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In a world where time travel in the past is possible but deletes the current future/ present, there is a person named Sarah who does not worry about the ripple effects her actions might create. Sarah chooses to travel back in time by 5,000 or more years, to be one of the most powerful women in the past, and to have a family.

Sarah is searching for a romantic interest. She has journeyed so far back in history, it's almost certain that all potential partners are directly related to her. In a situation like this, she wonders about the possible consequences of inbreeding. How cautious does Sarah need to be when selecting a romantic partner, given that she could potentially be romantically involved with a very distant ancestor?

The main question of this query relates to the potential effects of inbreeding in this type of situation. If Sarah were to have children with a distant ancestor, would the impacts of inbreeding be as severe as they are known to be in more immediate familial relationships, such as siblings or first cousins? When do the effects of inbreeding with a great great ... great grandfather not affect the condition of the child, how many years should she go back in time for it not to matter?

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    $\begingroup$ Whatever happens, make sure your protagonist doesn't step on a butterfly!! $\endgroup$
    – RonJohn
    Jul 17, 2023 at 13:41
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    $\begingroup$ @RonJohn question states "but deletes the current future/present"... she's deciding specifically to change the past. She's not worried about butterflies... she's stepping on dinosaurs in a push to become Cleopatra. $\endgroup$
    – WernerCD
    Jul 17, 2023 at 16:22
  • $\begingroup$ Is there a chance she could become her own ancestor? $\endgroup$ Jul 18, 2023 at 5:18
  • $\begingroup$ With sufficient time, either she will become her own ancestor or her line will become extinct. $\endgroup$
    – pjc50
    Jul 18, 2023 at 14:08
  • $\begingroup$ There is an error in your belief. After so much time, while there will be some people who are the ancestor to most everyone alive in the current day, there will also be a significant number of people who have NO descendants in the present time. $\endgroup$ Jul 18, 2023 at 14:37

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Not at all.

The problem with inbreeding is that "bad" recessives might combine. By adding several hundred generations in between, the risk of that is exceedingly small. No larger, really, than having a child with a partner from the same city.

There is the quip in Europe, 'everybody here is a descendant of Charlemagne.'

  • You have $2^1 = 2$ parents.
  • You have $2^2 = 4$ grandparents.
  • You have $2^3 = 8$ great-grandparents.

Assuming $30$ years per generation, and $40$ generations between you and Charlemagne, you get $2^{40} \approx 1100 \: billion$ great38-grandparents. That's way more than the global population at the time.

The only possible explanation is that your great-whatever-grandparent in one line is also your great-whatever-grandparent in another line. This concept is known as pedigree collapse. Humanity is coping reasonably well, as long as this pedigree collapse isn't too recent and too often.

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    $\begingroup$ Back of the napkin, I reckon the amount of inbreeding between Sarah and the ancestor will be similar to the amount of inbreeding between Sarah and a partner from her time, going by the normal standards of partner selection. $\endgroup$
    – Flater
    Jul 16, 2023 at 7:23
  • $\begingroup$ "you get 240≈1100billion great38-grandparents" which begs the question that inbreeding is happening in enough capacity that after a dozen or so generations is already a non issue $\endgroup$
    – WernerCD
    Jul 17, 2023 at 16:23
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    $\begingroup$ @Ed999 In-breeding doesn't cause population die-off. It is recessive traits that cause it. In-breeding just increases the chance that recessive traits are going to cause an issue. If you start with a population with no recessive traits. In-breeding won't cause problems (excluding the ick factor/psychological problems, as human psychology prevents in breeding by making the thought repulsive). $\endgroup$
    – Questor
    Jul 17, 2023 at 19:03
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    $\begingroup$ @Questor That's incomplete. Inbreeding also matters because it makes a group of people vulnerable to the same disease; instead of having a bunch of diseases that (e.g.) don't affect half the population, that make 40% of the population sick and kill 10% of the population, 10% of the diseases kill everyone. $\endgroup$
    – prosfilaes
    Jul 17, 2023 at 19:19
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    $\begingroup$ Putting some more advanced genetic mechanisms aside, 23 of your 2 x 23 chromosomes come from each of your parents, approximately half of that number (so about12) from each of your grandparents, and you very quickly end up with "less than one" chromosome from each ancestor (which is to say: with high probability , no chromosomes at all). Already 7 generations ago, there are so many more ancestors than chromosomes that it would hardly matter if a few of those ancestors are the same person ... $\endgroup$ Jul 18, 2023 at 12:58
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I voted for @O.M.'s answer and you should to. I only have an idea that builds on what @O.M. said.

This is an opportunity to do cool things that you can use in your story

Here we're treading the line between worldbuilding and storybuilding, but I believe it's reasonable here so you can see how your rule — time travel and the issue of avoiding the consequences of inbreeding — could be used for remarkable literary effect. Let's start with a little science:

Over the past decade, whole genome sequencing data of healthy and tumor tissues have revealed how cells in our body gradually accumulate mutations because of exposure to various mutagenic processes. (Source)

Now let's set a rule:

A consequence of accumulated mutations is that some of those environmentally-caused mutations become hereditary.

Storybuilding application?

An important character in the "present" happens to have a rare form of cancer, judged to be hereditary. That person — and the pain/desperation caused by the cancer, is part of the plot process that sends your heroine back in time.

Your heroine doesn't realize it, but she has contracted that same rare form of cancer that can only be found after a specific time in history because of its dependency on environmental conditions. She carries that mutation back in time with her. Part of the romantic story line is her dealing with the onset of "wasting disease" and the eventual realization that it was she who introduced the mutation into humanity before its time — and the final realization that it was her own great-great...grandson who compelled her to time travel.

Combining a dash of science with a world rule, the idea would provide a platform to discuss in your story the complications of time travel and the ethical obligations time travelers could and should have.

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    $\begingroup$ That would make for a very engaging story :-) $\endgroup$ Jul 16, 2023 at 17:49
  • $\begingroup$ She should also watch out not to bring other diseases, since 5000 years ago the effects of bringing modern pathogenes might have devastating effects. Most of the people will not have developed the immunities stemming from life in cramped cities with animals. She could wipe out a whole civilization while seeking romance. $\endgroup$
    – Falco
    Jul 17, 2023 at 9:59
  • $\begingroup$ Another interesting effect are her modern "mutations" which include adaption which took 100eds of generations. Her descendants will introduce these genes into the pool much earlier and probably jumpstart a lot of adaptions, which will influence the human species. This might spread her genes faster (since her children have a genetic advantages) and make her genes quite universal over the world 5000 years later. $\endgroup$
    – Falco
    Jul 17, 2023 at 10:04
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I would add another factor that wasn't addressed in other answers : Her partner's gene pool might never have made it to present times anyway.

There have been many wars, illness, accidents etc... that caused deaths to humans before they could procreate. It is very likely that 5000 years from now, a minority of humans were the ancestors of today's populations. All others either died before procreating in their time, or procreated but at a later time all their descendants died, effectively eliminating them completely from today's gene pool. So it is entirely possible that your Sarah is actually creating a family with someone whose genes won't make it to our present. Even if she becomes the Queen of her time, her gene pool might still vanish sometime in later history.

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    $\begingroup$ And even if there is a lineal relation, that doesn't mean the genes would carry. Each parent only gives half their genes to their child. After five generations (~100 years), only 1/32nd of your genes are still present. (For a single descendant - additional children increase the population total, but non-additively) -- That's a statistical average, though. There's a non-trivial probability that you'll receive no genes from a direct biological ancestor from even 300 years ago. $\endgroup$
    – R.M.
    Jul 17, 2023 at 2:30
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No significant danger

Populations have moved so much in 5000 years due to war, colonisation, migration etc,. that it's highly unlikely there is any risk.

Locale might make a difference in rare cases. I can't think of any off-hand. If she was in the USA and European ancestry then there were no Europeans there 5000 years ago. Just about all others have done a lot of mixing in the last 5000 except Australia, New Zealand, Micronesia etc, which did a lot of mixing in the last couple of hundred and wouldn't pose a risk.

She would probably pose more of a danger to the people back then than anything else if she carried some sickness they have no immunity to. Whole communities had no one left to bury the dead when measles first hit the Pacific.

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  1. Inbreeding is not all that bad, at least not in the biological sense. Yes, one might hit a recessive deleterious gene, but this is rare and is rather balanced with the risk of letting the broken gene in the population's gene pool.

It brings a lot more social and psychological than purely genetic issues.

  1. The risks related to inbreeding are directly related to the percent of genes shared between the partners. The shared genes are 50% in siblings sharing both parents, 50% between a parent and a child and halves at each generation of separation in each direction - it is 25% between a grandparent and a grandchild, 6.25% between first cousins (sharing two common grandparent), etc, etc...

Most cultures put the inbreeding limit at second cousins or even at first cousins (an the life could get really hard in a small village if the limit is at third cousins).

In other words, someone who is your 20 times grand- parent is generally as safe as someone who is your 10-ish cousin.

E.g. as safe as at all possible, because if someone is your 20th ancestor, chances are that they are your 20-ish ancestor in WAY more than one line.

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The protagonist Sarah should not have to worry about inbreeding affecting her children.

According to the answer of fraxinus a parent and child share 50 percent of their genes, and a grandparent and grandchild share 25 percent of their genes.

Extrapolating, a great grandparent and great grandchild share 12.5 percent of their genes, a great great grandparent and great great grandchild share 6.25 percent of their genes, a great great great grandparent and great great great grandchild share 3.125 percent of their genes, a great great great great grandparent and great great great great grandchild share 1.5625 percent of their genes, a great great great great great grandparent and great great great great great grandchild share 0.78125 percent of their genes, and so on.

An ancestor of Sarah born 5,000 years before Sarah would would probably be her ancestor about 200 generations earlier, so they should certainly not share any more genes, including the harmful recessive genes that make inbreeding dangerous, than any two randomly selected persons.

Of course Sarah might travel 5,000 years in the past and find some outstanding person to marry, not knowing that he is another time traveler and in fact Sarah's long lost brother who was separated from her when they were very young.

I note that if Sarah travels back in time about 5,000 years from the present time about AD 2023, she will appear in about 3000 BC. And about 3000 BC was about the beginning of recorded history in Mesopotamia and Egypt, and generations, centuries, or millennia, before the dawn of recorded history in other regions. So if Sarah travels back in time from AD 2023 she will might meet the earliest historical persons in Egypt or Mesopotamia, or might appear in a prehistoric - because preliterate - society elsewhere in the world.

Of course time travel is almost certainly impossible, and if possible almost certainly impractical. So if scientific advances ever permit practical time travel, it might be tens or hundreds of thousands or millions of years in the future. So it is perfectly possible that Sarah might travel 5,000 years into her past and arrive many thousands of years in our future.

And if Sarah travels from some time in our future 5,000 years into her past, she might take many future inventions along with her which might make her seem like a god to the people in her past, especially if her past is also far in our past when technology was less advanced.

Another reason why Sarah shouldn't worry about possible inbreeding of her children is that she is going to take billions of bacteria, viruses, etc. with her into the past. And they well introduce modern genes into the gene pools of thousands of different species of microscopic life. This will change the evolution of new species of microbes out of the past species of microbes.

Some species that produce deadly diseases in humans will not evolve, and so many humans will live who would have died, and some will marry people who otherwise would have become ancestors of Sarah.

And some species will evolve that cause deadly diseases in humans which never evolved in Sarah's timeline, and they will kill many people who would have lived longer in Sarah's timeline, including many who would have later become ancestors of Sarah.

So many of Sarah's ancestor's would never have been born, and so Sarah would never have been born. So if Sarah travels back in time she will certainly prevent herself from ever being born, and so won't have to worry about the possible genetic problems of her children.

Unless Sarah has found a a way to avoid being erased by changing history, she won't have to worry about her children having genetic defects because she will prevent herself from ever being born and so won't travel into the past to have any children.

Of course most time travel stories ignore the reasons why it would be totally impossible for a time traveler to avoid preventing their own birth. So you might just want to avoid all discussion of grandfather paradoxes and how Sarah avoids them.

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With 5000 years between them, there's probably not much overlap within their genome even if they are technically related. Like for twins it's 100% the same but a 50:50 chance whether the one from the mom or the dad is chosen, for siblings. And so with each degree of distance or respectively generation this becomes more unlikely, so that it's mostly only a concern for very close relatives or families with a history of inbreeding.

That being said with such a long distance between the couple you might already see signs of evolution kicking in. https://www.nature.com/articles/s41598-021-84583-1 . It might not be as big as between an ape or you'd still expect some mutations to have happened in between. So she might already stick out one way or another. Classical problem might be of height in terms of better nutrition.

Also it's quite possible that her immune system might be underdeveloped due to better access to modern medicine and the eradication of certain diseases that would now be present again.

And that's not getting into the social evolution which is likely not getting her a spot on the throne but rather one in a mental asylum or worse. But that's a different story.

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