That was sort of a revelation.
Nice to have a really clear definition:
"Inbreeding occurs when an individual has one or more common ancestors. A common ancestor is one that is present on both sides of the pedigree. I.e., in order for an individual to have a non-zero coefficient of inbreeding it is necessary for both father(sire) and mother(dam) to be descended from one or more common ancestors."
This is an obvious corollary:
"Consider further what happens when no ancestor appears on both sides of the pedigree - F drops to zero. Outcrossing to a completely unrelated individual always reduces F to zero at a stroke."
My maternal grandmother's parents were first cousins: Irving's and Amy's mothers were sisters, thus those women's parents appear on both sides of my grandmother's pedigree. My maternal grandfather's lines do not extend far enough back (yet) for me to detect common ancestry, but I'm highly confident it is there (Kleine Gemeinde Mennonite for the win). But just because each of my mother's parents were the product of inbreeding doesn't mean _she_ was as well: possibly there was no shared ancestry between her mother and father.
In fact, unfortunately, she had significant parentage from the Netherlands including some from Fryslan, and the Mennonites my maternal grandfather was descended from originally came from Fryslan (not all Mennonites do -- the Swiss ones wound up elsewhere). There were probably shared ancestors, but equally they were probably from before, let's say, the middle of the 1600s.
My paternal grandparents came from Fryslan and Zuid-Holland. Surely, a common ancestor at some point; equally, not anytime recently. Surely, again, through the Dutch heritage all around, there is a common ancestor between my father and my mother -- but again, a long time ago, probably from before the middle of the 1600s.
A recent shared ancestor is unlikely to be a strong cause for concern for my sister or I (or, presumably, for our children). There is, however, something else we could probably worry about:
But it, too, is not particularly worrying (except maybe the frontotemporal dementia one. That, I'm a little worried about, given what happened with my father's aunt and his sister. And we would appear to be mostly in the clear on diabetes I, as well, despite his brother). Working in our favor on the Dutch issue is the overwhelming longevity of so many of our ancestors.