your kit number if you match the search criteria they have entered for finding a match, you are under no requirement to have your name associated with your kit number. The use of an alias is permitted. You can also elect to hide your email address from display, although this can greatly hamper the opportunity for collaboration with previously unknown relatives since anyone who finds you to be a match will have no way to contact you.

2) Any health related information contained in your DNA is not displayed unless you specifically elect to do so. [Edit 31 July 2014: As kindly pointed out by CeCe Moore in her comments to this post, medical information being reported in genealogical DNA test results was only an option through the tests 23and Me offered, and only with tests purchased from them prior to November 22, 2013. My thanks to CeCe for the clarification.]

3) The specific makeup of your DNA is not revealed. A DNA comparison in a chromosome browser will only tell which portions of your DNA are in common with someone else. Here is an example of a GEDmatch DNA comparison between myself and my grandmother:

GEDmatch Screenshot

Some words of caution: If you begin using DNA testing in your genealogy research, you need to keep in mind the possibility of discovering a person who is not actually related to someone who they believe they are related to. For a variety of reasons, what is gently termed a “non-paternity event” (frequently abbreviated as NPE in genetic discussions) may have taken place. The revelation of such an occurrence can have devastating effects. Care and discretion need to be used if you accidentally uncover one of these situations. There are many people who would rather not know about such a situation. If you are one of these people, I would encourage you not to use DNA testing for your genealogy without giving it some very careful consideration.

I’m hoping this post will serve as a general overview to chromosome mapping, and help to answer the most frequent questions and concerns regarding using the GEDmatch website and tools to do DNA comparisons. My next blog post will cover the steps involved in getting your Ancestry DNA test results uploaded to the GEDmatch website. Please let me know if you have suggestions for improving this post, additional questions, or ideas for things you would like to see covered in future posts. Thank you for reading.

The next article in this series is How To Download Your Ancestry DNA Test Results.

This article last updated 7 Oct 2014

3 thoughts on “Chromosome Mapping And GEDmatch: An Overview Of What They Are And What The Benefits Are

  1. Hi Dan, Great post! I have shared it. In regard to item #2 under your privacy section, I think it would be good if you clarify that it only applies to 23andMe tests purchased prior to Nov 22nd, so people tested after that date and/or with other companies need not concern themselves about that aspect of testing.
    BTW, upon reading this, I came across your posts about the Oyster Bay Wrights. I am descended from Rachel Wright, b. 1755 who is supposed to be the daughter of Peter b. 1712 of Oyster Bay. I haven’t had much of an opportunity to work on this line, but I have had my doubts about some of the connections I have found online. Might we be related?
    CeCe Moore

  2. Pingback: How To Download Your Ancestry DNA Test Results | Adventures In Genealogy Research: No Stone Unturned/The Wright Stuff

  3. Pingback: An Example Of The Unreliability Of Ancestry DNA Shared Ancestor Hints | Adventures In Genealogy Research: No Stone Unturned/The Wright Stuff

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