Say you’re doing homework and you want to compare answers with a friend. But you didn’t do your own homework, you copied most of it off Dave. So you compare with Sara, and if there’s any errors that Dave made or Sara made you have a chance to catch and fix them. But if Sara also copied off Dave, you’re not gonna catch those mistakes.

Similarly, you have two sets of chromosomes, and for each “gene” (homework problem) you have genes (answers) that are more or less dominant. Bad genes that kill or impair you tend to be recessive, because if they are dominant the carrier doesn’t survive. So it’s all right if you have one copy, because it’s not expressed. But if you have two copies…

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.