Another thing to remember though is that recessive traits will likely winnow out of a population longer term. For every 4 children on average with matching recessive genetic deficiencies, one will die, two will be carriers, and one will be healthy. And of those carriers married to a clean person, odds are only one in 4 of their children on average will be carriers. Before modern medicine, a lot of these problems took care of themselves. I imagine by the time the population reached Easter Island, many problem genes had weeded themselves out. (That would be an interesting simulation to try to program).
More insidious is something like Huntingdon’s that doesn’t show up until childbearing is over. Those problems may not go away, not so easily bred out.
A recessive carrier marrying someone who is “clean” (homozygous normal) has a 50% for each child of that union being a carrier, not 25%.
Second, not all recessive traits are lethal. So there is no guarantee that a child who inherits two recessive alleles will not have children.
Third, some traits seem to have a greater survival advantage in the heterozygous state. So a child who inherits one “normal” allele and one recessive allele may, on average, have more children than a child with two normal alleles. A recessive CFTR gene, which may cause cystic fibrosis in the homozygous recessive state, seems to offer a survival advantage in areas where cholera is endemic. Sickle Cell trait is another such gene, offering survival advantage in the heterozygous state in malaria endemic areas.
It is true that modern medicine may allow persons with two recessive genes to live longer and have more children than they would have without modern treatments thus increasing the gene frequency of the recessive allele in the general population.
It was not unknown for islander cultures to practice some rather stringent eugenics - that is, they did not permit the significantly deformed to live. IF you’re willing to do that you can cut down on the bad genes floating around the population. It’s not a perfect solution but it might be one reason we encounter small populations that manage to survive for long time periods.
Disclaimer: I have no idea if the Rapa Nui actually practiced infanticide or not, I’m just say it’s a possible means to achieve a genetically healthy population with a small founder group. That doesn’t mean it was used in a particular instance.
It’s not a solution at all if the character is recessive. You could kill both the progeny and both parents without material effect on incidence.
Even for recessive lethals, the vast bulk of those genes are carried in the general population.
I suspect that in a lot of societies, most marriages involved people who were 3rd cousins or related more closely. I was raised in a rural area. When people were talking of someone, the conversation would almost always devolve in determining who is person was related to (“Wasn’t his aunt X, who was your husband’s cousin?”) and a family link was found pretty much all the time. I’m pretty sure I was third cousin of or more closely related to most of the local population. And the local people definitely married each other.
And there are societies, even nowodays, where first cousin marriages are commonplace. I imagine that 3rd cousin marriages there must be repeated pretty often.
Is there really a dangerously high level of birth defects related to inbreeding in these societies?
It’s not a modern mindset, it’s a cultural mindset. First cousin marriages aren’t perceived with horror over here as they are in the USA (not sure if it’s specifically an American thing or more generally an Anglo-saxon thing. Do British or Australians recoils at the idea of first cousin marriage?).
And I’m not sure how it’s “wrong”. Morally wrong? Because of the inbreeding? It seemed to me it was more a matter of “eww” factor than a matter of morals.
Well, yeah, like I said - it’s not a perfect solution. It’s actually pretty damn crude but if you’re operating at low levels of tech there may not be a heck of a lot of alternatives.
Island societies also use infanticide as a form of population control. Not all of them, of course. Other “solutions” we continental folks in the modern world might find abhorrent include putting the old/crippled into boats and sending them off to sea to “discover” new islands, putting the old and infirm on ice floes during hard times/famine, and other unpleasant things. When you start studying what people used to do to survive you gain a better appreciation for the modern world and its alternatives to such methods.
There is a detectably higher rate of birth defects/genetic disorders among societies with many first cousin marriages, but not to the point that those societies are in danger of extinction. The vast majority of people in such societies are fine and it’s a sustainable mating practice over the long term.
In Iceland they take the risks of cousin marriage very seriously. There is a dating app so you can avoid accidentally getting romantically involved with your near cousins. Iceland is a leader in the field because of its small population size, low rates of immigration and comprehensive ancestral records. They can make a good estimate of how closely related two people are.
Pretty soon genome decoding will be cheap enough for everyone to have it as part of their medical record and it should be possible to scan for two adults who want babies to scan for dangerous recessive genes rather than using fairly rough measures like cousins.
Lots of interesting ethical questions arise from genome decoding but also some huge medical benefits.
I don’t take it for granted which is why I look these things up. Media and 2nd hand reports often don’t carefully specify these things. Looking at papers like the one I cited by Bennett and friends make this clear. They are specifying a percentage increase of overall risk.
If the overall risk was 3%, that’s 3 out of a hundred. A 3 percent increase of overall risk would be 3.09 out of a hundred.
But even if the risk is doubled, that’s just 6 out a hundred. Still not a huge amount, which is why statistics can be misleading even if precise terms are used.
My guess is that the intelligence and technological development of modern humans also reduced diversity levels by reducing the chances of separate populations staying isolated long enough to become substantially differentiated. And those same factors would prevent future populations from becoming substantially diverse. There is almost nobody anywhere anymore who is too far away to boink.
Another point to ponder is that the Sentinelese were likely trading with nearby islands until recently, which would we presume include “pedigree borrowing”. One history I read about suggested that about 1300 ocean-going traffic from Indonesia and India started raiding the islanders for slaves; hence the current extreme hostility to outsiders. But the original inhabitants of the archipelago, again, are the extreme end of a migration out along the land bridge during the ice age about 30,000 years ago so not particularly diverse to start.
The estimated 50 and 400 Sentinelese individuals is enough. As I mentioned before an effective population size of around ~70 individuals was enough to populate all of the Americas in the past.
The papers I’ve checked, including my cite, say things like ~3% in general, another 3% for a total of 6% among 1st cousin offspring. (Depending on what they are looking for, etc.)
This is not that hard to check. I’ve done it here and in previous threads on this topic.
If the risk goes from 3% to 6% that’s not a 3% increase in risk. It’s a 100% increase. But double is only a huge jump if it was largish to begin with. 10% to 20% would be a huge jump, but from 3% to 6% is not.
This is a syntax discussion, stating it is a 3% increase in overall rate is far less ambiguous, and it also assumes that there are not other confounding factors.
If the mean rate for women is 3%, but the women is over 40 and thus has an increase in the overall rate of 4% the first cousin risk as an additive factor is no longer an 100% increase.
As the important number is the overall risk and not expressly the delta from the existing mean rate I would argue that the overall risk percentage is far more accurate and much less likely to be misinterpreted. That said if one was advocating for legal restrictions on marriage of first cousins the 100% figure would be a better figure for making that argument.
To be honest, I wasn’t particularly aware of this specific issue (first cousin marriage in immigrant communuties).
As a more general view from the UK I would say, yes it does occasionally happen, but it does raise eyebrows - it’s tolerated but with a little bit of an intake of breath between the teeth, if you you know what I mean.