After 32 generations (~800 years) you have more genealogical ancestors than there are base pairs in human DNA. There literally isn’t enough resolution to store a “record” of each of your ancestors, even if you inherited exactly 1 base pair from each ancestor.
Additional complications make it an even shorter timeline. After about 8 generations, you share no more DNA with your ancestors than you do with a random stranger.
Politically this should be more well known. Of course, racists and fascists rely on “blood quantum” arguments to justify racial or ethnic oppression. But they don’t invent this idea of strict genetic identity. It’s latent in the population.
Leftists should more frequently call out genetic tests like 23andMe as inherently racist because it’s based in race science nonsense. It may not be as obvious as Nazis invoking aryan genes or whatever; but it’s still just as incorrect when your aunt at Thanksgiving talks about how she discovered she’s 2% Choctaw or whatever
800 years is just the hard cap, the theoretical maximum. In reality, not all 3 billion base pairs are relevant to distinguishing humans from each other. Much of the genome is just basic biology shit, which is why the human genome overlaps 70% with bananas, 96% with chimpanzees, etc.
This 800-year rule also ignores the fact that there were not 4 billion people on earth in 1250 AD. There is a lot of… ahem… crossover within our lineage.
The point is that the way many people think about identity is rooted in a wrong understanding of genetics. The whole point of these genetic tests is to discover a supposed “true” identity that is ostensibly encoded in one’s DNA. Closely related with this belief is a strictly biological view of race and ethnicity.