Why Don’t I Match My Cousin?

no cousin

I receive this question regularly from people who have taken one of the autosomal DNA tests and who expected to match a cousin, but don’t.

Of course, the Jeff Foxworthy in me wants to say, “Because he’s not your cousin,” but fortunately, I never let my inner Jeff Foxworthy out in public.

Actually, that’s often their biggest fear – that they are uncovering a very unpleasant family secret – but Jeff Foxworthy aside – that’s generally not the case.

Let’s take a look at why.

According to Family Tree DNA’s FAQ on the subject, combined with the percentage of DNA shared with each type of cousin, we find the following.

Relationship to You Likelihood of a Match % of DNA Shared
1st Cousin (common grandparents) 100% 7-13
2nd Cousin (common great-grandparents) >99% 3-5
3rd Cousin (common great-great grandparents >90% .3-2
4th Cousin (common ggg grandparents) >50% <1%
5th Cousin (common gggg grandparents) >10% Sometimes none detectable at match threshold
6th Cousin (common ggggg grandparents) <2% Often none detectable at match threshold

If you don’t match your first cousin, then you need to start thinking about Jeff Foxworthy or you’re simply extremely lucky, or unlucky, depending on your perspective.  Buy a lottery ticket. 

In all seriousness, if you don’t match a first cousin, consider having your sibling (or parent) or your cousin’s sibling or relevant parent test as well.  In some cases, two people simply inherit different DNA and even though they don’t match each other, they do match other people in the same family. 

However, if you’re going to go down this path, be prepared that the answer may be that you really aren’t genetic cousins.  By the time you get to this point, you’ve already peeked into Pandora’s box though, so it’s kind of hard to shut the crack and pretend you never looked. 

Another option for determining whether or not you really match that cousin is to download both of your results to GedMatch.  The testing companies have pre-set match thresholds that determine what is and is not a match.  That’s a good thing, but what if your match is just slightly under that threshold, and there aren’t other relatives to test?  GedMatch allows you to match at very small segment levels that would generally be considered population matches and not genealogy matches.  

Judy Russell had the perfect example of just this situation in her Widen the Net blog.  Her mismatch was with a 3rd cousin.  According to this the chart above, she stood a greater than 90% change of matching, but she didn’t, so she’s in the special 10%.  And that 10% gets left wondering.  Fortunately, Judy had tested aunts, uncles and another first cousin, and her cousin who did not match her did match them. 

The moral of this story is:

  • Ignore Jeff Foxworthy when he starts to whisper in your ear, at least initially
  • Test as many family members as you can
  • Don’t jump to conclusions
  • Utilize third party tools like GedMatch if necessary
  • Understand that if you test enough family lines, you will eventually find an undocumented adoption someplace

 

 

 

 

 

17 thoughts on “Why Don’t I Match My Cousin?

  1. Hi Roberta
    This is Melba McGee Niemuth. When my DNA results came back, (I am NA (C), my younger brother, Robert (Bob) McGee (Q) didn’t show up. Neither did my 1st cousin, nor two of my 3 second cousins. The 3rd cousin that did—his mother ‘s father was a brother to my grandma. Melba

  2. Hi,

    I was running my husband’s matches and found a Roberta (Buster) Estes on his matches. So, I am wondering if you could give me an area where the Estes might have come from so that I could check for a pattern with terms to location, such as where the came from and where they landed.

    Thank you,
    Tia Gray

  3. Hi Roberta,
    I hope you are enjoying your trip. I have a couple of comments:
    From above: “In all seriousness, if you don’t match a first cousin, consider having your sibling (or parent) or your cousin’s sibling or relevant parent test as well. In some cases, two people simply inherit different DNA and even though they don’t match each other, they do match other people in the same family. ”
    If you don’t match your first cousin or even your second cousin, then there is no gray area, you are not genetically related. The ambiguity doesn’t occur until you get to the third cousin level, as in Judy’s example. It is impossible for first cousins to simply inherit different enough DNA from their shared grandparents to not be detected as a match by FTDNA’s Family Finder or 23andMe. (This is likely true at AncestryDNA as well.)

    Also, the chart isn’t broad enough. Second cousins can legitimately share much less DNA than 3%. (I need to update the Wiki, I see.) I have proven full second cousins in my family study who only share 1.07%, 1.15%, 1.3%,1.62%, 1.65% and 1.87%. I have tested many additional family members to confirm the actual relationship.

    • Does this mean that every person then has to inherit some autosomal dna from each of their 4 grandparents? How about siblings inheriting completely different dna from one of their parents? I understand it would be very unlikely but it is actually not possible? I wish I had paid more attention in school ;) Thanks!
      Casey

      • Yes, each person has to inherit some DNA from each grandparent. Siblings each get exactly half of their parents DNA. I’ve never seen a scenario where 2 children get the exact opposite half of millions of DNA locations.

  4. I love this explanation Roberta! & also the one on triangulation! This is so true & happened to us. Two cousin kit comparisons (1st cousins) was just not enough for us to find our surname matches, though we matched each other big time. We then then found a 4th cousin to test. I don’t match him, but my 1st cousin does. Between the 3 kits, we have found descendants of 5 children out of 8 , of our brick wall 3rd great grandfather. I don’t think that is so bad for a beginning. The sad thing is how many relatives refuse to test.

  5. Roberta, In trying to verify who is not the father of my cousin we have this information. J and B are 1st cousins on their maternal side, their shared cM is 779.51. J and B mother’s were sisters who married twin brothers, Ju and Jo. Since J and B have a set of common grandparents on each side, shouldn’t their shared cM be approximately double what it is? Are these FF results enough to make this conclusion. The paternal grandparents are P&A and the maternal grandparents are G&E. There are reasons other than DNA to support a NPE for J and Ju.
    We have a child, all 1st cousins, from each of 4 siblings, the two sisters above, 1 more sister and a brother(JA) who have tested. Of the 4 siblings, only the brother was able to be tested and all are now now deceased(the siblings). The other two 1st cousins, V and B. V shares cM 992 with J and 788 with B. N shares 816 with J and 814 with B. Ja is father of V and they share 3382. Ja is Uncle of J, N, and B, they respectively share, 1763,1549, 1658 with Ja.

  6. Roberta, I found the link to “percentage of DNA shared” above after I posted regarding my cousins J and B and the father of J. I think that answers my query, J and B would be “double cousins” and because their shared DNA is not double what they share with other 1st cousins, not possible for Ju to be the father of J. Still would be great to have a confirmation, thanks for all of the insight you post.

  7. Pingback: 2013’s Dynamic Dozen – Top Genetic Genealogy Happenings | DNAeXplained – Genetic Genealogy

  8. I want DNA Test match of my uncle or cousin.my father is dead in 28 years ago.now what i can do?my grandfather is two married.first marriage my grandperents.2nd married my uncle mother.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s