Basically the title, and it’s out of pure curiosity. I’m not inbred, and don’t know anyone who is, but what I’m not entirely sure about is why inbreeding (including breeding with cousins) causes issues like deformities and internal body issues?
I’m not a biologist, so could someone help me out? Thanks.
Comments
Reproducing with a person with an entirely different set of chromosomes reduces the chance that a deleterious mutation in your family gets passed down. There will be a 50% chance at each step of getting rid of it (oversimplifying).
When people breed with family or cousins, the genetic diversity is reduced, so any ‘bad’ mutations will be compounded over generations rather than being eliminated.
Edit: as someone mentioned below, an entirely different set of chromosomes can actually be bad, for reasons better explained below. It would have been better to say that you want a certain level of genetic diversity/similarity to most effectively screen these things out, since completely disparate populations could end up introducing entirely new issues to each other.
The chances of both people carrying a defective gene increases with inbreeding. Hence, the chances of passing this down to the offspring is much higher.
Some genetic traits are highly recessive meaning a more dominant gene will prevail if present. If the dominant gene doesnt exist, then the recessive trait becomes present. Most people in general have dominant genes expressed, with recessive ones not being a huge problem otherwise it probably wouldn’t have been passed from generation to generation. When you shrink the pool, you overexpress certain conditions bc there arent other genes to dilute the recessive ones. Sometimes it’s no big deal. But genes mutate, and not for the better. And if that mutation gets passed on and on, then that’s when it becomes harmful. Im sure there’s way more nuance than this explanation, but i think that’s it in a nutshell.
If both parties are perfectly healthy, then the offspring should be fine. However, the social and weird mental implications are still there.
Theres a lot of genetic mutations that WOULD cause serious health issues but are a) very, very, rare and b) recessive, meaning that you need to get the same gene from both of your parents for the effect to be present.
Imagine someone who has one of these very rare “faulty” genes. He gets two children, both will get that gene with a 50% chance.
Now if those thwo children were to get another child together, both have a 50% chance to pass the gene if they have it, making it a 25% percent chance for both “lineages” from the original carrier totalling to a 6.25% chance of the child getting two identical copies of that gene.
It doesn’t seem THAT much, but you increased the chance of passing a very rare genetic issue from near zero to 6.25% just within two generations.
If everyone has a few of these “faulty” genes, it will likely never cause any issue if the genetic diversity stays high, but as soon as two closely related people reproduce, you suddenly increase the risk of giving someone two copies of the same faulty gene by a lot.
The other two comments address this pretty well, but to really ELI5 it: yeah, you do know someone who’s inbred, I guarantee it. Probably several. They are dogs, not humans, though, but the effects are still super obvious and easy to think about. Every time breeders breed two “sames” to get a bunch of cute “same” puppies, they’re not just propagating the cute gene. They’re also by default propagating the breathes-so-loud-you-can-hear-it-next-door gene, the hip dysplasia gene, dwarfism, you name it. As soon as you mix things up, though, it’s easy to tell a puppy is no longer “pure”. Easy visual analogy for what’s going on under the hood, too. Looks like a mix on the outside, is a mix on the inside as well, and therefore less likely to be stuck with every single shitty disorder the parents had. Dominant disorders only, and after a few more generations of non-incest, perhaps even those will disappear.
Think of our genetic code like a story book, they’re made by listening to someone tell the story and trying to write down exactly what you hear. For the most part people are pretty good at this, but every once in a while someone makes an error. They write down a word wrong, or leave one out, or make a spelling mistake. Now this isn’t usually an issue because when you and your partner want to make a baby you write the new book together and you look at both of your copies of the story when doing so. This lets you catch the vast majority of the little spelling mistakes because it’s unlikely you both separately screwed up in the exact same place. But inbreeding is like trying to work with two very similar copies of the story. You both made the same spelling mistakes so when you go to write a new copy together that spelling mistake is copied into the new book instead of being corrected. Sometimes it’s just a little spelling mistake and nothing much goes wrong. But enough generations of uncorrected little errors and the book has some serious flaws.
Imagine you have a favorite t-shirt. You think it’s great, and everyone around you had a similar shirt, so you don’t feel the need to change it up fashion-wise. Now imagine the same shirt 20 years later: it seems OK at a cursory glance, but the colors have faded from repeated washes, the threads have come loose, and now there are holes in it. The shirt has degraded and is no longer a functional garment. Everyone around you, meanwhile, has bought new shirts from time to time and they look similar to the old ones, but they are in much better shape because they’re new.
Genes are similar – they can get wonky if reused. If you only mate from with someone of the same genetic background for generations, eventually the genetic threads come loose, and things start to break down. If new genes aren’t introduced to correct the problem…The result is offspring that seem ok but get more and more rundown as time goes by, resulting in health issues and genetic mutations.
When two people have a baby together, the baby inherits half of its chromosomes from each parent. Chromosomes are basically packets of DNA code – genes – that tell the body how it should be put together.
Humans have 46 chromosomes, arranged in 23 pairs. Sometimes, a given human will have a broken gene in one of the pairs, but this is usually fine as long as you have at least one “correct” chromosome for each pair.
Damaged or incorrect genes are very common, and usually cause only mild or unnoticeable effects.
Problems begin to arise when people start repeatedly passing the same sets of chromosomes around.
Let’s talk about cystic fibrosis.
Cystic fibrosis is an incurable genetic disease caused by a baby being born with both copies of a certain gene in chromosome 7 broken. This can only happen if both parents had at least one half of the chromosome 7 pair broken, and by chance (in this case, 50% chance) both passed broken copies to their baby.
Those parents are considered carriers of cystic fibrosis. Because only one half of the gene pair is broken, they don’t have the disease, but have the potential for any children they have to be carriers.
They probably inherited the broken gene from their parents, who probably got it from their parents. This means that their cousins are also likely to be carriers. This means that marriage within that family group, where the entire family are carriers of CF, are all very likely to result in babies with CF. And if those babies ever have kids, unless their partners are definitely free of CF, they’re highly likely to pass it down to their kids in turn.
In order to break the cycle, at least some of their cousins need to marry someone outside of the family who has not inherited the CF gene from shared grandparents, to bring new, unbroken chromosome 7 genes into the family.
The cystic fibrosis example is the simplest way to explain it, but genetic disorders are many and complex. Many broken genes don’t manifest as defined illnesses or disorders. People with genetic problems more often manifest as nonspecific flaws and problems – lower intelligence and learning difficulties, physical deformities, lower fertility, stuff like that.
By repeatedly breeding within the same small genetic pool, these issues are concentrated, and the chance of having babies with these problems rises with each generation of inbreeding. Every baby is a dice roll, and every failed dice roll means passing an increasingly damaged set of genes to the next generation.
DNA is essentially a series of pairs of data. You can visualize it as whatever you want. 0 and 1’s whatever works.
Evolution tries to eliminate ‘bad options’ so they are almost always a ‘recessive gene’ which needs both sides of the pair to have it in order to manifest itself. Let’s pick something super ridiculous. …the Habsburg jaw which made it hard for them to do basic things like eat.
If you carry a recessive gene in your pair it normally isn’t a problem because whoever you mate with likely doesn’t have it at all because even if they had an afflicted grandparent it is almost certainly ‘bred out’ entirely. Meaning that even if you have the Habsburg jaw it is incredibly unlikely that you and the unrelated individual will even have the chance of having a child with it (because your spouse wouldn’t have the gene at all most likely which means the ‘bad pair’ cannot exist at all)
Now with turning your family tree into a family tumbleweed is that even healthy members of the family who are carriers but not afflicted by the jaw carry a 50% chance of giving that gene to each offspring (while someone afflicted is going to pass it guaranteed) so you were essentially rolling the dice on if a child would be afflicted. You have to remember until recently it would have been impossible to know if someone was a carrier or not in large part because genetics wasn’t understood but even if it was there was no way to test their DNA so you’d have to calculate based on known lineage to find out what chance each person had of being a carrier before they would breed (which you could do, but even then you’d have to make assumptions about anyone ‘outside of the descendants’ that they have/do not have the trait – sure math nerds would get a kick out of it but most people would just glaze over out of boredom)
Very basically- variation is good because there’s lots of pro and cons in evolution. Tall and slow breeding with short and fast can make a child who’s average which can be advantageous. Close interbreeding can exacerbates flaws, so a propensity for an underbite magnifies and becomes the Hapsburg Jaw in one real world example. Various genetic disorders require both parents to carry the gene that triggers it so breeding at random means a good chance that only one parent is a carrier, brother and sister interbreeding virtually guarantees that the “bad” gene is passed down.
We reproduce sexually for genetic diversity. Genetic diversity helps our bodies correct for weaknesses and abnormalities, at least most of the time.
Most defects are recessive (simplification), so they won’t be passed on to offspring if the other partner doesn’t have that same gene – they receive the healthy, dominant gene instead.
If you breed with your siblings, your chances of passing along defective traits increases fairly significantly because you’re sharing most of your genetic material – you have the same parents, after all! First cousins are actually only slightly worse than any random pairing. Breeding with a first cousin one time isn’t a big risk, but if your grandparents were first cousins, and your parents were also first cousins, now your family tree is less diverse, so that becomes a problem. Each generation of close breeding increases the probability of passing alobg negative recessive traits to offspring.
2nd and 3rd cousins are almost completely negligible with inbreeding risk.
Any bad genes that exist in a family have a chance to actually grow to the point of becoming permanent fixtures in the family genome. It’s just as likely that bad genes will be passed down indefinitely as they are to disappear.
And there’s likely more than one bad gene being potentially passed down. If there’s a family history of lung disease, breast cancer, heart defects, and alzheimer’s, then you have a good chance of getting into a scenario where at least one of those becomes a permanent fixture in every future generation.
So, first, a short (and very incomplete) primer on how genetic illnesses work:
Genes are the instructions that your cells use to make proteins, and proteins do a whole lot of different things to keep your body alive. They make chemical reactions happen, they act as structural elements, they carry oxygen in your blood, and a whole load of other things.
A mutation in a gene is an error in those instructions, and very often that leads to a protein that either doesn’t do its job as well, or that doesn’t even work at all.
But you have two copies of each gene (or almost every gene if you have XY sex chromosomes, since the X and the Y chromosomes carry different genes). So often, if you have one mutated copy of a gene, but one that is fine, you won’t notice anything wrong because the good copy can still be used to make good proteins.
When your body makes gametes (egg and sperm cells), each one gets only one copy of each gene, so that when they combine together that once again makes a full set of genes. If you have a child with someone who is not closely related to you, it is extremely unlikely that they have mutations on the same genes that you do, and so your offspring will probably be healthy. But if it is with someone closely related, they do have a similar set of mutations to your own, and if you both have one mutated copy of the same gene, then it’s a 1 in 4 chance that the child ends up getting both of the mutated copies.
In a lot of cases it is more complicated than this, but this is the rough gist on why having children from incest is bad.
So, one off breeding with a cousin isn’t usually an issue. It’s when your whole family has exclusively bred with their cousins (and uncles/aunts/nieces/nephews) for years and years to the point that, genetically, your cousin might as well be your sibling.
Suppose there’s a deadly disease (Z) that’s recessive. If one parent is a carrier and one doesn’t have it, they have a 50% chance of having a baby who’s an unaffected carrier of the disease and a 50% chance of a baby who doesn’t carry it at all. So far so good.
But suppose the parents are both unaffected carriers of Z gene. They have a 25% chance of having a baby with no Z gene, a 50% chance of having a baby who carries the gene but isn’t affected by it, and a 25% chance of a baby having the disease.
Let’s say that Aaron and Beth marry, and Aaron is a carrier while Beth is not. They have four kids: Catherine, David, Fiona, and George. Only Catherine and George are carriers.
Each of those kids gets married to a non-carrier and has kids. Again, George has a 50% chance of passing his Z gene on to a carrier. If George’s son Harold wins that lottery and doesn’t have the Z gene, and marries Ingrid, his uncle David’s daughter, there’s no chance of their kids having the Z gene. They’re totally fine. And if Harold marries Jane, a girl from three towns over who happens to have the Z gene, their chances of having a baby with two copies of the Z gene just went up, even though they’re not related. In this situation, marrying his cousin is fine.
But if they successively intermarry within their own family, the likelihood of two cousins who are carriers increases (remember, there’s a 50% chance of being a carrier even if one of your parents doesn’t have it), thus increasing the likelihood of passing the gene on.
If you start to complicate this by adding more genes that could be passed on, you can see how the likelihood of something going wrong increases.
Incidentally, this is one of the problems with eugenics and dog breeding. The smaller the pool of prospective partners is, the higher the likelihood of one recessive gene becoming an issue in that tiny population. Sure, you might have a purebred golden retriever, but if you only breed that dog with other goldens, one recessive health problem might have a much larger impact (this is one of the reasons why certain breeds have certain health problems associated with them). You could breed them with another breed that definitely doesn’t have that problem… but then their puppies aren’t “purebred” anymore.
When human eggs and sperm form, they each carry one-half of the DNA required, the instructions for building the new person.
If the egg has a broken gene that means that it doesn’t make, say a protein right, but the sperm has a working copy of the gene to make that protein, chances are that the person that that DNA combination made will end up being able to make that protein correctly.
If you have people who are closely related, they’re much more likely to have both gotten a copy of that broken gene, and then there’s no good gene to make up for it, so the person experiences the consequences of having that fundamental building block broken their whole life. If they have more than one gene broken, the problems often become even worse, because they stack.
Let’s see you have a 3 buckets of apples. 2 of the buckets contain a mix of good apples and bad apples. And the 3rd has nothing but good apples.
You have to mix two of the buckets and want the most good apples you can.
In addition to everything else, we humans as species are already remarkably inbred from the get-go due to the genetic bottleneck our species passed through.
https://www.nhm.ac.uk/discover/news/2023/august/human-ancestors-may-have-almost-died-out-ancient-population-crash.html
We are not the only species to go through this kind of bottleneck, cheetahs for example barely avoided extincition two times already.
Genes can be recessive or dominant. Recessive genes are not inherently ‘worse’ (ie blue eyes etc), but recessive genes that DO have negative qualities are less likely to eliminated from the gene pool than dominant genes with negative qualities.
They are also less likely to be expressed in the offspring of two distantly related parents.
They are more likely to be expressed in the offspring of two closely related parents, as the chances of both parents posesssing the recessice gene (non-expressed) are higher.
Your genetics are based on the genes of your parents.
If your parents both have a gene that causes a weird abnormality in your heart, you’ll also get one. But when you have a kid, if your spouse doesn’t have an abnormality, it’s likely your kid could be fine.
Now, if you instead fuck your aunt who has the exact same heart condition from grandpappy, your kid’s gonna get that heart condition. They’ll also get that weirdly prominent jaw your grandpa & mother had.
That is to say, inbreeding causes relatively minor negative variations in genes to become more prominent over time. Stuff that would’ve been bred out, or not become problematic compounds, and gets worse each generation.
Some great explanations of why it happens- but to really see the reason you could look up the Hapsburg Royal line. They didn’t want to dilute their royal blood so there was a looooooot of cousin marriages.
The Hapsburg Jaw is believed to be from the inter breeding
Posted by a guy who has a BIG crush on his cousin.
Because they both could have the same disease, and they don’t cancel each other out.
Part of gene diversity is actually what builds a healthy immune system! When you have a variety of genes, you’re more likely to be resistant to a lot of things in multiple ways.
Theres health disorders that can impact this of course, but if you think about a Tower Defense game, the more variety you have the more likely you’ll be able to take out different types of threats. You can’t just rely on one or two types in most circumstances.
If all of your defenses are the same/the more similar they get, the more susceptible you will be to new and unique threats, especially when all it takes is to figure out how to get past one type of defense instead of a bunch of types of defenses.
Inbreeding depression also makes you more likely to get cancer, die young, have fertility issues, and more. Its just not good.
Imagine making babies is like rolling a pair of dice. If you get only ones you lose. When you marry your sister (obligatory roll tide). You are only rolling with one dice.
Basically you are doubling the change of a bad gene getting through.
The short version is that rare recessive conditions can become increasingly prominent.
Imagine that you carry a recessive gene for some disease (Dd, D – Dominant non-disease gene, d – recessive disease causing gene). If you have kids with someone with DD genes, your offspring have a 50% chance of carrying the recessive genotype Dd. Now if two people with Dd genotype have offspring, there is a 25% chance of the double recessive dd genotype. If you are not inbreeding, and the Dd genotype is fairly rare, the odds of producing dd offspring is very low. But in an inbreeding situation, there is a much higher chance of producing dd genotype offspring. Going through the math, assuming your offspring randomly pair together to produce the next generation, 1 in 8 will have the dd genotype.
Every human has 20 or so recessive defect genes that only happen when 2 people hit the bad genetic lottery and the child gets both genes from the parents. Most of these are so rare that it’s very unlikely for non-related people to have both genes and breed a child that has both genes. However, if 2 people are related the probability that both are carriers is orders of magnitude higher.
You’re a little young for Punit squares (that’s more for 11 year olds), but since you asked I’ll try. When organisms breed their offspring gets half their DNA from the father and half from the mother, and that DNA determines eye color, height, skin color, and any genetic defects for which the parents give a building block for that trait. Some traits are dominant, meaning that if that block is present the offspring will have that characteristic, whereas a recessive trait means that both blocks have to be present for the trait to appear. If someone has 2 dominant genes, their offspring won’t ever have the recessive gene. If someone has 1 dominant gene and 1 recessive gene, they will have the dominant trait but be “a carrier” for the recessive trait (if both parents are carriers, there’s a 25% chance the offspring will have the recessive trait). If someone has 2 recessive genes, that person will have the recessive trait.
Most genetic defects are “recessive genes,” meaning that without gene sequencing one can’t know if they’re a carrier or don’t have the recessive gene at all. Once a recessive gene is introduced into a population that is originally “pure dominant,” it’s not really possible to estimate who is a carrier and who isn’t, and it is then assumed that everyone is a carrier, but an “outside pure dominant” individual has no risk of their offspring having a recessive trait since the most that can happen is the offspring becomes a carrier. With inbreeding there’s basically a 25% chance that the offspring will have a genetic defect since it’s assumed that everyone in a family is a carrier (25% chance dominant from both parents, 25% chance dominant from father and recessive from mother, 25% chance recessive from mother and dominant from father, 25% chance recessive from both parents), whereas with outbreeding and assumed 1 parent isn’t a carrier (2 dominant), there’s a 0% chance that the offspring won’t have the recessive defect (50% dominant from both parents, 50% chance dominant and recessive with the recessive coming from the parent).
imagine you have two special decks of cards. your deck is magical, what you look like will be determined by these cards. both are unique, maybe one of them is dragons themed and the other one is space themed.
lets say we shuffle the two decks together and then split them in half. now you’l have 2 decks that are a bit space and a bit dragons themed.
now put the two decks side by side and flip one card from each.
higher card gets to stay, the lower card gets discarded, keep the higher card.
do this for both of hte entire decks and ul be left with a discarded deck and a kept deck
you will never show your discarded deck to your friends, it will always stay in your back pocket. the higher card deck is the one you show.
however when u wanna pass your cards down to ur kids, you shuffle your kept deck with your discarded one, split em in half, and give it to your kid. Your kid will get one of these from their mom and one from their dad. So notice how its possible for the kid to have gotten one of those cards from the discard pile.
so when ur kid goes to do the same excercize with their mom’s and dads decks, its possible a low card from the moms discard pile as well as the dad’s discard pile gets included into the kids final kept deck. Perhaps this low card is nice(blue eyes). or perhaps its ugly(disease).
Its possible for ur dad to have had some ugly cards in his back pocket all his life and never showed any disease because he had some other high card that won out over it. If you inbreed theres a higher chance you end up with 2 of the same cards from the discard pile since ur related and have very similiar cards.
A lot of genes requires a copy of the gene from each parent to take effect. The set of genes you get is from your parents.
Think of it like everyone gets 10 random scrabble tiles in a bag. Half of those come from each parent. You and a sibling are likely to have half of your scrabble tiles match. Your mom has a J in her bag, and there’s a 50/50 chance that you and your sibling both have Js.
Js are only bad if you have 2 of them, and only 1 percent of the population has Js, so the chances you’ll find 2 people who both have Js who decide to breed is very unlikely. But 2 people who draw from the same bag and each have a 50/50 chance of having a J are much more likely to draw that pair.