system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why babies are born all messed up when a a brother has sex with a sister? Like an inbred child?

I just saw a post that had a picture of a pretty fucked up baby and it said "this is what happens when you knock up your sister"

Well I never really knew why that happens, can someone explain? I remember hearing how it's because of certain genes.
gpt
What you're talking about is "deadly recessive" genes.

Sexual reproduction (rather than asexual reproduction, where a creature does not need a mate to create an offspring) has one big advantage: it combines the DNA of two different creatures.

So let's say there's a recessive gene in your DNA.  It's not a good gene, it'll mess up your heart, or cause brain development issues.  Fortunately, tho' you have this gene, it is recessive, and you also have a perfectly functional gene which is dominant.  So the bad gene does no harm to you.

Now, your sister also has these same two genes.  No big deal for her either.

But if you two both produced a child, that child will inherit one gene from each of you.  If the child inherits the bad gene from you both, then it doesn't get the good dominant gene to overwrite the issue, and ends up with the brain development issues or the malformed heart etc.

Now, there's a chance of picking up a bad gene in any kid.  But the chances of getting two copies of the same bad gene is considerably greater when both parents have it.  And in the case of siblings, they're much more likely to share the same genes.

Make sense?