A few years ago about this time of year, when independence is an important word, I wrote a post about data independence—the idea that only independent data improves our knowledge. When data all derives from the same source it is dependent. More dependent data does nothing to help us move our research forward.
I’ve been thinking about that again both because of the time of year and because of the talk on DNA that I’m working on.
One of the things that makes DNA so useful is its independence from documents and the human memories that produced them. DNA is also free from questions of tampering (at least for now) and provenance. Virtually every document we use is a written version of a memory. The memory may be correct or incorrect. It might be fresh or faded. It might have been checked or corroborated at the time or it might not. It might be an attempt to relate what happened or in a few cases it might be intentionally altered, bending the truth or altering it beyond all recognition.
DNA is a chemical memory, not a mental one. A DNA test might give a false positive or a false negative. It might even give correct and yet truly bizarre results in those rare cases of chimerism in which a person is their own twin and passes on DNA to their children that does not at all correspond to what was found by the familiar swab of the check. For the most part though, DNA correctly tells us about the biological relationships that form that part of our personal past. It yields that information in a way that is independent of of any memory or document. It does not come filled with names, dates and places as documents do. Without help it is silent on those points but it does tell an almost unerring story of the the links between us.
Yet the whole point of DNA evidence is dependence. Your DNA depends on your parents DNA, which depended on their parents DNA. If you find a match, the reason that you have done so is that your match’s DNA depends on the DNA of an ancestor you have in common. It is the lack of independence from person to person that gives DNA its value to genealogy. My DNA and my distant cousin’s DNA both depend on one or two long ago people who passed on the DNA that doesn’t just partially determine who we are, it identifies the link between us.