Can seventh cousins share 54.7 cM of DNA?

Question

I've found two people reported to be seventh cousins who share two autosomal DNA segments:

Chr Start Location  End Location    Centimorgans (cM)   SNPs
11  124,632,262     131,124,570     14.4                1,029
19  49,942,996      63,776,118      40.3                2,020

Is this even possible or must they have a closer connection that is not yet known?

According to this diagram (found here found here: http://isogg.org/wiki/Autosomal_DNA_statistics#mediaviewer/File:Shared_cM_Project_v2_updated.png):

Seventh cousins would only share up to 10 cM, yet strangely it says 8th cousins would share up to 16 cM. Is that an absolute upper limit or just the normal amount expected in most cases? I found this match searching across 38 kits, so there were 38*37/2 combinations to compare or 703 pairs, so a relatively low probability event is conceivable.

Note that these two people are also related more distantly in other lines as follows according to the paper trail:

• Eight cousins twice removed
• Tenth cousins once removed
• Tenth cousins twice removed

Answer 1

Yep. Unfortunately, everything is possible. The genetic genealogy is very young field of human knowledge. And the provided graphs, comparisons etc. usually describes only very simple cases.

For example, why do you think that you have ONLY ONE set of common ancestors between seventh cousin? It is possible that they are related through two bloodlines and the amount of total shared DNA will be greater than in provided diagram. Or we may say it in other words - if the parents of people, who were tested, are related, the numbers may be greater, than you expect.

The second thing is that this diagram shows us that inheritance is statistical process. It means that we share precisely 50% DNA with our parents, but beginning from grandparents the things become more complicated: we DO NOT share precisely 25% of DNA with them. The amount may vary in wide ranges. Of course, we can calculate them in simple cases and you are right, that 7C will have shared DNA in range ~0-15cM.

The last problem is the artifacts of matching algorithm. The algorithm calculates so called half-matches on the chromosomes. It means that when we took test and got results, we don't know for what chromosome belongs particular test location. So algorithms tries to find matches on both chromosomes. It is not totally incorrect, but usually gives results of reasonable quality. But sometimes it leads to false-positive matches. Also there is such problem that one particular person may have more shared DNA with somebody, than his parents. This may happen between tricky overlap of parents' segments. So it is ALWAYS good idea to have parents tested and then split the matches to the paternal and maternal lines. It is made automatically in services like FTDNA. Or you can do the same on services like GEDMATCH. The process is called "PHASING". I want to emphasize that it not only split the matches into paternal and maternal lines, but also effectively eliminate false-positives.