system
Given the following post, respond as if you're commenting on Reddit:
human
Why does incest have a negative effect on offspring, as oppose to just a neutral effect?

I understand the benefits of sexual reproduction; in a nutshell, diversity of genetics makes us more awesome. It makes sense then that reproducing with a sibling (for example) doesn't have any major *benefits* due to not expanding one's genetic diversity much, but I don't understand why it can have actively *negative* effects. What does it do to harm the offspring that makes it worse than, say, simply cloning some one?
gpt
One reason would be that genetic diseases are more likely in children of incest. Many people 'carry' disease-causing alleles of genes but do not show symptoms themselves because they also have a normal wild-type copy to compensate.
It is likely that closely related people would both be carriers of similar disease-causing alleles. Thus, when they mate to produce offspring, there is a good chance the alleles will combine in the offspring (i.e. offspring would get the disease allele from mum and dad) to cause the disease.

When unrelated people mate, there is a much lower chance that both will be carriers of similar disease-causing alleles. Thus, there is a lower chance of genetic diseases manifesting in the offspring.

Note that this is quite a simplistic argument. Most diseases are probably not caused single gene mutations but are the result of a complex mixture of several alleles (you could call this the genetic background) interacting with environmental factors. Nevertheless, the logic of reduced diversity in children of incest still holds.