system
Given the following post, respond as if you're commenting on Reddit:
human
If incest is wrong, why is it ok for people to knowingly pass hereditary diseases to their offspring?

From my understand, incest (besides being gross) is wrong because there is an increased risk of the offspring having developmental problems or diseases.

However, people who have all kinds of hereditary diseases are congratulated when they have children. Why is it considered ok, or even good, for them to have children? 

There is the chance that they won't pass the hereditary diseases to their offspring, but there is also the chance that two cousins would have normal offspring. 

I am sorry that I don't have a background in philosophy. I am also not trying to offend anyone...just trying to understand.

Edit: if this post offends you and you are down voting, please tell me why.
gpt
You are pointing out the rational/philosophical reason for people to believe that incest is wrong; however, people believe that incest is wrong for a variety of reasons.

It seems that most people have an innate aversion to engaging in incest.  (See the Westermarck effect.)  The incest taboo has likely arisen in most/all cultures as a generalization of this.  

There are also a number of religions that expressly forbid incest.

There also seems to be a general belief that incestual relationships are generally unhealthy, a position that could be true, but currently has no particular support from science.

Incest is generally considered to be wrong even in cases that cannot be explained by the increased risk of disease: in same-sex relationships, or where one of the partners is sterile.

If one were to claim that their only reason for promoting the incest taboo was because of disease, then to be logically consistent, they would have to condemn non-incestual relationships with hereditary diseases, and allow same-sex or sterile relationships.