I think the switch from Ancestry.com DNA test results, where your tree gets searched for you, making using DNA with genealogy easy – to GEDmatch where you have to figure out how to use the data yourself, is quite difficult. So this post is an attempt to help my cousins who have tested at ancestry and uploaded to GEDmatch. It might also help others new to GEDmatch who want to look at where they match a [possible] cousin by walking through that process.
- First make sure that you understand current DNA basics (click here for my page on that). Genetics have advanced greatly since my high school biology class and perhaps since yours too.
- Next realize that the raw data from your test is only a small part of your genome, a sampling. It is the SNPs that are currently considered the most interesting. They represent the most likely spots where we are different from each other. If a contiguous sequence of those SNPs is the same in two people for about 10 centimorgans (cMs) or more then they are expected to share a common ancestor. With a match of 7-10 cMs it is likely but not a sure thing. There is a good article in the ISOGG wiki on the likelihood of a match at different segment sizes.
- In order to see where your DNA matches someone else’s, you need your kit number and his kit number. Then you can use the one-to-one comparison to see on which chromosome(s) you match each other. Your kit number shows on your GEDmatch homepage. You can find the kit numbers of other possible relatives in the one-to-many display or perhaps your new cousin has sent you his kit number.
- I recommend that you keep a spreadsheet with the information on your matches, sorted by chromosome and start point, so you can see who else a new match might match. I have a number of posts on this blog about using spreadsheets and a template in my downloads area. Many people like to use the genomemate tool to organize their data.
The image above is from a recent new match to my Dad uploaded from ancestry.com. The blue rectangle shows where there is a DNA match. The numbers in the box are what I cut and paste into my master spreadsheet for Dad.
Looking at my spreadsheet I see about 15 other folk matching on that segment, none of whom I have found a known ancestor with, but wait! Further up the sheet is a 37 cM match that continues down to this one. It is with my known 3rd cousin Katy so that puts this match on my WOLD line if he also matches her there. It is called triangulation when you have a three-way match. So next I have to do a one-to-one comparison of my new match with cousin Katy:
It’s a match! Now I will compare him to some of my other cousins on the WOLD line to see if he matches any of them elsewhere. I actually have a few distant cousins descended from the ancestors of my WOLD gg-grandparents so I might be able to further narrow down the possible common ancestors. None of them match this specific segment but a double 6th cousin on gg-grandma Anna’s line matches the segment just before it, that Katy has also, and segments do like to stick together.
When I am done with my analysis I will send Dad’s new match an email that includes a link to this article and to my online family trees and I will offer to look at his tree. In my experience on my Norwegian lines, a single segment match of this size will be for an ancestor in the late 1600s or early 1700s. Closer relatives will usually have more segments and larger ones.
Hopefully going through this process with me has been helpful. By the way I found cousin Katy with DNA and blogged about it of course.
For more of the fun things you can do at GEDmatch, like look at ancestry composition or use the chromosome browser, read some of my older posts about that site or view the slideshow I made for a GEDmatch presentation. There is also a manual for using GEDmatch in my downloads area.
Last but not least, I have a page here with answers to the most frequently asked questions on the DNA_NEWBIE list, which can help you with the terminology and methodology of genetic genealogy.