system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are members of the same family more prone to producing offspring with biological issues?

Also, bonus related question: Is the fact that we aren't normally attracted to the members of our family due to a biological inclination, or is that more affected by our environment (being taught that it's wrong)?
gpt
There are 2 basic concepts when dealing with genes - they can be dominant, which means they are usually shown, or recessive, which is the opposite. Dominant genes can be shown even if you only get the gene from one side of your parents (For example, being brunette). However, with recessive genes, you need that gene from both of your parents (like having red hair).

Say there was some defect that ran in your family. Alot of defects are recessive genes. In other words, they can run in your family, and when you breed with someone who does not have that same defective gene, your kids will most likely not show it. However, if you breed with someone in your family, they would also carry that gene, making it much more likely that your child will have that same defect.