Caution should always be taken before pursuing an answer to a genealogical question. Ask yourself (or your relative) before diving into the research whether you really want to know the answer, and whether you think you are prepared for it. Just because you as a genealogist may be prepared for the result does not mean everyone in your family will be.
This is as true with DNA evidence as it is with any other type of genealogical evidence. Just as a census record or birth certificate can reveal a family secret, so can the results of a DNA test. The Genetic Genealogy Standards, for example, make clear that unexpected results can occur:
12. Unexpected Results. Genealogists understand that DNA test results, like traditional genealogical records, can reveal unexpected information about the tester and his or her immediate family, ancestors, and/or descendants. For example, both DNA test results and traditional genealogical records can reveal misattributed parentage, adoption, health information, previously unknown family members, and errors in well-researched family trees, among other unexpected outcomes.
This is especially true when testing two people who are second cousins or closer, as there has NEVER been a single demonstrated case of second cousins or closer who fail to share DNA. Let me re-phrase that: no one has ever demonstrated second cousins or closer that fail to share DNA at a level detectable by one of the three testing companies. There are claims out there, but they are completely unsupported at the current time. (Also see “Are There Any Absolutes in Genetic Genealogy?” for more).
What would be needed to prove a case of actual second cousins or closer that don’t share DNA? You would need ALL of the following elements; without any one element, you have NOT proven the case:
- Test results for second cousins or closer that don’t share DNA according to 23andMe, AncestryDNA, FTDNA, and/or GEDmatch (if at AncestryDNA, the result would need to be confirmed at GEDmatch to ensure that Timber isn’t having an impact); AND
- Test results for each of the related parents of the second cousins or closer, showing that: (1) the two parents share detectable amounts of DNA in common with each other at a level commensurate with their genealogical relationship; (2) that parent 1 matches child 1 at the parent/child level; AND (3) that parent 2 matches child 2 at the parent/child level.
So, in the following diagram, if first cousins Angela and Bill don’t share DNA, then the following elements would be necessary to support the claim that they are actually first cousins rather than a case of misattributed parentage:
- Angela and Bill do not share detectable DNA;
- Abel and Bruce share DNA at an appropriate sibling amount;
- Angela and Abel share DNA at an appropriate parent/child amount; AND
- Bill and Bruce share DNA at an appropriate parent/child amount.
Without any of the above elements, you have NOT proven the case. This is a very high burden, but that is required to establish that the case isn’t actually the MUCH more common occurrence of misattributed parentage. After all, Occam’s Razor suggests that misattributed parentage (no matter how much the parties may protest) is significantly more likely.
If someone tells you that they have a case of second cousins or closer not sharing DNA, ask them for the GEDmatch kit numbers, and confirm that the above requirements are met. Otherwise, be a skeptic. Unfortunately it seems sometimes that our culture has taught people that skepticism is unhealthy, so fight that urge. Be a skeptic, ask for the evidence!
P.S. – note that it is technically possible that a second cousin or closer wouldn’t share DNA, but the odds against it are astronomical. You likely have a significantly better chance of being struck by lightning twice in your lifetime (1 in 9 million).