This is a sort of offshoot from the “Is there any practical way to increase the birth rate?” thread. Someone pointed out it’s not impossible that increased medical knowledge will probably increase life span and extend how long we remain fertile, and talked about the possibility of populations with lots of fertile people who are hundreds of years old.
How would laws about incest have to be changed to adapt? Genetically the point is to lessen the chances of recessuve ‘bad’ genes from being matched up to the harm of the baby. So parent/child, and sibling crosses are pretty much universally taboo. Some groups bar first cousin matches, and I expect somewhere even second cousin matches are discouraged. I imagine there have been a few grandfather/granddaughter matings over the span of history, but I bet vanishingly few. And menopause pretty much takes care of grandmother/grandson having children.
But what happens if we get that suggested boosted longevity? I can perfectly well image that people grow less interested/involved/informed about who their descendants are, when the generations start piling up.
Of course, with each generation the number of shared genes decrease. Half your child’s genes came from you. One quarter of a grandchild’s are yours. One eighth of a great-grandchild’s. and so on.
Plus, all humans are basically related. Pick a random girl from China and a random boy from Sweden. Some percentage of their genes will match, well,just because they are both humans, regardless of how far back you’d have to go to find a common ancestor. (Genghis Khan, maybe?)
So, basically, how many generations apart would two people have to be that their genes have no more in common than those randomly picked pair?
It is seldom possible to identify fourth-degree cousins, since few people can trace their full family tree back more than four generations. (Nor is it considered important, since fourth cousins tend to be genetically no more similar to each other than they are to any other individual from the same region.) [26]
Most laws allow 2nd cousins to marry, iirc. And some allow 1st cousins to marry as long as they are not ortho-cousins.
*An ortho-cousin is a cousin who is the child of a parent’s same-sex sibling /mark.
the calculation of genetic relationship comes out to 0.1% for fifth cousins (if I’ve interpreted it right).
But in many communities of the modern day (and even more of the past), we can’t assume that the common ancestors in the past weren’t related to some extent.
OP: Don’t forget it’s not number of generations but who’s in them that matters
If your kids marry & breed with each other & their kids do the same, generation after generation, more generations are not better.
What you need for diversity are people not in your nearby family tree. Which is also why isolated villages historically turned into inbred basket cases.
Why would they have to be changed to adapt simply because people live longer? Sure, there will be more generations alive at once but my state’s current definition of incest includes
“person whom he or she knows to be related to him or her, whether through marriage or not, as an ancestor, descendant, brother or sister of either the whole or the half blood, uncle, aunt, nephew or niece”
so any ancestor - not just a parent but a grandparent or great (however many times) grandparent. I checked a few states and more than half used “ancestor, descendant” . The other half used “grandparent , grandchild” - but it’s possible that “grandparent” might legally include “great grandparent”
I see you’ve never met a 28 year old grandmother.
I should have mentioned this first but the point of laws/customs prohibiting incest is not just to prevent recessive genes from being expressed. It can’t be - different cultures have different rules and there were rules before anyone knew anything about genetics. There’s also the issue of family dynamics - even where first cousins aren’t permitted to marry, it often isn’t legally incest ( a crime) if they have sex. The definition of the crime of incest involves someone who was likely in a position of authority over the other person when they were a child. Usually an aunt/uncle has some authority over a niece/nephew and the same for parents/children and all types of grandparents and their grandchildren. The only sort of exception is siblings and even in that case one often had some authority over the other as children. Which is why the definitions vary - different cultures define family and who is allowed to marry differently - if it was just genes, there would never be a difference between parallel cousins and cross cousins.
There are no restrictions on cousins marrying in Canada or England, although there is a bill in the UK Parliament which would prohibit first cousin marriage.
And that’s why the laws might need to be changed. Genetically, there’s no issue if Lazarus Long has kids with his great^n granddaughter, and sociologically, there’s no issue either, since he never even met her until she was a full adult. But legally, under the current laws, it’d still be incest.
The sire and dam our of our shetland sheepdog (AKA sheltie) were basically American on one side and Canadian on the other. There are no pedigree repeats until seven generations back. While I have not calculated his coefficient of inbreeding, this means it would be very low compared to any purebred dog, and perhaps also low for a mixed breed.* I also think our dog’s coefficient of inbreeding is probably on the low side for a human. That’s because the standard formulas, even if you can put in a full twelve generation pedigree, give very little weight to repeats in the distant past.
HOWEVER, if you do go back more than, say, ten generations, the number of repeats in the pedigrees would start being very high – probably a lot higher than with people. That’s because there was a time, maybe in the late nineteenth century (or a lot more recently for some breeds – could be right after WWII), when the breed was being started up by a small number of hobbyists, so that there were not all that many individual dogs
The question then is the same as with the thread title, just for a different species. Is the standard formula correct to almost totally ignore repeats in the fairly distant past?
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* Wondering how this can be? If the sire and dam were both purebred, the mixed breed coefficient of inbreeding would indeed be miniscule. But most mixed breeds are not the product of purebreds AKA F1.
Based on British literature and biography first cousin marriage was not considered note-worthy in 19th century Britain (probably because it conserves estates) - Darwin married his first cousin and the heroine of Pride and Prejudice turned down a proposal not because ir was from his cousin, but because he was a no-hoper
The question is fundamentally different with nonhumans, because there are things we do with breeding dogs that wouldn’t be ethical with humans.
Let’s say that you start off with two closely-related organisms, who both have some bad recessive gene. If you breed them together anyway, about one fourth of their offspring will show the recessive trait, half will be carriers, and a fourth will be free of that gene. But if you cull the ones that show the recessive trait, and then breed the rest of them together, the rate of that gene is now decreased. Keep doing that, and eventually you’ll breed the trait out of the population entirely. Cull all of the negative traits that you don’t want, and eventually you’ll breed all of those negative traits out of the population. And once you have all of the negative traits bred out, inbreeding doesn’t matter any more.
For most purebred domesticated animals, this process has largely already occurred, so even though all purebred animals of the same breed are extremely closely related, it’s not as problematic to breed them with each other as it would be for wild animals that closely related. But there are still a few negative traits left, so you have to know what the negative traits are in your breed and watch out for them, and continue to cull them.
Biologically, of course, all of this would work just the same way for humans. But we don’t just cull humans.
When he wanted to marry Catherine Howard there was an ecclesiastical bar because she was a cousin of Anne Boleyn. That would be too close for matrimony, because Henry would be a cousin-in-law to Catherine.
So Henry got Parliament to abolish the bar on cousins marrying. It ceased to be part of English law, and likely in lots of other Commonwealth countries as a result.
Then Henry married Catherine.
Poor Catherine. Would have been better for her that the law not be changed.
As mentioned, some jurisdictions allow first-cousin marriages. The link earlier points to first cousins having about 1/8 common genes. SUpposedly, the implication is their children would have approximately 1/16 genes in common. Humans have 23 pairs of chromosomes, and receive half from each parent. So the odds that a child of cousins has significant number of two same chromosomes is fairly low; plus, chromosmes can “mix and match” during cell splitting, so the chromosomes may not be completely identical. Get beyond first cousin and the odds get worse.
Not sure what the laws are in Iceland. It’s noted for its interesting social situation. From what I’ve read - a relatively small secluded population, less reliance on traditional marriage so there are a lot of children of couples who co-habit, have a child or two, and then split up, to go on and cohabit or marry someone else and have children. Sounds a lot like North America today, except for the much smaller, less diverse gene pool. As a result, some people may not be terribly familiar with their extended families.
Fortunately there’s a well documented historical geneaology; and now they have an app that can tell people who meet the extent to which they are related, should they feel the urge to carry on the relationship further. (“Want to get it on? There’s an app for that…”)
Also brings to mind the history of Easter Island, where supposedly one or two large canoes landed a thousand years ago, and by the time European sailing ships encountered them, they’d populated the Island with up to several thousand people before a serious ecological collapse. No indication that those inhabitants were any less robust than a random pool of humans.
I recall a guest lecturer in a university course I took on Science Fiction. (Hey, they offered it). His concern was not inbreeding or such, but that modern medicine was allowing people with defective genes to survive that we would not have in pre-civilization times. I noted that he and I both wore glasses. And presumably the twist is that today we have genetic correction technology to some extent.
A lady I worked with became a grandmother at 35. (Her daughter was born when she was 16)