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.

Many birth defects are rare, and require 2 copies of a defective gene to show up. Most "normal" people will be carrying a few defective genes (out of thousands of pairs), but are fine because they have a good copy still working.

Family members tend to have similar genes.

The chances of you and a family member having the SAME defective gene are massively greater than you and some random stranger.

Thus any child would also have a massively greater chance of inheriting 2 identical copies of the defect.

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...

Swiss cheese theory.

Genes code for all the differnet protein molecules in your body; if any of them are damaged, they don't produce that particular protein and your body is all messed up.

This is bad. Luckily, you have two strands of DNA, one from either parent - and a copy of each gene on either strand. If the copy of the gene you need on strand A is broken, no problem, you just use the copy from strand B (or vice-versa).

However, this relies on both strands having all their broken bits in different places.

Think of a slice of swiss cheese - if you need 100% cheese cover on your sandwich, and you have two randomly-selected pieces of swiss cheese, all the holes are going to be in different places, and you'll probably be fine. Maybe there'll be a couple of tiny gaps, but you'll probably get by.

However, if both slices come from right next to each other n the same block, then most if not all the holes are going to line up with each other, and you are fucked.

I am a carrier of CFTR mutations. If I knock up a random woman theres a 1 in 80000 chance of the child having cystic fibrosis. If I knock up my sister theres a 1 in 16 chance instead.

I have a disease that is autosomal dominant (and absolutely sucks.) That means only one gene (the one I got from Dad) was needed to give me the disease.

Now I have 2 of these genes. Everybody does. You get one from your mother and one from your father. So I got the bad one from Dad, and a good one from my mother.

It's a super rare disease! If I nail somebody at random in the population, they have 2 good genes they can contribute. My one bad one and one good one flip a coin; our kids could have a bad one from me (and die young) or a good one from me (and be fine!)

But if I nail a brother who had the bad gene, there's a coin flip for both of us. Even if I give a good gene, he might give a bad one! Way more likely the bad thing happens.

(So, so grateful none of my brothers have it so I don't have to get all squicked imagining that but it makes my point.)

Not all bad things are as easy to see as my disease. Lots of them "hide" until you get two bad things. But it's more likely two siblings have similar hidden bad things, which makes it more likely the bad things will show up in their child.

I'll try to keep it relatively simple - your cells contain chromosomes that contain your genes. You usually* have two sets of every chromosome.

These genes come in different variations/mutant forms called alleles. Most alleles function more or less the same, but some malfunctions result in deformities.

If a malfunctioning allele results in errant gene inactivation, it is known as recessive, which means as long as your other copy works, you're all good.

If a malfunctioning allele results in an errant gene activation, it is known as dominant, which means if you have the allele you get the deformity regardless of if your other copy works or not.

Fortunately for life, most malfunctioning alleles are recessive, so as long as you've got high genetic variance (a lot of alleles) in a population, the chance of two people meeting with the same recessive malfunction is low.

Incest can result in a drastic decrease in genetic variation, which can result in malfunctioning alleles becoming much more prevalent than they usually would be, resulting in many more cases of recessive deformities than in the wider population.

*For males this is not true of their sex chromosomes. Many genes present on the X chromosome are missing on the Y chromosome, which can lead to sex exclusive traits and diseases.

For example, it is the reason why there are almost no calico/tri-colour male cats, as the genes for it are in X but not Y chromosomes.

Genes mutate.

If you have one copy of a mutated gene, it could be bad. However, one mutated gene is usually recessive or will be co-dominant with the other gene so it isn't that bad. If you have two copies of that gene, it can be a lot worse.

Two close family members are more likely to have the same mutated gene, so the odds of getting that same mutated gene twice are a lot higher.

It's mostly a game of chance.

Grossly oversimplified: Say you have black eyes, but carry a blue-eye gene (which doesn't show because black is dominant). There's a very good chance your sibling carries the same gene. If the two of you have a child together, there's a higher chance that the kid will carry double blue genes and has blue eyes.

Had you had the kid with another black eyed person that's not a sibling, there's a higher chance that they may carry different genres (black, grey, brown, green eyes). So much lower chance of having a blue-eyed kid.

Imagine the same but with all sorts of physical and mental defects. They may not show in you, but if you carry the genes, there's a good chance your sibling does too. Hence the higher chance of it showing up in your kids.

Even if the trait doesn't show in your kids, over generations of inbreeding practice (common in old royal families) the bad genes would be so concentrated in the pool that so many defects will start showing up.

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.

First you need to understand the difference between a dominant and a recessive gene. Dominant genes manifest if they're present, recessive genes manifest only if there are no dominant genes present. A quick example is blood types, 0 is recessive, both A and B are dominant, you have two genes that specify your blood type, if both of them are 0 you are type 0, any other combination with 0 you're not 0, i.e. 0A or A0 are A, while 0B or B0 are B. This means that a person with blood AB can't have a son with blood 0, because his son will either have one A or one B inherited from that person.

Genetic diseases that happen because dominant genes are hard to miss, if you have the gene you have the disease, however genetic diseases that need recessive genes can be carried for generations without anyone manifesting symptoms. But of two persons have the same recessive gene it's quite possible that their children will have both of the genes be that one and manifest the illness. The chances of two random people having the same recessive genes are quite slim, but the closer people are genetically the higher the chances that they have the same recessive genes. Using blood type as an example, if a parent is AB and the other is B0 their children have 0% chance of being 00, but they have a 25% chance of being A0 and 25% chance of being B0 (the other 25% being AB and BB). Now if their children A0 and B0 have a child of their own that child has a 25% chance of being born 00, whereas if any of them had a kid with a AB, AA or BB the chances would be 0%.

Humans are diploid. We have 2 independent copies of every chromosome in each cell, one from each parent. They function as "backup" copies. If an error is on one, as long as the other one has a good copy of the effected gene, nothing happens. However, if parents are related, it becomes possible to receive two identical (or nearly identical) copies, and lose that backup. In that case, any error/mutations will cause problems.

Resistive genes play a huge part, plus any transcription errors the are passed down don’t have a chance to get skipped with inbreeding

I manage a small closed queen breeding program and we have to try very hard to keep as much variation as possible

Every person gets their genes from their mom and dad, however certain traits (defects, if you will), are present in everyone. Thats why we have mental illness and birth defects. However, you isolate a gene pool, and those bad traits get more and more focused. For example, their is a family known as the Blue Fugates who have been doing incest since the 19th century, and now the family has a blood defects that makes their skin blue.