To date, there has never been a confirmed case of second cousins (2C) or closer that don’t share DNA. There have been a few rumblings here and there, but nothing proven. See “Second Cousins (Or Closer) That Don’t Share DNA?” for more details.
But what about second cousins once removed (2C1R)? That’s only a single meiosis away from a 2C relationship. Is it possible to not share DNA at that distance? If I don’t share DNA with my 2C1R, should I suspect a misattributed parentage event, or is that normal? Is there anything I can do to give myself some peace of mind?
According to data from the 2016 update to the Shared cM Project, several submissions to the project reported no shared DNA between 2C1R. The histogram shows the distribution, and how often 0 cM shared is for that relationship:
Sharing no DNA at a distance of 2C1R appears to be possible, assuming the submissions were correct. But is that the end of the analysis?
A Real-Life Example
I was recently corresponding with someone on a mailing list who had a 2C1R relationship that doesn’t share DNA. She had tested a handful of other close relatives who all matches that 2C1R, but she didn’t.
It was a perfect case to examine this relatively rare situation, and she was kind enough to let us examine it on the blog for everyone to learn! Thank you!
Mary [all names are changed!] has tested herself, her 2C1R, and six other members of the family that we’ll look at in this blog post. The family tree looks like this, where the people in red are tested, and their segment data is available:
The key relationship here is Mary Smith and Benjamin Johnson, who are 2C1R. They share no DNA in common, which as noted above is relatively rare but not impossible. Every other tested person in the tree shares DNA with Benjamin, ranging from a low of 16 cM to a high of 177 cM:
Mary’s three siblings, for example, share between 56 and 99 cM with Benjamin, but Mary shares no DNA with him!
Mary’s Relationship to Her Father and Siblings
The very first thing to check is Mary’s relationship to her immediate family (father, siblings, uncle, and 1C). If there is a misattributed parentage event that explains Mary’s lack of sharing, this comparison will make it clear. However, the comparison shows that Mary is related to her immediate family as expected:
Clearly, there are no problems with any of these relationships. The shared DNA with the uncle is on the high side, but not outside the range of the Shared cM Project (range 1301 – 2193 cM, average 1744 cM).
Accordingly, just this analysis alone suggests that Mary’s lack of sharing with Benjamin is pure chance.
To the Segments!
With the segment data, we can examine this phenomenon in greater detail.
For example, let’s test this situation using Mary and her three siblings. We know that the siblings all share between 3 and 6 segments (56 and 99 cM) with Benjamin, the 2C1R. And we know that Mary shares no DNA with Benjamin. So what can we hypothesize with regard to Mary and the segments her siblings share with Benjamin?
We can hypothesize that Mary should not share any of those segments with her siblings. If she did, we would expect Mary to match Benjamin on these segments.
This will be a little difficult to test with siblings since we can’t tell which segments came from which parent, and we’re only looking at the paternal side of the family. But we can hypothesize two things:
- Mary and her siblings cannot share fully-identical regions (the green regions using the GEDmatch One-to-One tool) at any location where her sibling matches Benjamin; and
- If Mary and a sibling share a half-identical region (the yellow regions using the GEDmatch One-to-One tool) at any location where her sibling matches Benjamin, that segment must be a maternal segment!
So let’s do a comparison of Jim and Benjamin, and then Jim and Mary. Jim and Benjamin share DNA on chromosomes 8 and 11:
As expected, there are no HIRs shared between Jim and Mary at the locations where Jim and Benjamin share DNA. On chromosome 8 it is clean and simple; Mary and Jim share no DNA where Jim and Benjamin share DNA.
On chromosome 11, there is about 10 Mb of overlap between the large segment shared by Benjamin and Jim (top image) and the segment shared by Jim and Mary (bottom image). This region is likely a maternal match between Jim and Mary.
Second Cousins or Closer?
The process above is what would be needed to prove that there are 2C or closer that don’t share DNA. It may happen eventually, if millions of second cousins are tested, and hopefully this will provide some insight into how to analyze a suspected case.