You and I are related to each other, but the question of how closely we are related has two very different answers. One genealogical, one genetic. 1/n
To see why, consider a simulation of my family tree back over the generations, my number of genealogical ancestors doubles every generation. I soon have a lot of genealogical ancestors 2/n 
An overlapping simulation of your (blue) and my (red) family trees. Because of the doubling each generation back, we soon have many overlapping ancestors (circles) just short while back. We're close genealogical cousins. In fact we're distant cousins many times. 3/n
Yet in a region of my genome, say the tip of my maternal copy of chromosome 11, I inherit this DNA from only a particular ancestor in a given generation. That means that we can trace a single line of my ancestors back that are my genetic ancestors in that chromosomal region. 4/n
We can do the same for you, the 2 (dark) lines don't intersect. We're close genealogical relatives sharing many ancestors, yet we've inherited this chromosomal region from different ancestors and so aren't genetically related to each other over these small # of generations 5/n
How far do we have to go back till you and I find a shared genetic common ancestor for our chromosomal region, i.e. where those lines intersect tracing each of our genealogical ancestors coalesce? 6/n
Well for two people of european ancestries we only have to go back 600-1000 years untill they share (nearly) all of their genealogical common ancestors. Yet it is very unlikely that they've inherited the same region of their genome from a shared ancestor in that time 7/n
In fact, we have to usually go back 500,000 years till we find a shared genetic ancestor for a specific region of the genome, i.e. where those lines meet and our genetic ancestry in that region coalesces.
You and I are related to each other. Genealogically we're closely related, cousins many times over just a few centuries into the past. But in any region of our genome we're only very distant genetic cousins. 9/n
Even 2 people whose ancestors lived on different continents are distant cousins many times over just a few 1000 years back. Yet it's still ~550,000 years back till they share a CA for a particular genomic region (about 10-15% longer than the 500,000 years for 2 Europeans). 10/n
This is a snapshot of a long 1/2 written blog post that I'll finish up in the new year. Much more to write on CAs along the genome, and who your recent genetic cousins are. But there were cookies to ice, pressies to wrap, & pork chops to grill. Merry Christmas & Happy Holidays.
Here's the last of the old series of blog posts ( gcbias.org/2017/12/19/162… ), which ended pretty much when kid #1 stopped napping on weekends. And more on genetics and genealogy: gcbias.org/category/genet…
