Many families have recessive fucked alleles that cause all kinds of problems. An allele is a variant of a gene. You need two copies of a recessive allele for that gene to be expressed (do something).

Every fucked allele the parents share has a 25% chance of being expressed in the child. The more DNA the parents share the more fucked alleles they share. Usually… sometimes the opposite happens and the number of bad alleles is reduced in the child. Inbreeding can lead to fucked animals and “perfect” ones. Weird stuff.

Now you might be thinking “why do families have so many fucked alleles?”

That’s a good question. The thing about carrying a single recessive, fucked allele is that it doesn’t do anything to you. Mutations happen all the time and so these alleles are floating around and multiplying constantly.

It’s only a problem on the odd chance that they get expressed which is most often when family members produce offspring.

So it’s worth noting that this is the type of thing that we can check for these days. In some cultures it’s acceptable to marry a cousin and I’ve heard that they check for any genetic issues before proceeding when they live in a first world country.