Blog post

Tuesday, August 30, 2011

Analysis of a second cousin DNA match

Recently, one of my known cousins took a DNA test with Family Tree DNA. He’s a half second cousin to me. We have a great-grandfather in common but descend from different wives. I’ve been looking forward to seeing the results and figuring out what they reveal.

Direct comparison

The DNA test came back as a match, showing us as predicted second cousins with a range of second to third cousins. Spot on. We have a total of 141 cM in common over 20 segments of shared DNA, with the longest block being 33 cM.

Here is FTDNA’s representation of where my known cousin and I are half identical (only half due to the other half of our DNA coming from the other parent). Our shared segments of DNA are marked in yellowy-orange. Only segments of at least 5 cM are shown in this view.


This tells me that half of those yellowy-orange areas of my DNA came to me from our shared great-grandfather.

I think that’s a pretty cool thing to know.

Matches in common

My cousin has confirmed our relationship as second cousins in Family Tree DNA, and I can now see who we match common. I have 63 matches in total on Family Tree DNA, three of whom also match my known cousin.

I can add our matches in common to the comparison. One match shares a segment of DNA in common with both myself and my known cousin. The other two must obviously match my known cousin elsewhere, and I assume that they match each other at the location shown, although it’s possible they don’t. I think it would be reasonable to pencil that location of my DNA in as (half) coming from my great-grandfather.


Our three matches in common are unfortunately predicted to be fairly distant cousins so I’m not optimistic that we will work out our connection. I do hope to contact each of them before too long and tell them which branch of my tree our common ancestor falls in, though. Perhaps I will be proved wrong. That would be good!

Matches not in common

Finally I looked at anyone who matched at the same location as my known cousin, but wasn’t a match in common with him.

Unfortunately the relevant segments for the people who met these conditions were quite small. I can’t draw the conclusion I had hoped to – that my common ancestor with these people is on my mother’s side. Because the segments are only small, they may be related to my known cousin but not meet FTDNA’s threshold for declaring someone a match.

Here’s an example of a comparison of my known cousin with another of my matches (who doesn’t match him) that almost shows what I was hoping to find. This time segments of 3cM and larger are showing, as this match didn’t share any segments of 5cM or more with my known cousin:


All in all, I think this new information has given me a few edge pieces of the DNA jigsaw puzzle.

Is there anything else I could have drawn from this or should have considered?


  1. This is interesting.  We recently found a second cousin, once removed through testing at 23andme - 9 segments in common, 135 cM in total.  Our longest segment in common was ~26 cM.

  2. Susan (Nolichucky Roots)September 1, 2011 at 1:30 PM

    Great post, Shelley. I've been reviewing information on my father and a cousin of his but had not considered your matches not in common scenario. Back to the spreadsheets!