The same question seems to come up over and over again among those new to autosomal DNA testing. If I match A and B on the same segment why is that not enough to prove they match each other and we have a common ancestor?
The reason the ancestor is not proven is that you have two strands of DNA on each chromosome (remember there are 23 pairs of chromosomes) and the testing mechanism cannot differentiate between the two of them. So A could match the piece from your mother and B could match the piece from your father or one of them could even be a false match to a mix of alleles from both parents (see my post on IBC for more on that concept)
The way to prove the common ancestor is to see if A and B match each other in the same place that they match you. This is what we call triangulation.
Kristine’s shared DNA with other Wold descendants, relationships are to her (to me in parenthesis)
About a year ago I blogged about how, after many years, a change in spelling on the paper trail had led fellow genealogist Dennis to think his wife Kristine was perhaps descended from my great-grandmother’s brother Carl. To prove this I suggested he test her autosomal DNA.
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The current technology for personal genome testing cannot tell you which of the two chromosomes, maternal or paternal, in a pair that an allele comes from. It can tell you that there is an AG at a specific position and a CT at the next position but not whether the A came from your mother or your father. This leads to much confusion about DNA segment matching.
Kitty and Shipley; siblings sharing 47% of their DNA
The matches that these testing companies find are for stretches of DNA that are half identical regions (HIRs). This is due to the fact that a relative who shares a DNA segment from a common ancestor with you will match you along the chromosome you got from the parent who is descended from that ancestor. Thus your new relative will match you for half the alleles in those positions. Only a sibling will share fully identical regions of DNA. Click here for a page that has a picture of the DNA I share with my brother Shipley.
For example, if my Dad gave me AAAAAAAAAAA and my Mom gave me CCCCCCCCCCC then I would seem to match absolutely everyone on that segment because every position has both an A and a C. So an ACACCAACCAC or a CCAACCCACA looks like a match, but only those with an AAAAAAAAAAA or a CCCCCCCCCC would be real matches. This is simplistic and the segment runs used for matching are much longer than this to try to avoid that sort of false matching. Also note than when your testing company shows an AC it is really an AT and a CG but just one of the known pairing is shown for brevity.
The term for a real match is IBD, which is an abbreviation for Identical By Descent. The term IBS means Identical by State which would apply to any false match. So in our example, the CCAACCCACAA match would be considered IBS.
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Sometimes I wonder if the interest in genetic genealogy runs in our DNA. I have found many more new cousins with autosomal testing that are descended from my WOLD line than in any other family. Yes they all had lots of children until recently, but so did the Munsons and the Skjolds.
So I decided to make a picture of the HIR (half identical region) DNA segments that I know come from my great-great-grandparents Jørgen and Anna Wold of Drammen, Norway. To do this I made a CSV file with a list of all the segments that are just from those ancestors. I put the first names of the group of matches in the column that would be the MRCA in the usual style segment map. I have to give credit to my distant DNA cousin (on the AJ side) Israel Pickholtz (he blogs too) for this wonderful idea of making a reverse segment map. Below is my picture of Wold DNA created with my DNA segment mapper tool. Click the image to go to the actual html page which will show the centimorgan values and names when you put your mouse on a colored block.
The use of two lines is arbitrary; I could have used three or four. The DNA segments shown are not separated into lines for Anna and those for Jorgen. Where that was possible, I had intended to do it with colors, but did not get to it, next version. Knut, OK, Nancy and Aaron are on Anna’s side while Susannah, her mother, and Lester are on Jorgen’s side only.
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So many of these one segment DNA matches that I find with other testers are too far back to find the common ancestors. However those of us with Norwegian ancestry often have very deep trees thanks to the good records and the many farm books for each locality. So when you find a fellow genealogist with Norwegian roots who matches your DNA you get quite hopeful, and sure enough …
I had four matches, the colored lines in the picture below, for my Norwegian descended Dad on chromosome 16. However they span the centromere which is from 35335801 to 38335801, so initially these segments did not seem that promising.
Four people overlapping on Chromosome 16
from my one segment mapper tool
Luckily the most recent match had an extensive tree of which only a small piece was Norwegian. So perhaps it would be easier to find our common ancestor with less tree to look through. Note that using a tool to compare Norwegian GEDcoms does not usually work so well because of differences in naming conventions (patronymic, farm name, Norwegian characters, etc)
Here are the details of these segment overlaps from my master spreadsheet for Dad:
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By Jim Bartlett
Impressed by Jim Bartlett’s prose on various message boards and mailing lists, I asked him to do a guest blog post on using spreadsheets with autosomal DNA results, here it is – Kitty
Using autosomal DNA testing can be a challenge – but it doesn’t have to be. It can be intimidating – but by taking it a step at a time, you can break it down into bite-sized pieces that are much easier. When you decide to use autosomal DNA (atDNA), and to get the most out of it, I recommend three broad areas of focus right from the start:
- Learn all you can about DNA testing for genealogy and particularly about autosomal DNA (atDNA). The ISOGG wiki is a good place to find good articles, tools, blogs (to keep you up to date), etc. Join email lists and read and ask questions. This is definitely a “continuing education” hobby. We are on a frontier with genetic genealogy – and we are pushing the boundaries every day!
- Create as robust a Tree as you can – stretch as much as you can to 12 generations, or more. This is the net you need to catch cousins and find your Common Ancestors. This is very important – if you don’t have the ancestors in your Tree, you cannot expect to find a Common Ancestor with a Match.
- Set up a process for your autosomal DNA project. To determine Common Ancestors you have to share ancestry info with your DNA Matches – you’ll be sending (and receiving) a lot of emails and messages. You’ll want to keep track of what you do; to find info on your Matches; to remember the Common Ancestors you determine; new names, new emails, new links to Trees, etc., etc. You may want to use a spiral notebook as a Diary or Journal of your notes. Some people keep a notecard for each Match, or a folder. I now have over 3,000 matches at FTDNA and 23andMe, so I need something that can handle that many (and more) Matches. Many of us use a spreadsheet – read more to see how to set up one.
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