system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what actually causes genetic issues if breeding pair are for example siblings?

i guess we've all seen the whole 'little hickville town with mutated villagers' horror scenes in series and movies. would human beings (or other mammals) actually mutate into barely recognizable as humans like that if for example siblings had babies together for several generations? 

from the very little i've read, i just came to the conclusion that inherited health issues would be more likely to manifest themselves but beyond that and the whole social stigma thing the offspring would be relatively normal. if i'm wrong, what causes it?
gpt
Our DNA has redundancy built in: it comes in two halves.

Each half can potentially bear faulty copies of a bunch of genes - but so long as there's a working copy in the other half, you're fine. 

Think of two pieces of paper, each with a random set of holes in it. Put them together, and so long as none of the holes line up, you effectively have a complete sheet. 

When two organisms breed, they take half the DNA from each parent - one sheet of paper each in this analogy.

If the two parents are only very distantly related, it's a pretty good bet that they'll have few or no faulty genes in common - any holes they have will likely be in completely different places, and so won't line up. 

If the two parents are very closely related, however, those two pieces of paper will have a very similar pattern of holes, so at least some of them will very likely line up, and the offspring will get two broken copies of the gene. If they're lucky, this doesn't make a whole lot of difference.... but if they're not, really important stuff can get fucked up.