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This question already has an answer here:

Outfitting an generation ship. How many unrelated people would you have to have in order to have a big enough genetic pool for mating to prevent inbreeding?

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marked as duplicate by James, Frostfyre, Hohmannfan, Jim2B, fi12 Mar 14 '16 at 20:50

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    $\begingroup$ Possible duplicates: What is the minimum human population... or How many people are required.... $\endgroup$ – Frostfyre Mar 14 '16 at 18:28
  • $\begingroup$ @Frostfyre, I disagree with the first being a duplicate, as it very well could be that the number by which the population is sustainable (minimum viable population) differs from the number of families one would need here. The second is closer to a duplicate... but I don't know what they meant by "clean genetics." That said, my question is... Is there enough functional difference between the word "family" and these other questions that use the word "people"? $\endgroup$ – The Anathema Mar 14 '16 at 18:36
  • $\begingroup$ @TheAnathema "Family" is a poorly defined term in general (immediate, extended, etc.) People could come from the same family, so long as they don't breed within a certain degree of separation. So, if we're talking long-extended families, then the answer is, technically, 1 family. $\endgroup$ – Frostfyre Mar 14 '16 at 19:18
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    $\begingroup$ Welcome to the site J.k, while an interesting question we have talked about it many times and have several good answers. I would suggest searching for minimum viable population to find questions and answers that should help you out. $\endgroup$ – James Mar 14 '16 at 19:41
  • $\begingroup$ Thanks James. I didn't know the right words to use. mea culpa. $\endgroup$ – J.k. Miles Mar 15 '16 at 20:18
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Practically speaking? A couple dozen unrelated women should be enough. Freeze a few million sperm samples and you're good to go. No inbreeding at all.

If you really really insist on ignoring sperm banks then according to this:

https://www.newscientist.com/article/dn1936-magic-number-for-space-pioneers-calculated/

You'd need about 160 completely unrelated, very healthy people.

You'd probably want to screen then beforehand for serious recessive genetic diseases and you'd need to keep careful records of family trees and screen for serious mutations.

Caveat: this is for a limited number of generations(10) and have some additional plans for when you arrive at the destination.

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    $\begingroup$ Murphy, I used the same answer to a question a while back. Be sure to read the assumptions/caveats with that number and include them here. Basically it assumes that it's a voyage of a specified time (10 generations?) AND there's good genetic diversity at the end of the voyage. So 160 genetic samples isn't enough for a viable colony but is enough to keep the generation ship crew healthy during the voyage. $\endgroup$ – Jim2B Mar 14 '16 at 20:24
  • $\begingroup$ Agreed. I tried to find how long sperm stays viable in storage and found: "Frozen semen seems to have no expiration date as long as the storage environment is well maintained and stable." I am guessing it actually means we just haven't been doing it long enough to run to the actual limits, but it would imply it is counted in decades at the minimum. So there really is no point in having actual men on a generation ship and you can have as much genetic variation as you have storage space for. $\endgroup$ – Ville Niemi Mar 15 '16 at 15:00
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    $\begingroup$ But with sperm, you'd get 1 million fathers and a dozen mothers. Frozen embryos are better for genetic diversity. $\endgroup$ – Madlozoz Jul 25 '16 at 8:31
  • $\begingroup$ I am super against frozen embryos. Even more so than embryonic stem cells which already takes the life out of a tiny developing ball of cells. If anything, embryos would fare the worst when frozen because there are so few cells compared to even a newborn baby. So even with cryopreservation, you would still get a ton of dead embryos after a slow thaw, not what you would want, especially on a generation ship. $\endgroup$ – Caters May 26 '18 at 3:01
  • $\begingroup$ @Caters Frozen and unfrozen embryos have pretty much equal chances. Embryos are actually pretty capable of surviving freezing. nejm.org/doi/full/10.1056/NEJMoa1705334?rss=mostEmailed Even if you have 100 dead for every one which survives, they're tiny so you can store a lot of them with very little weight or space requirements. We're already talking about a scenario where generations of people are born to die in a glorified tin can during transit to fulfill their grandparents desire to travel. $\endgroup$ – Murphy May 30 '18 at 12:13

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