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If, for some reason (global nuclear war and necessity to stay in a closed vault for several hundred years, for example), a group of people will be forced to live within an isolated community without the possibility to invite anybody from outside for a very long time, after a few generations everybody inside it is going to be blood-related to everybody else.

How should the community plan their reproduction to survive for a maximum amount of time without eventually becoming too closely related to each other and gaining various genetic defects as a result? And would letting in a few survivors from the outside once in 1-2 generations help a lot?

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  • $\begingroup$ This relates very heavily to the concept of a minimum viable population. See also the Wikipedia article. $\endgroup$ – a CVn Apr 10 '16 at 16:37
  • $\begingroup$ Hahaha well, I don't think a few hundred years would be a realistic timeframe, since a normal human could possibly live for a hundred years. And you'd need at least 4-5 generations first, before inbreeding becomes a problem... $\endgroup$ – ASH-Aisyah Apr 10 '16 at 16:49
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    $\begingroup$ @ASH-Aisyah - Longevity is irrelevant. Female fecundity is about 15 to 45. That is what determines generation time. $\endgroup$ – WhatRoughBeast Apr 10 '16 at 18:27
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    $\begingroup$ Note this isn't a yes/no issue, as in, there is enough genetic diversity or there isn't. You can't say, 114 people is not enough and they'll all die but 115 is plenty and everything will be fine. It's, The more diversity the better. I suppose up to the point where every allele in existence is present in your gene pool, but that would be a lot of people unless you were able to choose them very carefully after thorough genetic tests. $\endgroup$ – Jay Apr 11 '16 at 13:28
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100-150 individuals with equal sex ratios is the number I have seen thrown around for human survival while avoiding serious inbreeding issues. This number could be lower for more diverse populations and higher for more homogeneous populations.

This group could keep track of their family histories strictly to try and maintain genetic diversity for as long as possible. Initially, this could start specifically by mating with those as un-like you as possible. Letting in a few outsiders every few generations would be helpful if they were genetically fit, or if they were genetically very different from the population inside. Or, if they had some kind of genetic resistance to a deleterious genetic mutation occurring inside the original group. Your group could also maintain records of genetic abnormalities to prevent two recessive carriers of a mutation from breeding. Yay punnette squares!

Having kids with your cousin (even your 1st cousin), genetically speaking, isn't actually such a big deal - the aversion to that is mostly cultural. Problems arise more so when families repeatedly marry their first or second cousins, or their brothers and sisters (think ancient Egyptian royalty or the last Russian Czars). So in some ways, it depends on your time scale, and the number of people you start off with.

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    $\begingroup$ Is a "number you have seen thrown around" really the correct answer to a scientific question? Care to give any reasoning or citations for why it isn't any of the other many numbers people throw around? $\endgroup$ – Mike Nichols Apr 11 '16 at 16:00
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If they see the war coming and have chance to plan ahead, they should get hold of lots of diverse genetic material in the forms of eggs and sperm, or maybe ready-fertilised eggs. Then they could implant these into women when they reach the appropriate age, or better still use incubators at the best sustainable rate. If they keep this up until they have a big enough gene-pool they can re-establish the human race.

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  • $\begingroup$ Doesn't this assume access to highly advanced medicine, which sort of runs counter to the premise in the question of the people involved needing to remain in a "vault" for several hundred years? $\endgroup$ – a CVn Apr 11 '16 at 13:19
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    $\begingroup$ Living in a vault for several hundred years needs a lot of resources too. They will at least have a freezer. With a medical textbook and a lot of initiative they could manage to use frozen sperm at the very least. $\endgroup$ – RedSonja Apr 11 '16 at 13:23

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