I am now 2 weeks into the “21 Day Extreme DNA Challenge” that Scott Fisher of “Extreme Genes” is hosting on Facebook. I have a rudimentary understanding of how DNA works, but I don’t think I’ve spent very much time really thinking through what it all means and how it can help my research.
If you are new to using DNA to help you figure out your family tree, there are very few relationships that are more helpful than 2nd cousins. According to the Genetic Genealogist (2016), there has NEVER been a single demonstrated case of 2nd cousins or closer who fail to share DNA. It IS possible for 3rd cousins and even 2nd cousins twice removed to share no common DNA with the test taker.
Most people who are not searching for birth parents know who their grandparents are/were. But unless you have been doing family history research, you may not know who your great-grandparents are. You and your 2nd cousin share the same great-grandparents!
But not all 2nd cousin relationships are the same. Second cousins once removed (1 generation before or after the DNA tester) MIGHT share the same great-grandparents. This is the case for all children of full 2nd cousins. But if a 2nd cousin once removed is one generation older than the test taker, then THEY are part of your family through a SIBLING of your great-grandparents. THESE matches can help you find your 2x great-grandparents. These great-grandparents and great-great-grandparents should have census entries that can help you find dates and locations to keep you moving in the right direction for your research. Or if you already have information for your great and great-great-grandparents, finding DNA matches will help to confirm the information. This can be a HUGE help if you have ancestors with common names or if there’s more than 1 person with the same name in the location you are researching.
FamilySearch has a nice chart that you can download for free here.
But what if you don’t KNOW how a cousin is related to you? Then you need to look at the number of centimorgans (cM) you share plus do a little detective work. There are MANY possibilities for relationships based on centimorgans alone, but to keep this simple, let’s just think about 2nd cousins and 2nd cousins once removed.
2nd cousins will share between 41-592 cM with the test taker with the average amount being 229.
2nd cousins once removed will share between 14-353 with an average amount of 122.
You can take a look at this interactive chart showing all of the centimorgan amounts for many, many, MANY relationships on dnapainter.com. A REALLY cool feature of this interactive chart is that you can enter the number of cMs shared with a match in the box at the top and they will give you percentages for how likely the potential relationships are. Give it a try with 300 cM to get a feel for the information you can get with this tool.
Most of the websites that have DNA information will give you an estimate of a relationship, but don’t take those as gospel. For example, Ancestry never gives you an estimate of a half-relationship or a “removed” relationship.
If you are trying to figure out who a test taker’s great-grandparents are, begin looking at matches that have between 14-592 cM. This should find all of the 2nd cousins and 2nd cousins once removed. But be aware that it will also find other relationships as well. If you have many matches in that range, begin with the matches that are closest to the average amount of 122-229 cM. Focus on the matches that have trees attached to their results and look for surnames that you recognize. Keep good notes – whether it’s online or on paper. Sooner or later you’re going to be saying, “Now where did I see that name?” and you’ll be happy that you took those notes.
So how does it help you to know which of your matches are 2nd cousins?
- It can confirm the research that you have been working so hard to collect.
- They may have branches of your tree that you have not discovered yet. (Never accept someone else’s information without good documentation! But DO use all information as clues to be followed up on!)
- You might find photos or documents in a cousin’s public tree that you’ve never seen before.
- If you can communicate with your cousin, you might find that they have more information or photos, etc. that they haven’t put online in their tree.
- They may want to work together on researching your family – and it’s pretty fun to have someone to share the highs and lows of family research!
Take a look at your matches now and good luck with your search!