Monday, December 7, 2015

Degrees of consanguinity and DNA

For some background you can read two posts I did in 2012 on Recording kinship by the law and When is your great-uncle not really your great-uncle?

So what does this have to do with DNA?  If you are looking at a DNA site and they have predicted that your relationship to a certain person is a 3rd cousin you have to understand that what they are really telling you is that you and this person are predicted to be 8 steps apart.  It is the number of steps it takes to go up the chain to the common ancestor and back down the chain to the other person.  There are a lot of relationships that will give you 8 steps and all of these are possibilities.  The DNA website can’t list all of these so it lists the most direct relationship. 

To see this in action take a look at Blaine Bettinger’s Shared cM Project Charts.  Blaine created these charts from actual data sent in to him from people that have had their DNA tested and know the exact relationship to other people that have had their DNA tested.  For example, I have my DNA, my mother’s DNA and a paternal uncle’s DNA.  I was able to send Blaine how many cMs I share with my known mother and my known uncle. I also sent more data on known cousins that I could map out on a pedigree. Blaine did a statistical analysis on shared cMs using known relationships so that you can predict what relationship two people have based on these numbers.

Here is a very simple example.  2nd cousins are 6 steps.  1st cousins twice removed are also 6 steps.  On Blaine’s chart the average 2nd cousin relationship is 245.98 cM.  1st cousins twice removed average 239.72.  That’s pretty darn close considering the limited pool. 

Nutshell version: If one of the DNA websites tells you that you and another person are predicted to be 2nd-3rd cousins that means you are predicated to be 6 to 8 steps apart.  If you look at the two pedigrees thinking that you must straight 2nd or 3rd cousins without investigating the other possibilities then you just might miss the connection/common ancestor.


Copyright © 2015 Michèle Simmons Lewis

1 comment:

  1. And this is why I will be meeting with a DNA specialist next month Michele. I want to sit down with my family's tests and find out how to better interpret what it all means. I went to CeCe Moore's talk here in San Diego a couple of months ago. She was wonderful and gave great examples of how helpful DNA can be. Which, of course, is why many of us have run the tests. Thanks for sharing this with us.