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At one point in the past, a man had a mutation on his Y chromosome. Anyone with the mutation would only produce Y sperm, and only have sons. My plan is to have this have a big impact that threatens to wipe out humanity, but does not succeed. Society should eventually recover and reach something comparable to modern technology, but the impact of the mutation should still be felt - either through some carriers still remaining or having had a big impact on history, preferably both.

Assuming history is the same until the mutation is introduced, when and where should it appear? Anything from broad place/time to naming a specific person.


Hopefully distinct from What is the result of a genetic mutation that disallows female offspring? by that question assuming some type of modernity with government and officials, while mine doesn't.

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    $\begingroup$ Have you done any modeling, even rudimentary? Why on Earth would such a mutation spread to reach significant levels in the population? (Hint: what are the chances of having grandchildren for a man who has only sons compared to a man who has both sons and daughters? Take into account that almost all women reproduce, whereas for men the proportion is significantly lower; and even more so in older times.) $\endgroup$ – AlexP Nov 12 '19 at 12:04
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    $\begingroup$ @AlexP such genes do spread in a population, we see it in mice. but they also can't take over a population and you get boom bust cycles. in humans these cycles would take a tens of thousands of years. $\endgroup$ – John Nov 12 '19 at 12:23
  • $\begingroup$ @AlexP My rudimentary model is that the frequency of the gene doubles every generation, assuming men with it are not disfavoured as partners. Normally a gene is passed to half your children on average. This gene is passed to all your children. Of course this assumes the gene survives a one or two generations by pure chance, otherwise there is no story. $\endgroup$ – Grollo Nov 12 '19 at 13:01
  • $\begingroup$ "A gene is passed to half your children on average": true. Now, if the child in question is a girl and she survives to adulthood, she has almost 100% chances to reproduce and to pass the gene to your grandchildren. If the child in question is boy and he survives to adulthood, he has much lower chances to reproduce and pass the gene to your grandchildren. For the purpose of passing genes to your grandchildren it is very much better to have girls than boys. $\endgroup$ – AlexP Nov 12 '19 at 13:20
  • $\begingroup$ @AlexP That does't actually matter though. If fewer males have children at all, this is balanced out by some males having a lot more children. Also, have you heard of the Fisher Principle? By default, sexes are in equilibrium. $\endgroup$ – Grollo Nov 12 '19 at 13:30
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As the top answer in the linked question indicates, this sort of mutation is a very serious problem for humanity which fortunately can be quite easily mitigated with modern national- and international-level identification and amelioration programs. So it is essential that your story begins prior to the discovery of modern genetics at the beginning of the 20th century. Scientists of the Mendel-era might have been able to identify the mechanism behind the illness, but would have been powerless to identify which of a given collection of human males had, or did not have, the mutation.

I'd say any time before 1900 is fair game for the mutation surviving and posing an significant threat to humanity. But you want the mutation to threaten all humanity, and that requires a very broad dispersal across ethnicities and cultures as well as geographically.

The best vector for such a dispersal is a large body of men with a high proportion of 'infected', travelling into new populations and procreating with a large number of native women, disregarding normal monogamous family groupings within that population, and leaving the male offspring in the destination population. That unfortunately describes pretty accurately the conduct of all armed forces prior to the Second World War, and a depressing number since. Armies being the main vector makes a lot of sense in all sorts of ways: infected monogamous families will have a large population of sons, and 'second-plus sons' are more likely to join the army. Meanwhile soldiers prior to the 20th century would spend years at a time away from their families, making the increasing gender disparity harder to identify, while the wives of soldiers away for such a long time might have daughters from, ahem, 'other sources', further complicating discovery.

My suggestion is that the mutation breaks out in early- to mid-18th-century Western Europe at the height of the Age of Empire. At this time the European nations were rapidly expanding their military and economic tentacles into the New World, to either civilise and enlighten, or rape pillage and plunder (depending on your perspective) these new lands. While the mutation hollows out the demographics of the Old World populations it would be spread to all corners of the globe by sailors and soldiers.

The consequences of this could be quite wide-ranging, and would certainly make a big impact on history. The empires of the old world would probably collapse from within as their demographics skewed beyond recovery, leaving the fledgeling colonies independent much earlier than in our history.

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    $\begingroup$ Worth noting that at that time contemporary scientists probably would be capable of recognizing the skew in birth rates but many leaders would likely view it as a good thing. More men mean more soldiers and sailors. Once women became noticeably scarce it would be too late. The excess of men without access to women would drive further expansion and fuel the military with excess manpower leading to conquest and colonization of the whole globe. $\endgroup$ – Mike Nichols Nov 12 '19 at 16:10
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    $\begingroup$ In the late phase there would also be a drive to 'import' women from the colonies/conquests to make up for the dwindling female population at home. I initially thought that this situation might actually end up good for female empowerment, but on further reflection I suspect it's likely to be as bad, if not worse, than real history... $\endgroup$ – Stephen Nov 12 '19 at 16:47
  • $\begingroup$ such a gene is not spreading in any meaningful span of time. you need to introduce it when people are all still hunter gatherers to have an noticeable effect on modern populations. $\endgroup$ – John Nov 13 '19 at 14:02
  • $\begingroup$ @John The way I'm thinking about this is that if we assume that the allele frequency doubles every generation then it should only take ~20-25 generations for the allele to "explode" in a region with an initial population of a few million. What leads you to suggest the mutation would have to arise so long ago? $\endgroup$ – Mike Nichols Nov 13 '19 at 16:16
  • $\begingroup$ @MikeNichols because it doesn't double, keep in mind a side effect is reducing the fecundity of the carrier. and more importantly as it spreads basically stalls your population growth, so your population is limited by your immigration rates which is never high. in mice the R2d2 genes spreads at about about 0.1% of the population per generation and crashes if it becomes too common in a local portion of the population. projected estimates are around 1000 generations to fixation which would be about 30,000.years for humans. $\endgroup$ – John Nov 13 '19 at 16:32
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Specific Person: Genghis Khan. It's late enough in humanity development to have continents not affected by it but his impact on child bearing is noticeable. Not to mention that people think that after wars there should be increase in boys being born (they also think increased boys being born means war but whatever).

A place in time could be right after Black death plague: very weak Europe, Marco Polo and his father exploring China. Golden Horde wouldn't be affected first so they would/could lanuch an attack on weakened Europe taking captives and further spreading the mutation.

Yes, this is Europe/Asian centric but the goal is to have similar technology after all. By that I assumed that place that need to take the biggest hit is the one that had big impact on said technology.

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Boundary conditions:

1) You need centuries for the mutation to spread. Assuming that population is stable (actually not so problematic assumption, because the calculation would remain the same, if both populations were growing with identical speed)

20 years per generation, 2 surviving sons => 2^5 per century. So just from mutation to producing a million of carriers you would need 4 centuries.

For year AD 1000 there were pending on source 250-300 mln people. So in pure number game you'd need this mutation to happen roughly counting 560 years earlier to be able to engulf whole mankind. In half millennia someone would notice the trend.

2) You need this group of angry incels to be sufficiently mobile, so they would be able to fight their way to find any mate. In Africa and Eurasia this one is not the case, as already in ancient times there were invasive nomad tribes. Even then, people in Americas or Australia would remain unaffected.

3) Infighting - While hard to stop in direct invasion, a big group of frustrated men does not need any external enemies to get in to a fight, so their mortality rate would be excessively high.

Conclusion: any moment in historical times, after a few centuries it may snowball all over Eurasia and north Africa, however it should fizzle out quite quickly and only holdouts of this mutations would remain, while even premodern people would look at it as divine curse and would not let their daughter marry such aberration.

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