On Genetics

[Stormcrow]

Oki Dokie. To move away slightly from the usual;

I'm working on a campaign world and my question is thus, can you take four families of 50-100 members each and have them breed nigh exclusively and have each family survive viably. Or is one hundred not enough people to make each family genetically different enough.

In summary.
Each clan is genetically pure. Just that clan's bloodline, is 100 people enough to start that process?

<If you don't know genetics, please don't comment>

[Iku Rex]

Each clan is genetically pure.

Huh? What does "genetically pure" mean?

[Stormcrow]

To explain more clearly.
Say an extended family of one hundred walks into a forrest together, are they enough building blocks for a society to flourish there, genetically. Without people from outside of that original pool coming in.

[Mewtarthio]

I believe he means that nobody has any significant genetic disorders that would preclude breeding.

[horseboy]

I think it's 50/500
50 individuals for short term survival
500 individuals for long term viability.

[Prometheus]

Other than the societal taboo of inbreeding, I don't think medieval people knew of the negative genetic effects of inbreeding. Therefore as a DM you can just say that there are no negative genetic effects or that genetics doesn't work the same way.

Probably the better question you want to ask is if the PCs will see it as distasteful inbreeding, and the answer is probably not exclusively, but it will make them a little uneasy. If that is the effect you are going for then you've got it perfect. If you want no element of disgust, go with 1000, if you want a high level of disgust, go with 30.

[Rex Blunder]

Wonder which genetics thread is the fittest

What's the scenario? Does each family breed with members of the other families, or amongst itself? If the latter, what's the definition of "family"?

Pitcairn Island started with 25 unrelated people. After 70 years, their population was 200, and I assume that at that point, everyone was related to each other. Their colony has kept going until today without much new blood. It's a good way to collect genetic defects though.

[Stormcrow]

I've got a cultural anthropologist in my gaming group so I try to get stuff right as often as I can.

[Rex Blunder]

Any chance of letting the cultural anthropologist design some of the campaign world? It'd give him/her some ownership and give you a well-researched world. Of course, if you need to spring surprises on the party, that might not work.

[Lord Tataraus]

If it is a Lawful group (alignment-wise), I suggest doing what the aborigines of Australia did which is ingenious and I'll ways remember it. First, its been awhile since I went Down Under and learned this, so this is the best I can remember:

The whole group splits into 8 different subgroups A-H.
Males stay in their subgroup, females are married out.
Females from group A marry into group B, females from group B marry into group C, etc.
So basically you get something like this:
A -> B -> C -> D -> E -> F -> G -> H -> A...

Due to this organization, the small isolated groups of aborigines survived without any genetic disorders with relatively small numbers.

[Xefas]

And this anthropologist isn't more concerned about the fact that every living thing in D&D can be half something else, even if they don't even use the same breeding method? or are even of comparable size?

Where do the half-dragon badgers come from? Where?!

[BizzaroStormy]

First of all, are you wanting some specific trait to be passed on to every member?

[Stormcrow]

You did catch I'm... _from_ Australia yeah?

But I'm not touching that one with a thousand yard pole.

Basically the idea is that having rifted from their society the four effected families move to a provence called Voetaari and there is some issues with locals etc but basically each family moves into a different area of the country and venerates a different nature spirit that dwells there.

The familes of 100ish people mostly breed amongst themselves minimum distance of cousins however, and ocasionally other tribes send people for alliances and stuff. I want to know if its viable or if I need more principle people before I write their history.

[StickMan]

I would like to note here that 25% of all marriages world wide are between cousins. I think your going to need more than 100 but it also depends on what your family set up is. If all of these people are related with in one generation your in trouble i.e. they are all first cousins, brother and sister your doomed. If there is more diversity than that such as there are 3 cousins and such in large numbers you might be OK with that small of a number.

Odds are the family would be very careful about who is married to who, even early people figured out that mating with close family is bad that is why its taboo in many cultures. When you want to marry or mate with someone your going to need to be at least second cousins for it to be OK with the clan.

This is only if your talking humans some species do better with inbreeding some do worse. Especially in a DND world.


Well this is large than 100 people but look how the European royalty look. Waits to be killed by European Ninjas.

[HidaTsuzua]

200-400 people is likely enough to keep a population going. Minimum Population Size is based off of probability of extinction at that size (usually 5-10%), not a guarantee. Polynesia studies and theories are a good source of ideas about colonization. Here's a paper that gives around ~70 women for the founding of New Zealand. There are others that give as low as 20-30 people total. So it's okay especially if you up the numbers a bit. Throw in some interbreeding and things are even better.

However you will likely have a huge amount of genetic drift. With such a small population, the chances of any given allele (variations of a certain gene) becoming fixed (everyone has it) or gone (no one has it) will be great [this is the usual fate for most alleles, but the speed depends on chance and effective population size]. So people will look similar (not identical) and often have stuff rare in other populations (6 fingers, a genetic disease, the Habsburg Jaw, and more).

Lately please ditch "genetically pure." While I think I know what you're talking about, it sounds really bad. Also really isn't a factor anyways (the bottleneck of 100 people will do it).

[Lord Tataraus]

You did catch I'm... _from_ Australia yeah?

O.o


Oh...*hides*

[OneWinged4ngel]

As far as I know, inbreeding doesn't cause much problems in and of itself (contrary to popular belief) so much as simply causing recessive traits to show up more often (which can be beneficial OR detrimental.)

[AtomicKitKat]

Inbreeding is both beneficial and detrimental. And it's the frequency of ocuurence of recessive traits which provides both. Assuming that a recessive trait is "obvious" and the families are very strict about enforcing "purity", they will exile the rejects into cloistered "villages"(possibly separated by gender, or neutered, or even outright killed, depending on alignment). Over time, only the fittest will survive, in much the same way that dogs have been forced into very specific breeds over millenia.

[Ulzgoroth]

Loss of heterozygosity has reliable bad consequences. There are just so many places for it to go wrong, since it lets any defect in the ancestor get a chance at homozygosity. Yes, you might come across a useful recessive trait, but the bearer would probably be nice and messed up in all kinds of ways.

Also tends to reduce fertility, because there's a big reservoir of prenatal (often conception-level) recessive lethal alleles that get a much better chance of coming up than usual.

[Chronos]

...so much as causing recessive traits to show up more often (which can be beneficial OR detrimental.)

Usually, if the recessives are beneficial, they'll tend to become more common anyway, and if the advantage is significant, will quickly (as evolutionary timescales go) spread through the population.

Really, though, you can almost go to an arbitrarily small seed population, if you're willing to accept very high rates of defects. If you just have lots and lots of kids, and maybe cull out (by death, exile, or sterilization) the ones which show undesired traits, you'll eventually get a population without those traits. Of course, this is morally and ethically questionable for humans.

[StickMan]

People above bring up good points. If the starting group is very healthy you should be good, but if they do have a negative trait it will become pronounced in the population. I think your safe with the group size honestly.

[Rex Blunder]

If you just have lots and lots of kids, and maybe cull out (by death, exile, or sterilization) the ones which show undesired traits, you'll eventually get a population without those traits.

I don't know if that's true. Aren't there hereditary diseases like cystic fibrosis which are genetic but usually fatal in childhood? I think recessive genes can survive in the population even if, when they cause their trait, they prevent breeding.

[Sundog]

I don't know if that's true. Aren't there hereditary diseases like cystic fibrosis which are genetic but usually fatal in childhood? I think recessive genes can survive in the population even if, when they cause their trait, they prevent breeding.

Actually, such recessives can only survive in an "open" populaton. In a closed population, lethal recessives are either bred out or become dominant - in which case the population crashes to either a much smaller population without the recessive, or to extinction.

[Nerd-o-rama]

Well, the only thing I've ever heard is that you need about 200 genetically distinct men to repopulate the planet. That was with the better part of 3 billion women to breed with, however. Also it was in a comic, but Brian K. Vaughn seems to do his research.

500 sounds like a good estimate.

[Dervag]

Pitcairn Island started with 25 unrelated people. After 70 years, their population was 200, and I assume that at that point, everyone was related to each other. Their colony has kept going until today without much new blood. It's a good way to collect genetic defects though.

The Pitcairn islanders do have some issues with genetic defects, though not crippling ones.

I would like to note here that 25% of all marriages world wide are between cousins.

For what value of 'cousin'?

Well this is large than 100 people but look how the European royalty look. Waits to be killed by European Ninjas.

Actually, most of the European Ninjas ended up backing various republican or socialist movements, not least because the alternative was all those inbred monarchs.

On the other hand, European royalty tended to marry cousins more often than populations would indicate, because they kept trying to keep various noble titles in the family. Perversely, they actually had an incentive to engage in mild inbreeding (i.e. not brother/sister, but second cousin/second cousin or occasionally first/first).

As far as I know, inbreeding doesn't cause much problems in and of itself (contrary to popular belief) so much as simply causing recessive traits to show up more often (which can be beneficial OR detrimental.)

For a group with reasonable distributions of genes, it tends to be detrimental. Many recessive gene combinations are fatal; few recessive gene combinations are so beneficial that it offsets the increased risk of death.

People above bring up good points. If the starting group is very healthy you should be good, but if they do have a negative trait it will become pronounced in the population. I think your safe with the group size honestly.

The problem is that they can be perfectly healthy and still carry nasty recessive genes.

Actually, such recessives can only survive in an "open" populaton. In a closed population, lethal recessives are either bred out or become dominant - in which case the population crashes to either a much smaller population without the recessive, or to extinction.

The risk of this happening is, of course, the entire problem with inbreeding as a practical matter.

[Clementx]

For the majority of human cultures for most of human history (lots of generalization there, but it bears out), the preferred mate was the 1st cousin, usually for reasons of inheritance. Combine that with the lack of mobility and low-population density for most of history, you can see that humans do just fine in small clusters. The whole incest taboo is purely subjective. Western modern cultures think that 1st cousins are gross. Unilinear cultures would think nothing of marrying a cross-cousin, but be just as repulsed by a parallel-cousin.

One hundred gives you plenty of familiar clusters, and as long as they are grouped into moieties, exogamous clans, or other divisions, describing them as non-Dunwichean won't raise any flags for verisimilitude.

[Nonah_Me]

I want to link this thread to the thread where the guy says most people on the boards are commoners with less than a 13 INT.

[dragonseth]

Hooray for the founder effect! Inbreeding doesn't cause mutations by itself, but it will increase the rate of homozygosity in the population. It is important to note, though, that allelic frequencies do not change, only genotypic frequencies do. And so, individuals who are homozygous recessive for some genetic disorder become very common. A good example is the Amish. EVC syndrome and glutaric acidurea both have very high rates of occurence in some Amish communities.

On a related note: Support Hardy-Weinberg Equilibrium! Practice random mating!

[Nonah_Me]

On a related note: Support Hardy-Weinberg Equilibrium! Practice random mating!

That, sir, is going in my sig. Thank you.

[dragonseth]

That, sir, is going in my sig. Thank you.

Our branch of Tri-Beta sells shirts with that on them. It's really funny. They also have shirts with a cell undergoing cytokinesis that say "Stop staring at my cleavage!"