Step 3 - Generating a Shortlist of possible candidates

(by identifying and/or eliminating ancestral lines which are “unlikely” or “highly unlikely” to contain the common ancestor)

If you and your match have extensive family trees and you have been lucky enough to trace every one of your ancestral lines back 10 generations, then it should be a simple matter of comparing your list of ancestors and picking out the one you have in common.

However most people, especially when it comes to researching Irish ancestry, can only go back to the early 1800’s or the mid-1700’s at the earliest, and then not in every ancestral line. In that situation, you will be looking for a common ancestor who does not currently appear in your tree. It is important in these circumstances to narrow down the number of ancestral pathways down which the shared DNA could have passed. This helps focus your research and eliminates dead ends and ancestral pathways which are not contenders.

Several techniques allow us to identify ancestral lines that are either “unlikely” or “highly unlikely” to be those of the common ancestor. By eliminating non-contenders, we reduce the number of possible candidates for common ancestor, and reduce the number of ancestral lines down which the shared DNA could have passed. This process of elimination increases our chances of identifying the common ancestor, or, at the very least, the ancestral line on which he sits.

We will deal with each of these techniques in turn:

3.1 Can you tell if the match is on your maternal or paternal side?

3.2 Do you match each other on the X-chromosome?

3.3 What is the likely ethnicity or nationality of your common ancestor?

3.4 How to do Phasing and use it to identify which grandparental line contains the Common Ancestor.
3.5 Other factors?

I liken this process to playing a game of cards. In the example below, imagine that the suggested relationship for you and your match is 2nd cousins. This predicts that you share a common great grandparent - but this could be one of 8 people in your tree and one of another 8 people in your match's tree. Using the above steps, we can whittle down the list of possible candidates to just one single person  in your tree - your mother's father's mother (ahnentafel number 13).

You start off with a full deck of ancestors, but by a process of elimination you can (for example) …
get rid of all your paternal ancestors (step 3.1)
then all your “non-X” ancestral lines (step 3.2)
then lines of improbable ethnicity/nationality (step 3.3)
and then one of your grandparental lines (step 3.4)

So this step is very important for getting rid of ancestors and ancestral lines that should not be given further consideration. This becomes even more important when the relationship with your match is quite distant (e.g. 4^th or 5^th cousin) and the number of possible candidates is quite high.

Now let’s look at each of these techniques in closer detail and explain a little of the science behind each one, followed by some instructions on how to do them in practice.