Inbreeding and loss of genetic information

[Talakeal]

In a recent RPG game I introduced a dynasty of horrific monsters that reproduce exclusively with their own offspring. One of my players said that this was impossible because inbreeding degrades the genetic code, and after a certain number of generations the offspring would no longer be viable.

The player couldn't actually explain the mechanism for this, and I didn't want to stop the game for a debate about genetics, but this didn't seem correct to me.

I understand that with this system the species would have little to no genetic diversity and that harmful mutations wouldn't be difficult to breed out of the line, but I am not aware of any mechanism by which genetic information is actually removed through inbreeding.

Am I missing something? If so, would anyone care to educate me?

Or is my player just misunderstanding the difference between loss of genetic diversity in a population vs. loss of genetic information in individuals as I suspect.



Thanks, and please try and keep the conversation scientific!

[Gnoman]

I suspect your players are misinterpreting the problems faced by purebred dogs, which have lost significant amounts of genetic information because they've been selectively bred for specific traits.

While there are no historical incidences I know of where the sort of breeding you're talking about happened without highly-specific direction, we do have a number of demonstrated populations -in both the animal and plant varieties of life, and with humans specifically- where the breeding pool was very shallow. In all such cases, the problem is "bad" traits getting preserved rather than anything getting lost.

[Mechalich]

Or is my player just misunderstanding the difference between loss of genetic diversity in a population vs. loss of genetic information in individuals as I suspect.

Probably this, with the caveat that 'genetic diversity' and 'genetic information' are in some sense the same thing. Specifically, as allele diversity is lost due to fixation the total amount of genetic information in the population has dropped because there's now only one version of the respective gene in circulation. Inbreeding creates problems in this regard because in a small population selective pressures are reduced, which makes it significantly more likely that deleterious alleles will become fixed and also that a dangerous mutation that occurs in a single individual with high reproductive success will easily spread throughout the entire population.

Now, genetic information can be said to be 'lost' if a non-functional allele becomes fixed in a population, effectively turning the gene off entirely for the whole population (lactose intolerance in human populations operates largely in this fashion due to the presence/absence of a specific allele of a regulatory gene). However, this can only happen for non-essential genes, otherwise since non-viable offspring die before being able to pass on their genetic information. Inbreeding can result in lethal alleles being carried at high frequencies in heterozygous individuals, which may result in a very high rate of miscarriages and stillbirths.

An exclusively inbreeding population is likely to reach a point where reproductive success is very low, especially if the population is small. This is observed, as Gnoman mentioned, in artificially bred organisms, but also in island-dwelling organisms with very small populations and even some widespread species that went through a major population contraction at some point in the recent past - ex. Cheetahs. This can make a species or population much more vulnerable to extinction - especially from disease, commonly observed in extremely low-diversity agricultural plant cultivars, ex. the Irish Potato Famine - but would not cause extinction outright.

[NichG]

If the creatures don't have multiple copies of genes (e.g. multiple chromosomes), then inbreeding should be much less relevant. Basically when you have redundant copies, you can accumulate a lot of things that would harm you a little bit if you had double-recessive, but which are basically safe if you only have single-recessive (because the dominant gene provides the function you need). As long as you're breeding with other individuals chosen at random, only a few of those things will end up being double-recessive among the offspring. So you might get some harms, but not much. Now if instead you have an inbred line, the recessive genes are all correlated - you're much more likely to produce offspring with someone else who also has a recessive gene in the same slot. So instead of a few random little harms, you get a large sum over many little harms, leading to life-threatening harms.

But if you only have a single copy of each gene, those harmful recessive traits will be under much stronger pressure to be excluded from the population since there's basically no redundancy to hide behind. So in that case, inbreeding should make less of a difference.

[Vahnavoi]

As a corollary to what NichG said, there isn't a single answer to the question, because it varies by genome and organism. Some genomes are more resilient to change and corruption than others. This is especially visible in organisms that reproduce by mitosis, but even in reproducing plants and animals, we have examples of self-fertilizing organisms where each new generation consists effectively of clones of their parents.

As Mechalich pointed out, this may create a single point of failure across a population, something that has happened, but it is not itself a cause of extinction.

[Mechalich]

If the creatures don't have multiple copies of genes (e.g. multiple chromosomes), then inbreeding should be much less relevant. Basically when you have redundant copies, you can accumulate a lot of things that would harm you a little bit if you had double-recessive, but which are basically safe if you only have single-recessive (because the dominant gene provides the function you need). As long as you're breeding with other individuals chosen at random, only a few of those things will end up being double-recessive among the offspring. So you might get some harms, but not much. Now if instead you have an inbred line, the recessive genes are all correlated - you're much more likely to produce offspring with someone else who also has a recessive gene in the same slot. So instead of a few random little harms, you get a large sum over many little harms, leading to life-threatening harms.

But if you only have a single copy of each gene, those harmful recessive traits will be under much stronger pressure to be excluded from the population since there's basically no redundancy to hide behind. So in that case, inbreeding should make less of a difference.

Haploid organisms are rare among animals, being largely limited to males of social hymenoptera (bees, wasps, ants, etc.) which function broadly as sort-lived sperm-dispersion mechanisms. Plants, fungi, and certain algae groups switch between haploid and diploid states across an alternation of generations, but the diploid state is dominant in most larger organisms. Having only a single copy of each gene produces vulnerabilities to DNA damage, and resulting failure states such as cancer, which seems to select against this in larger organisms. A more likely option would by polyploidy - 3+ copies of each chromosome in some or all cell types. This is quite common in fish and amphibians.

[NichG]

Haploid organisms are rare among animals, being largely limited to males of social hymenoptera (bees, wasps, ants, etc.) which function broadly as sort-lived sperm-dispersion mechanisms. Plants, fungi, and certain algae groups switch between haploid and diploid states across an alternation of generations, but the diploid state is dominant in most larger organisms. Having only a single copy of each gene produces vulnerabilities to DNA damage, and resulting failure states such as cancer, which seems to select against this in larger organisms. A more likely option would by polyploidy - 3+ copies of each chromosome in some or all cell types. This is quite common in fish and amphibians.

I guess you could also make it work if you didn't have crossover with one chromosome out of each set being from each parent, but instead had all sets of each chromosome from one or the other parent. Or if you had some sort of more integrative approach like plasmid sharing or homologous recombination.

[Anymage]

I guess you could also make it work if you didn't have crossover with one chromosome out of each set being from each parent, but instead had all sets of each chromosome from one or the other parent. Or if you had some sort of more integrative approach like plasmid sharing or homologous recombination.

At that point you're still vulnerable to genetic damage as mutations pile up with no outside sources to bring in fresh genetic material. Plus you have the issue where since you basically have a bunch of clones, an inconvenient disease could run rampant across the population. In ideal circumstances you could get away with a straight up parthogenic species, but there are too many things that can go wrong and it's all a question if when something happens rather than if.

Although skipping the science talk, and I'm aware who the OP is, the specifics of DNA combination in a fantasy world sound a touch like asking the DM to explain all the details of his world in ways that fit with (what the player understands of) modern sciences. Even if inbreeding did actively cause DNA degradation somehow, that's about as relevant to a fantasy world as whether giant animals could function given the square-cube law. Table management and telling a player to just roll with it are probably better things for Tal to focus on than the fine details of real world genetics.

[NichG]

At that point you're still vulnerable to genetic damage as mutations pile up with no outside sources to bring in fresh genetic material.

Well the point is to fail fast, rather than be robust through redundancy. If you have redundancy, then basically the population ages closer and closer to the edge of failure. If you just fail immediately when something bad enough enters the genome, then your rate of successful replication is lower, but you can't actually pile up deleterious mutations. You can still have neutral or beneficial mutations of course, and that prevents the population from becoming completely stagnant or homogeneous, at least as long as the environment fluctuates enough or contains enough niches that you don't get competitive exclusion.

[Mechalich]

At that point you're still vulnerable to genetic damage as mutations pile up with no outside sources to bring in fresh genetic material. Plus you have the issue where since you basically have a bunch of clones, an inconvenient disease could run rampant across the population. In ideal circumstances you could get away with a straight up parthogenic species, but there are too many things that can go wrong and it's all a question if when something happens rather than if.

Straight up obligate parthenogenic species do exist, notably in whiptail lizards, and they can perform very well for a long time, so long as the environment is stable. Facultative parthenogenesis or other forms of cloning are more common, wherein the organism produces clones for as long as the environment is stable and then switches to sexual reproduction when the environment changes. This is very well known among aquatic crustaceans, which often squeeze out a whole bunch of clone generations throughout summer and then switch to having sex right before winter. The deleterious impacts of inbreeding probably could be mitigated in this way, since the clone generations would essentially halt the fixation process while they were ongoing.

Well the point is to fail fast, rather than be robust through redundancy. If you have redundancy, then basically the population ages closer and closer to the edge of failure. If you just fail immediately when something bad enough enters the genome, then your rate of successful replication is lower, but you can't actually pile up deleterious mutations. You can still have neutral or beneficial mutations of course, and that prevents the population from becoming completely stagnant or homogeneous, at least as long as the environment fluctuates enough or contains enough niches that you don't get competitive exclusion.

The trick here is selective pressure. A population bottleneck driven by intensive selective pressure is liable to result in fixation of beneficial mutations (assuming the heterozygous state is not superior, but that's fairly rare). If those pressures ease post-bottleneck but remain more or less the same - for example, if the population resides in an environment where the big stressor is seasonal drought, the population that lasts through a really, really bad drought will probably have the right mutations for more modest ones going forward, then that's fine and inbreeding won't be a problem until either conditions change or enough mutations accumulate to allow new alleles to develop and circulate through the population. In both cases quite a few generations are likely to pass before this becomes an issue.

And, that may be all the justification the OP needs. Assuming the monsters in question are long-lived with a lengthy generation time and the dynasty descends from founders who had broadly beneficial alleles fixed for all the important genes, then inbreeding simply might not have become a problem yet. Alternatively, there's also the solution of artificial selection pressure. Meaning, murdering or otherwise preventing reproduction by individuals who display deleterious traits. Now, in the pre-industrial world this was hard to detect - infanticide of newborns with severe, visible physical defects is believed to have been common in some parts of the ancient world, but there's a lot that can't be seen easily - but magic might get around this issue in a fantasy.

[Tvtyrant]

In a recent RPG game I introduced a dynasty of horrific monsters that reproduce exclusively with their own offspring. One of my players said that this was impossible because inbreeding degrades the genetic code, and after a certain number of generations the offspring would no longer be viable.

The player couldn't actually explain the mechanism for this, and I didn't want to stop the game for a debate about genetics, but this didn't seem correct to me.

I understand that with this system the species would have little to no genetic diversity and that harmful mutations wouldn't be difficult to breed out of the line, but I am not aware of any mechanism by which genetic information is actually removed through inbreeding.

Am I missing something? If so, would anyone care to educate me?

Or is my player just misunderstanding the difference between loss of genetic diversity in a population vs. loss of genetic information in individuals as I suspect.



Thanks, and please try and keep the conversation scientific!

Eventually the inbreeding would kill the descendants with genetic disorders altogether, leaving only those completely bereft of them alive. This is how pure breed animals made by humans can exist, you essentially purge the bloodline of disorders by making them so lethal none of them can survive.

This is necessarily true from an evolutionary point of view. Most species have at one point or another lost the majority of their individuals in cataclysms, if inbreeding were as bad on a species level as on the individual there would be no species.

[Quizatzhaderac]

There is something similar in realty: Adactylidium mites mate only with their siblings. For added yuck, the mating occurs before birth and the daughters eat their way out of the mother. This is not a "smart" design (parthenogenesis would probably be more efficient), but it's not fatal.

To understand this, it's important to know that there are generally two kinds of animals: those that rarely inbreed and those the often inbreed.

For species that often inbreed, new genes that are a problem for inbreeders are select against almost immediately.

For species that rarely inbreed, these genes can build up, and sometimes propagate through accident, or may be beneficial if a person has only on allele (like the sickle cell anemia gene).

What's dramatically bad is when a species that rarely inbreeds, suddenly starts doing so. Genes that were 1 in 100,000 suddenly to double up in one individual.

[Squire Doodad]

Although skipping the science talk, and I'm aware who the OP is, the specifics of DNA combination in a fantasy world sound a touch like asking the DM to explain all the details of his world in ways that fit with (what the player understands of) modern sciences. Even if inbreeding did actively cause DNA degradation somehow, that's about as relevant to a fantasy world as whether giant animals could function given the square-cube law. Table management and telling a player to just roll with it are probably better things for Tal to focus on than the fine details of real world genetics.

As an addendum, it's also worth noting that even if this would doom the population of any realistic population, this is a fantasy world with magic.
It is perfectly reasonable to hand wave the genetic questions away by saying their ancestors experimented on themselves with magic that was both accursed and forgotten. It's not why they're so bizarre or reproduce in that fashion, but it facilitates it and there's no way to figure out how it happened and any records of how it worked are lost.
Overall, exactly as you said - real world genetics need not apply. The evolutionary biology of this group isn't important to the campaign, just that they are currently horrible. If why they are horrible is relevant, it doesn't need to be related to the genetics sides of things.

[sihnfahl]

This is how pure breed animals made by humans can exist, you essentially purge the bloodline of disorders by making them so lethal none of them can survive.

With the caveat that the genetic disorder, when in effect, has to be lethal.

Exceptions like...
Hip dysplasia in purebred American German Shepherds. What makes the especially egregious is that this 'genetic disorder' was selected for because of the aesthetics!
The facial structure of pugs and English Bulldogs that makes it hard for them to breathe ... and see above.
Golden Retrievers and their skin allergies. Not intentional, but ...

[Mechalich]

With the caveat that the genetic disorder, when in effect, has to be lethal.

It doesn't have to be lethal it just has to apply selection pressure. The reason inbreeding is such a problem in animals kept by humans as pets/livestock is that natural selection pressures have been largely removed and been replaced by artificial selection pressures that fit criteria set by humans that usually deviate significantly from traits that would provide success in a natural environment. That includes even animals that are extremely athletic, like racehorses.

[Tvtyrant]

Lethal, or prevents successful reproduction* An animal can certainly have a non-lethal genetic disorder that keeps them from mating and so removes them from the genepool.

[Trafalgar]

As an addendum, it's also worth noting that even if this would doom the population of any realistic population, this is a fantasy world with magic.

"This is fantasy, forget about the science"

This is the mantra I repeat to myself every time I watch a show that ignores basic science. I said this to myself a lot while watching House of the Dragon where the Targaryen's regularly marry brother to sister. The Hapsburgs regularly married their cousins/uncles/nieces and look how they turned out.

I imagine Game of Thrones would be a lot less popular if Daenerys was shorter, had an unusually pronounced jaw, and non functioning kidneys.

[Willie the Duck]

Beyond all these lovely replies, its also worth noting that there's no reason that this monster couldn't have developed this trait recently, and perhaps be on the way to extinction.

[sihnfahl]

It doesn't have to be lethal it just has to apply selection pressure. The reason inbreeding is such a problem in animals kept by humans as pets/livestock is that natural selection pressures have been largely removed and been replaced by artificial selection pressures that fit criteria set by humans that usually deviate significantly from traits that would provide success in a natural environment. That includes even animals that are extremely athletic, like racehorses.

Since the comment was in regard to human selection for traits, yeah, we're down to mostly artificial pressures.

Lethal, or prevents successful reproduction* An animal can certainly have a non-lethal genetic disorder that keeps them from mating and so removes them from the genepool.

The main issues there would be sterility or 100% fetal mortality.

Functional issues can be addressed through additional artificial means. On the male side, humans have been pretty good about artificial insemination techniques for breeding animals with ... less than good traits. And I daresay we've even gotten tricking eggs on the female side as well...

[DavidSh]

In a recent RPG game I introduced a dynasty of horrific monsters that reproduce exclusively with their own offspring. One of my players said that this was impossible because inbreeding degrades the genetic code, and after a certain number of generations the offspring would no longer be viable.

I suppose I have to interpret this as "either with their parents or with their offspring", as both parents can't be offspring of each other, barring time travel. Assuming two sexes, an individual would have one set of children with the parent of the opposite sex, and another set of children with various children of the opposite sex

[Talakeal]

Do note that there was no argument or hostility at the game, one of the players just mentioned that he thought it was biologically impossible.

I suppose I have to interpret this as "either with their parents or with their offspring", as both parents can't be offspring of each other, barring time travel. Assuming two sexes, an individual would have one set of children with the parent of the opposite sex, and another set of children with various children of the opposite sex

They are hermaphroditic and always have a single child which they impregnate before dying.

[Mechalich]

They are hermaphroditic and always have a single child which they impregnate before dying.

Okay, that's not inbreeding, it's a highly convoluted and inefficient form of cloning. The offspring are getting 100% of their genetic information from the single parent, it's just alternating across generations.

[DavidSh]

Okay, that's not inbreeding, it's a highly convoluted and inefficient form of cloning. The offspring are getting 100% of their genetic information from the single parent, it's just alternating across generations.

If you have the diploid genetic structure typical (although not universal) of Earth animals, and the first two generations started out as genetically distinct, you should eventually converge to all generations being genetically identical and homozygous in all genes. Unless the mutation rate is sufficiently high. Unless they are invulnerable, you'd expect the line to die off some time, from accidents, predation, or diseases. It takes only one death before reproduction to end the line.

A sufficiently powerful creator, such as a wizard or deity could create a new line from scratch whenever necessary, I suppose.

[OracleofWuffing]

They are hermaphroditic and always have a single child which they impregnate before dying.

But... Where did the first child come from?

[tyckspoon]

Do note that there was no argument or hostility at the game, one of the players just mentioned that he thought it was biologically impossible.

Well, they're not wrong - even if the reproductive method is 100% infallible and never results in a stillborn, failure to conceive, or a youth that has some fatal flaw that prevents it from maturing to adulthood, a species that only ever has one-to-one replacement is doomed to extinction sooner or later, because every single member that dies prior to initiating their clone represents a permanent irrecoverable loss to the population. It's a dead end, evolutionarily speaking, and not likely to be a method of reproducing that would arise naturally because of it. It's not the inbreeding that does it, necessarily, although if the species -is- prone to random mutations or can have their reproductive capability harmed by environmental factors the inbreeding/cloning aspect will tend to make that worse (and if each individual only clones themselves and doesn't mix with other members of the species, eventually what you'll see is either they all die out, or you get a bunch of distinct species-of-one that have common ancestry.)

But being a 'dynasty of horrible monsters' in a roleplaying game, I would assume these things did [B]not[B] arise naturally, and if they're -created- monsters rather than -evolved- monsters then their reproductive biology doesn't actually matter, because if whatever mad wizard, god, demon, or extradimensional alien decided they wanted these things to exist needs there to be more of them then they'll just make more of them.

[Talakeal]

Well, they're not wrong - even if the reproductive method is 100% infallible and never results in a stillborn, failure to conceive, or a youth that has some fatal flaw that prevents it from maturing to adulthood, a species that only ever has one-to-one replacement is doomed to extinction sooner or later, because every single member that dies prior to initiating their clone represents a permanent irrecoverable loss to the population. It's a dead end, evolutionarily speaking, and not likely to be a method of reproducing that would arise naturally because of it. It's not the inbreeding that does it, necessarily, although if the species -is- prone to random mutations or can have their reproductive capability harmed by environmental factors the inbreeding/cloning aspect will tend to make that worse (and if each individual only clones themselves and doesn't mix with other members of the species, eventually what you'll see is either they all die out, or you get a bunch of distinct species-of-one that have common ancestry.)

Oh sure, its an evolutionary dead end as the line will one day die out (indeed, that was the inevitable result of coming into conflict with the PCs).

That was not what the players were saying, rather than their method of reproduction alone would eventually whittle their genetic code down to nothing absent all outside factors.

[Vahnavoi]

"This is fantasy, forget about the science"

This is the mantra I repeat to myself every time I watch a show that ignores basic science. I said this to myself a lot while watching House of the Dragon where the Targaryen's regularly marry brother to sister. The Hapsburgs regularly married their cousins/uncles/nieces and look how they turned out.

I imagine Game of Thrones would be a lot less popular if Daenerys was shorter, had an unusually pronounced jaw, and non functioning kidneys.

Targaryens are not the worst example. They have massive problems within that family that could be realistically attributed to inbreeding - just different problems from the Hapsburgs. From the unusually fair hair and complexion, to notably high incidence of infant mortality and mental instability.

In real life, some Egyptian dynasties did the same thing as Targaryens. I don't remember for how long, but in the short term, it's viable.

[Draconi Redfir]

i can't find anything that goes into detail on it without some stupid "Enter your email to continue reading!" popup blocking the text, but i think there's some species of Black Crazy Ants that through some genetic trick, is able to produce males that are unrelated to their sisters, allowing them to reproduce with their siblings and keep the colony going effectively indefinitely even in complete isolation.

it's not "breeding with your offspring", but it's pretty close. Think it might even still be possible for them if i'm reading this right.

Mother–offspring analyses demonstrated that these allele frequency differences resulted from the fact that the three castes were all produced through different means. Workers developed through normal sexual reproduction between queens and males. However, queens were produced clonally and, thus, were genetically identical to their mothers. In contrast, males never inherited maternal alleles and were genetically identical to their fathers. The outcome of this system is that genetic inbreeding is impossible because queen and male genomes remain completely separate.

queens may mate with their brothers in the parental nest, yet their offspring are no more homozygous than if the queen mated with a male randomly chosen from the population. The complete segregation of the male and female gene pools allows the queens to circumvent the costs associated with inbreeding and therefore may act as an important pre-adaptation for the crazy ant's tremendous invasive success.

some research would be needed, but theoretically it can be done.