system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If two people had to repopulate the world, after how many generations would serious inbreeding deformities arise? Would the problem worsen into perpetuity?

Thought of this while watching the South Park episode 'Casa Bonita.' I'm not on this subreddit much, so if this has been asked before, please don't go nuts. Just downvote or something.
gpt
Short answer is that it depends, and serious deformities may or may not ever actually arise.

The thing is that if the initial two had "perfect" genomes with no mutations corresponding to serious defects, there is no reason that their breeding (or that of their descendants) would necessarily give rise to a population with those defects. If they aren't there to begin with, they wouldn't be created by inbreeding. The founder effect is the formal name for how low diversity groups starting a population can lead to high incidences of genetic defects. But that is only because the genes for those traits were brought by a founding member of the population. Inbreeding NEVER creates a genetic mutation. It only increases the chance that one already present will be expressed.

And anyways, if the two people happened to carry in some serious problem traits, as in serious to the point that they would kill off the affected person, then natural selection would act to start pushing the gene's prevalence downwards anyway. Simulations with programs mathematically assigning fitness levels to mice show how when a given gene gives a higher probability of successfully breeding, that the inferior gene is purged to a minimal level very quickly (described in Reece and Campbell, "Biology", 7th edition)

So in short, assuming that these two individuals were healthy to start with, that pretty much guarantees that there are enough "healthy" genes in play to rebuild the population. Situations like the Kentucky families that took on a blue skin tone was because that trait didn't really reduce fitness levels by much (if it did, the blue ones would die and that gene would fade out). You would have a low level of genetic diversity, yes, which would leave humanity vulnerable to being ravaged by a disease or other pressure of that kind. So you could very easily get some strange traits popping up, but if they were "serious deformities" to the point that they mess with a person's ability to propagate themselves, then no, that trait wouldn't show up and stay around.