I don’t understand that - what does the size or diversity of a gene pool have to do with knowing your extended family? Plenty of people in the US barely know who their first cousins and don’t know second cousins at all. The size and diversity has a lot to do with why any two random Icelanders are likely to be at least sixth or seventh cousins to each other - but that’s a different question than whether they know who their second cousins are. That well-documented genealogy means that two Icelanders can compare their family trees in a way two Americans generally can’t - but that just means an Icelander will be able to look at the website and find out if the new boyfriend is their third cousin once removed while the American might not know until the wedding where the bride’s grandfather recognizes his cousin who is the groom’s great aunt.
If carriers still have as many children as they would otherwise have, though, just with non-carriers, then this would have no effect on the incidence of the gene. At least, not by itself: It’s also possible that some carriers might choose not to have children at all, or might do genetic testing on gametes or embryos to ensure non-carrier children, or whatever.
It does not work this way, though. Unless you can identify carriers before they reproduce, the recessive allele will stay in the population. There are naturally occurring alleles where the recessive homozygous version is fatal, and those alleles persist in the population. Artificially removing the homozygote recessive individuals will change the ratio of alleles, but won’t (on its own) drive the recessive to 0.
I don’t think I can answer this directly, because it is going to depend a tremendous amount on the population you’re talking about. Even if we defined that, it would still take knowledge of that populations level of background inbreeding.
The modern way of thinking about this is runs of homozygosity (RoH) are stretches of DNA where an individual has inherited identical copies from both parents. Individuals who are more inbred will have longer and more frequent RoH. In humans isolated or bottlenecked populations will have long RoH, and admixed ( “mixed race”) individuals will have short RoH.
Different traits respond differently to different levels of RoH. If I recall, some traits like height are reduced (on average) in individuals with greater RoH, but other traits show no correlation with RoH. These are all empirical questions that can be tested in large samples, like the UK Biobank. A search for runs of homozygosity and inbreeding depression should find some articles.
Well, by the simple (continuous) models, the rate of the recessive gene would asymptotically approach 0, but in reality, you can’t have fewer than 1 individual with the gene, so an asymptotic approach to 0 will eventually actually reach 0. And it’ll happen much more quickly if you’re deliberately inbreeding the organisms, as is done with purebred animals.
Yes, you can use Hardy-Weinberg equilibrium to compute the decrease in the recessive (q) allele in each generation. If you have a small population and fast enough generation times, then you could eliminate it. Particularly if you can cull (or sterilize) entire litters and the parents whenever a homozygous individual is born to eliminate carriers from the breeding pool.
In humans though, with a generation time of one generation per generation, this could take longer than your thousand year reich if you just rely on eliminating the homozygous. There are also other troublesome issues in the real world, such as complex traits that involve many genes, de novo mutations, desirable alleles being in disequilibrium with undesirable ones, and loss of genetic diversity by removing haplotypes (and their people) from the gene pool.
But it could all be done viss a computer!
That’s true, too. I was just eliding over all of that because it doesn’t even come up unless you ignore the ethical problems with selective breeding of humans.
Er… no.
Let’s assume a family where the women have their first child at, say, 12. Definitely young by our standards, but all too often physically possible. So, a woman has a child as 12.
Her child has a child at twelve, resulting in a 24 year old grandmother.
The grandchild has a child at 12, resulting in a 36 year old great-grandmother
And the great-grandchild has a child at 12, so the 48 year old woman is now a great-grandmother. And quite likely has NOT gone through menopause and, though a bit unlikely, could potentially give birth to a child both her own child and her own great-great-grandchild. Although it’s actually more probable from a fertility standpoint that she’d give birth to a child/grandchild (at 24) or child/great-grandchild (at 36)
What about a child/great-great-great-grandchild? Under this scenario that would be when the originally woman was 60. Huh. According to the Guinness information I could google the oldest natural conception was at 58 with a child being birthed when the woman was 59. Um… extremely unlikely but actually possible in theory.
So no, menopause is not a barrier to a grandmother/grandson pairing. It is a potential barrier to hold old of a woman that could happen to, but not to the pairing occurring.
Yes, this post is full of squick. I am not off to find brain bleach.
Quite a few isolated populations in the Pacific where, shall we say, very active in eliminating bad traits. They were basically a breeding population of humans that culled themselves. Nomadic hunter-gatherers likewise did some culling (birth defects incompatible with their lifestyle, and in some cases always reducing multiple births down to a single child post-birth. There are documented instances of small groups practicing female infanticide to control the population).
This had the effect of keeping isolated populations that were so small inbreeding was inevitable healthy enough to continue to survive. You can still get recessive traits that are seen as negative (such as achromotopsia on the island of Pingelap affecting one in ten natives) but as long as people with the trait can survive and reproduce the trait won’t disappear.
But you’ll never entirely eliminate negative traits because there’s always a certain amount of mutation in each generation.
I’ve know a number of people who elected NOT to reproduce at all out of concern for passing on traits they felt undesirable.
I have personal knowledge of one couple who used in vitro fertilization and pre-implantation testing to avoid having a child with debilitating gene possessed by one of the parents. Otherwise, they probably would not have had children.
So, not only is it possible to “scrub” a gene from reproduction, thereby ensuring subsequent generations of a family will never have to worry about that negative thing ever again (outside of spontaneous mutation) it is actually happening now.
And the Spanish Habsburgs.
As for the Bill in the UK against first cousin marriage, it’s brought in under a provision that gives it no priority unless it either gets unanimous support or is adopted by the government. No chance, therefore, though I could imagine the government taking it up as a health/education issue rather than one of legal proscription.
Recent studies have shown that the Easter Island population wasn’t as isolated as previously thought (and that the collapse wasn’t as severe as previously thought).
https://doi.org/10.1038/s41586-024-07881-4
“Two prominent contentions have arisen from the extensive study of Rapa Nui. First, the history of the Rapanui has been presented as a warning tale of resource overexploitation that would have culminated in a major population collapse—the ‘ecocide’ theory2,3,4. Second, the possibility of trans-Pacific voyages to the Americas pre-dating European contact is still debated. Here, to address these questions, we reconstructed the genomic history of the Rapanui on the basis of 15 ancient Rapanui individuals that we radiocarbon dated (1670–1950 ce) and whole-genome sequenced (0.4–25.6×). We find that these individuals are Polynesian in origin and most closely related to present-day Rapanui, a finding that will contribute to repatriation efforts. Through effective population size reconstructions and extensive population genetics simulations, we reject a scenario involving a severe population bottleneck during the 1600s, as proposed by the ecocide theory. Furthermore, the ancient and present-day Rapanui carry similar proportions of Native American admixture (about 10%). Using a Bayesian approach integrating genetic and radiocarbon dates, we estimate that this admixture event occurred about 1250–1430 ce.”
Given that the nation of the remains and of the modern people have the same name, it sounds like this was something that was already well-known?
It’s common for laymen to overestimate the danger of cousin marriage. Marriage of 1st cousins does not have particularly high risk (unless part of a group where cousin marriages are quite common since consanguinity can “build up” in a complex pedigree). Marriage more distant than 1st cousins is seldom a problem. Mating with one’s great-grandparent yields the same inbreeding coefficient as 1st cousin mating. (Remember that both grandfather and grandmother contribute to inbreeding in marriage of full cousins.)
It is also common to over-estimate the inbreeding in royal pedigrees. Some accuse the British royal family of inbreeding. Nonsense! Of course some royal families were inbred: As a result of much incest, the King of Spain Carlos II “El Hechizado” was slightly more inbred than the product of sibling marriages; his inbreeding symptoms were extreme.
Note that if both mother and father are very inbred, but inbred from different families, the inbreeding coefficient drops to zero in their offspring! (In fact the offspring may have genes slightly better than average since inbreeding eliminates some bad alleles due to early deaths.)
Genetically speaking, “Variety is the spice of life.” Reproducing via a first or even second cousin is genetically unwise and markedly riskier in terms of possible genetic defects. I can’t imagine why it is legal. Political reasons, perhaps?
Political reasons in England and thus Commonwealth countries being that Henry VIII wanted to marry his deceased wife’s cousin five centuries ago.
And yet, the possibility of first cousin marriage within England and other countries does not appear to have contributed to major genetic issues over the past five centuries.
Correct- good point- unless you keep doing it, over and over.
While recent studies have cast serious doubt on whether cousin marriage is as dangerous as is popularly assumed… In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.1–2.0 percentage points above the average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40
Only if a family keeps doing it. If you marry your cousin, the risks are slight. However, if all your kids marry their cousins, and so forth, things can get bad.
Correct. Good point.
In some cultures, cousin marriage is allowed, and even preffered over marrying some who is not related. The reasons are complex, but a lot of boils down to not risking bringing an outsider into the extended family.
Surely, the simpler solution would have been to amend the inbreeding laws to apply only to blood relatives.
It’s interesting that, in some cultures, a widower could be prohibited from marrying his wife’s cousin, while in other cultures, a man might be obligated to marry his brother’s widow.
Sometimes practical - if you’re living on a low-population where distance travel is very difficult folks will usually wind up related to each other after a few generations people the gene pool is limited. This happened in some areas of Appalachia - mountainous terrain pre-motorcar made distance travel quite difficult so finding someone you were truly, completely unrelated to after a century or so became nearly impossible. Once automobiles arrived people traveled more and inbreeding dropped.
Religion and other cultural norms can also limit gene pools, even when living in densely populated areas.
There was a Native American tribe in an arctic area that practiced polyandry. Every woman had two husbands because (apparently) it took the efforts of two male hunters to provide for a woman and her children. They also practiced female infanticide to maintain the 2:1 male to female ratio. I don’t remember the name of the group or if they still maintain polyandry or have changed with exposure to the outside world.
Sorry - poor composition. the tendency to serial pairing (?) where the parents of a first child move on to marry/pair with others, often with little contact afterwards, means the children have little contact with one parent’s (presumably the father’s) extended family, and the relative connections become very tangled. If Ann and Bob have a child, Charles, then Bob moves on to Dora and has a child Ellie, and then moves on to Frieda… Other than knowing they have a half-brother or sister, the next generation may not know the connection. If Bob moves all over the country, New York to Chicago to LA, then the risk of the offspring of Charles and Ellie becoming “acquainted” is less; Or Bob’s sister and her offspring in the mix; etc.; in a small environment like Iceland, apparently this is a larger risk. Estranged baby-daddy and no regular contact with that side of the family is not a new thing or unusual.
One aspect of Icelandic socety according to the article on the “relationship app” was that a couple having a child early in life and then moving on to other partners later was not uncommon.
What makes Easter Island interesting is that the other islands were well aware of each other and traded regularly, so not really isolated gene pools. Easter Island seems to have been completely isolated - the articles suggest they managed to reach and trade with South America for a while, until they could no longer put together decent sized vessels. IIRC one of the early explorers mentioned when they arrived, the few small canoes were stitched together from pieces, since the large trees were all gone. Jared Diamond’s Collapse has a discussion of Easter Island.
Infanticide as a means of population control was common in many early societies. Sadly, female infanticide was more common because the number of women determined the number of births, while extra men were just more labour (or warriors).