Human genes only really “work as intended” when they are combined from very different sets.

So-called “recessive” genes are overruled by your partner’s different pile of genes. They are usually shit traits like soggy bones or hair growing backwards, but since they never dominate, they haven’t been naturally selected away. They’re just harmless baggage.

You can still get them because it’s all random, but the likelihood is generally low.

If you don’t have that difference in mating genes, more of these recessive genes get to have a say in building the human. This severely increases the likelihood of birth defects.

https://en.wikipedia.org/wiki/Inbreeding

Fun fact: This is one of the reasons why - when we start colonies on the Moon or Mars or wherever - it’s important that we send a fuck ton of people.