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.
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.
I also convinced another Wold cousin to test in order to help out. This was a good thing since Kristine shared no DNA with my nuclear family but she did share DNA with my first cousins (brothers Henry and George), my 2nd cousin (John), and much DNA with the tested Wold cousin Mike (my third once removed).
Every place you see two parallel colored lines in the diagram above, two of my Wold side cousins are matching Kristine’s DNA and each other in the same spot. That is triangulation. Because her matching DNA triangulated with several different cousins in three spots, we are now confident that Kristine shares our gg-grandparents Anna and Jørgen Oleson Wold (click here for the full story). [addendum 19 Oct 2015: this diagram was updated to the one used in my triangulation presentation which includes cousin, Katy, found via DNA testing after this article was originally posted. She is a 2nd cousin 1R to Michael. The details of this triangulation are shown in my triangulation presentation, slide 8 on]
Looking through the master spreadsheet for my Dad, I see a reverse case. Jeff matches my Dad and me from 46M to 57M on chromosome 2 for about 10cM while LuAnn matches my Dad and my brother from 46M to 58M also on chromosome 2. Even though they each match Dad in the same spot, they do not match each other there. Plus one matches me and not my brother while the other matches my brother and not me. This is a good example of a match that is proven to be from different sides, one maternal and one paternal, and thus different ancestors. In other words, a pair of matches at the same spot that did not triangulate with each other, but did triangulate with other family members.
So how do you do a triangulation?
The easiest place to do a triangulation is GEDmatch.com – there you can use the one-to-one comparison tool to compare A to B, B to you, and A to you and then see if all three share the same segment(s).
On 23andme it is also easy, provided you are sharing with both A and B. You use the Family Inheritance Advanced tool (under Ancestry Tools on Your Results) to compare all three. When you see that A and B match you, then you switch the person on the left to A or B and put yourself on the right with the other one. Now you can see if A and B match each other at that same spot. Below is the example with Jeff and LuAnn from the case previously mentioned. In the first one LuAnn is compared to my family and Jeff. Then I switched her and Jeff for the next compare.
UPDATE: 21-AUG-2020: You cannot do DNA triangulation at Ancestry because there is no display of the actual segments, known as a chromosome browser. However MyHeritage has automated triangulation included in its chromosome browser see https://blog.kittycooper.com/2018/04/myheritage-dna-matching-excellent-enhancements/
On Family Tree DNA it is more complicated to triangulate since you cannot directly compare A to B, you can only compare them to yourself or any other kits you manage. One possibility is that when both A and B match you on a specific segment you could contact each of them and ask them to check if they match the other at that location. A workaround that many use, however, is the In Common With (ICW) function. As in the example below, you can click below a person listed in your family finder match list and request to see all the people the two of you match in common by clicking Common Matches and then In Common With.
If A and B are in common with each other then it is likely that the segment where they both match you is a match. But it is no guarantee. The number I have heard is 90% but that is just an estimate based on experience; others say 99%. Below is a case where both Gunnar and Stephen match my Dad and they are each on each other’s in common with list. So a likely match.
Is there anything better? Well you can use other family members to check the match.
Since both my brother and I have tests at Family Tree DNA as well, I used the surname search to find these kits in my brother’s match list, then viewed them in the chromosome browser. If these two guys are a match, my brother will either match them both or none. Why you ask? Most likely, my brother only has either Dad’s paternal segment or maternal segment at this spot. DNA tends to travel in chunks and although it may get smaller as it gets passed down, it takes many generations for it to get more mixed up. Here is my brother’s view of these two in the chromosome browser:
By the way, neither of these fellows matched me. Have you noticed that this is the same spot on chromosome 2 that we were looking at for LuAnn and Jeff? So what else might we learn from this?
Well remember that I matched Jeff and my brother matched LuAnn so we each got different segments from our Dad here. Thus LuAnn must match Gunnar and Stephen as well. I am hoping Stephen and I will find this common ancestor for our 16.9 cM match, but no luck yet, and I have not heard back from the other two, a common problem …
Further reading on triangulation:
This blog post of Roberta Estes:
This article at the ISOGG wiki: