Important NOTE: paternity/parent DNA testing tends to look at only a handful of markers, usually anywhere from 13 to 26, sometimes as many as several hundred markers, maybe 200 – 400 or so. In comparison, genealogical DNA testing by 23andMe, AncestryDNA, FamilyTree DNA (FTDNA), FindMyPastDNA, LivingDNA, MyHeritageDNA, Geno 2.0, and Helix DNA tends to look at several hundred thousand markers at a minimum, between 100,000ish to around 1,000,000 or so, with an average of around 500,000 – 800,000 markers. End result – you are going to significant differences between a paternity/parent DNA test with maybe 13 – 400 markers than you will get with a genealogical DNA test that tests 100,000 – 1 million plus markers. Some companies, like DNATribes look at only CODIS (generally 13 – 26 markers) or CODIS plus a few other markers. HomeDNA offers some tests that look at 80,000 autosomal DNA markers. It is important to understand how many, or how few, markers are being looked at when doing any kind of DNA testing.
One of the most common explanations by non-experts for a DNA mismatch is Chimera or Chimerism – https://en.wikipedia.org/wiki/Chimera_(genetics) although a better article might be https://www.thisisinsider.com/what-is-a-human-chimera-and-how-does-it-happen-2017-11, or https://www.scientificamerican.com/article/3-human-chimeras-that-already-exist/. Exactly how common chimeras are is up for debate: https://bigthink.com/robby-berman/you-could-be-your-own-twin. I don’t think it’s anywhere near as common as some articles indicate, but it’s the leading answer by non-experts if you and a DNA match don’t match. I first became aware of the term through a CSI episode (2004) – https://www.imdb.com/title/tt0534653/ (NOTE: it’s an episode involving rape) long before I submitted my first DNA sample. I have yet to come across any reliable estimates on how common or uncommon the condition is supposed to be. This is different than chimera DNA resulting from bone marrow donation or the mother absorbing some of the child’s DNA during pregnancy. In the first case, your DNA may show as the DNA from your bone marrow donor or a mix of your DNA and the donor’s DNA. If it’s a close family member, then the chimera DNA would show as being related, but not necessarily as close as you expect. In the second case, it’s usually trace amounts of shared DNA although it should indicate a relationship.
It’s worth noting that if you or a relative has chimera DNA as a result of absorbing a twin, you should still show as related, but not necessarily as closely as you might expect. There have been several cases where this condition explained DNA results:
https://www.academia.edu/202539/Which_Half_Is_Mommy_Tetragametic_Chimerism_and_Trans-Subjectivity or https://web.archive.org/web/20060526105634/http://www.five.tv/programmes/extraordinarypeople/twininside/
Some other chimera DNA related links:
If you need a bone marrow transplant, you may want to consider getting DNA samples done before the transplant. If you are doing or about to undergo chemotherapy or anything that suppresses your immune system, DNA testing temporarily may not be an option once you start the procedure, or you may need to do it before undergoing the procedure. Also, if you are about to undergo chemotherapy, it’s best to either DNA test before the chemotherapy or wait until several weeks, possibly two (2) months, after the last chemotherapy treatment. It’s less an issue with cheek swab DNA testing than it is with spit kit DNA testing or blood DNA testing. Cheek swab DNA testing relies on the skin cells of your cheek to get DNA.