When traveling to an alternate Earth, how many generations would you have to breed with the natives to prevent inbreeding?

A group of 35 superhumans from the future get trapped on the alternative Earth still in the Dark Ages. Banding together they take control of a small country and establish themselves up as kings and nobles. They seek to preserve their superhuman traits, but at the same time they want to prevent inbreeding. Every generation that they interbreed with the natives, their children will be less powerful. How many generations would the group have to breed with the natives until they can begin to breed among themselves again?

Assume that someone with one full super power parent will be half as powerful has one with two full super-powered parents. And if the one with only one full super-powered parent breeds with another non super-powered native the offspring will be again half as powerful as there one super-powered parent.

• What do you mean by "a group 35 from the future"? Do you mean a group of 35 people? Are superhumans affected in a similar manner by inbreeding as normal people? (By that, I mean whether they share similar risks at similar levels of inbreeding as normal humans.) Also the dangers of cousin marriage are often overstated: news.bbc.co.uk/2/hi/health/7404730.stm – typesanitizer May 5 '17 at 0:00
• Yes the superhumans powers do not include and Immunity to interbreeding and they have the same vulnerability to it as regular people – Bryan McClure May 5 '17 at 0:07
• I think the 35 superhumans are more likely to compete for dominance than to cooperate. Very soon, you'll be down to 30, then 25. After a few generations, you may be down to six or eight...and most of the survuving bloodlines will be intermixed. Inbreeding starts to matter after that. – user535733 May 5 '17 at 1:26
• @user535733 assume that they are had already been had working together for years on their earth before getting stuck on the primitive one. – Bryan McClure May 5 '17 at 1:29
• Depending on how many of the 35 are female, they may not need to interbreed with the natives at all. Generally you'd want a ratio of 1 male to 3 females, and each female to have children by 3 different men. But... if you must breed with the natives, then three generations should be enough - sufficient to get everyone to 2nd cousin level. – Tim May 5 '17 at 2:37

Depends on how much power you want to lose

Based on the information in this answer, assume we need a minimum founder population of 400. Other answers suggest as low as 80 with good medical support, but that seems like it will be unlikely in your scenario.

35 of those 400 are your super-powered individuals. However, to prevent the genes from being too diluted, you need to spread that genetic variation among multiple generations. Lets say that every generation, you add ~30 new people into the breeding population for 12 generations. These people will bring their 0% power genes into the collective gene pool. Lets further assume that there is a 50% increase each generation: 3 children for each 2 parents. Here is how the gene pool looks each generation:

Gen  Pop %Powerful
0     35   100
1     97    53
2    191    41
3    331    36
4    542    33
5    859    31
6   1333    30
7   2045    29
8   3113    29
9   4715    29
10  7118    28
11 10722    28
12 16128    28
13 24237    28


28% is the asymptotic limit. Your population will stabilize there in time.

Now lets re-run the exercise, but breed in far fewer people each generation; only 10. It takes 36 generations now (actually kind of a long time, something like 900 years) to get enough people into the gene pool, but on the plus side you keep the power a lot stronger. For this second run, we'd have to tone down the birthrate some to keep from over-running the earth, but the asymptotic power level is 54%. So the slower you add people to your gene pool, the more power you can maintain.

So the answer is: As low as 10 generations, if you don't mind getting depowered, around 30 generations to keep the average power level to 50%

Good old fashioned racism will help keep your population powerful

There is another factor at work. The superhumans will presumably be ill-disposed towards those who show few powers due to the genetic lottery. IF they discourage those people from mating, then your powers will eventually be bred back into the population. If, once the population has expanded enough to minimize the genetic bottleneck, people with less than 50% power are simply kicked out of the ruling class and now allowed to intermarry any more, you will find that the average power level of your group will increase over time; to at least 50%.

Thus, you may only have to interbreed for the 10 generations, and then rely on 'artificial selection' to keep your power level higher. With enough time (and enough racism) you can get your power level back to near 100%.

An excellent example of this in practice is the country of Mexico. For 400 years in Mexico, everyone in the upper class insisted that they were all Spanish, and didn't have any Indian in them. The result is that even though the population has a pooled genetic material that is about 30% European, 65 % Amerindian, 5% Black, the skin color skews heavily white (graph shown on the link). This is basically because skin color is the easiest thing to be racist about, so there was artificial selection pressure for lighter skin for so long. So there are now a lot of light skinned half-Amerindian Mexicans running around.

You can use this effect to eventually (maybe after 400 years) get a population of 100% superpowered half-non-powered (muggle?) people running around on your alternate Earth.

• keep in mind at that tech level you are likely to lose half or more of your offspring each generation before they even reach pubescence. So to get 3 offspring to reproduce per couple they actually need to have closer to 6 or 7 offspring. – John May 5 '17 at 4:57
• @John Three is just the surviving number. It doesn't matter how many children you have, only how many survive to reproducing age. Also, superhumans may not have such mortality problems. Its hard to imagine young Clark Kent dying of rubella. – kingledion May 5 '17 at 11:12
• well the author never says what the super power is, so infant mortality could still be a big factor. I know that for your calculation it does not matter but when the calculation is turned into story it does. – John May 5 '17 at 13:32
• A super powered being can breed again with descendants of other super powereds to bump up the power ratio of the offspring. With calculated breeding and long-lived superpeople, you could keep the power level around 99.9% – T. Sar May 7 '17 at 11:15

It's a simple answer but your group of 35 don't need to out breed at all. Assuming they aren't closely related to one another and assuming their population expands with each generation they shouldn't have any major issues with inbreeding.

My reasoning is fairly straight forward. The population is of sufficient starting size that no incestuous offspring will ever be necessary. Assume 35 individuals pair off to form 17 unrelated families for the first generation. This is a sufficient number of families that no one will ever have to marry someone closer than a second cousin indefinitely.

Even if we allowed for random mating as a worst case scenario the relative frequencies of recessive alleles (barring extraordinary misfortune in the genetic lottery) will be rather small. As an example if two of your starting 35 humans happened to have recessive mutations for a serious disorder that disorder would still only have a prevalence on average in the population of 0.08% = (2/70)^2.

Beyond simple inbreeding there are other genetic phenomena at work on small populations. Genetic drift would be a cause for concern if your population was maintained at such a small number for many generations, but since it will likely be expanding exponentially genetic drift won't have any detrimental effects.

Genetic diversity could be an issue, but 35 people (and therefore 70 alleles for every locus) is still a large amount of diversity. If you do want your super humans to need to out breed you could make them susceptible to certain plagues that the locals have been selected to be resistant towards.

• Yes, 35 people is enough if they have sufficient diversity already. Biggest problem would be stopping them from breeding with the natives assuming they landed in a place with attractive females. – Kilisi May 5 '17 at 20:00

35 people... 2^5=32... So assuming perfect choice of sexual partner for 5 generations they would have no need to even think about mating a common man. After 5 generations - each person would be a bit related, but more some distant cousin... imperfect but acceptable.

If one accept slightly increased number of genetic illness like in Arab societies - then they are not in desperate need of local partners at all.

Concerning such mechanism, I wonder more about social institution:

-would one cull defective superheroes (or in other way remove them from gene pool)?

-would one keep blind eye to some extramarital affairs?

Because I can imagine a situation, where in long run:

-a few unviable offspring simply dies early, thus eliminating itself from gene pool

-a few would seriously ill would be kept celibate or sterilized

-there is a constant influx of superhero genes in to local population, both natural selection and some local rulings should favor that. In long run - a slightly inbreed group of heroes, and mixed blood masses, with slowly increasing hero percentage.