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I'm worbuilding a situation about a polygamous family, consisting of a father, his several wives and their many children. They are cut off from their tribe, by highland rebellion which closed the only mountain pass. As per their custom they don't intermarry outside of their tribe.

Could the family survive for two more generations if half siblings marry each other?

Per tribal code, members that present liabilities for tribe are disposed off, sword in a heart style. That includes deformed, crippled, ill with small chance of getting better etc.

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    $\begingroup$ Why wouldn't it survive? Is there any specific reason why you think that their survival is uncertain? Jealousy? Rivalry? Deadly feuds? Is there a shortage of arable land? $\endgroup$
    – AlexP
    Jul 12, 2020 at 23:03
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    $\begingroup$ I'd worry more about the tribe, which has been inbreeding for longer. Most real-world tribes have some mechanism for bringing in new blood. This includes slavery and bride-trades. $\endgroup$
    – NomadMaker
    Jul 13, 2020 at 12:39

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Contrary to popular belief, inbreeding doesn't radically increase the chance of offspring developing health problems in isolated cases--it takes many, many generations of close, in-family relationships for offspring to start having high chances of birth defects or other inbreeding-related health problems.

That said, In your tribe, not only do you have multiple wives to diversify the genetic pool, but you're also only concerned with two generations--a rather short amount of time in the grand scheme. As long as your patriarch and wives aren't all already blood-relative direct siblings, there shouldn't be any significant genetic problems in the first two generations above the norm.

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Inbreeding does not automatically result in birth defects or cripples. It's the method used to produce "purebreeds" and it's no different between horses, dogs and humans; it will increase greatly the chances that any negative trait that is present in the genetic stock, and is normally masked by genetic variability, will surface and breed true - resulting in birth defects, stillborns, and cripples (this plus controlled reproduction of the healthy individuals only, will allow to weed out genetic defects and select for desirable traits until they "breed true").

For example, the father is XY and the mother is xX. This is a typical situation for haemophilia. Neither parent is affected, but the mother is a carrier. The children might be Xx (carrier again), XY (unaffected), xY (affected), XX (unaffected). Now it is possible to get a xx offspring in the next generation from the Xx and xY siblings. Breeding the apparently healthy individuals with one another (Xx and XX vs XY) will yield a healthier second generation and allow surmising that one of the two females is unaffected, and stop breeding the other as well as the other's offspring.

Of course, the more the latent defects, the more difficult and lengthy the process (it's little use to have a child which is healthy for traits A and B if they die in their infancy because of trait C).

But if those defects aren't present, they can't come to the fore. In your case, with several possibly unrelated wives, it is unlikely they would come to the fore for several generations; the current stock has probably been vetted enough in the past, "sword in the heart" style.

There is such a situation, if memory serves (minus the sword part), in Charles Stross's The Family Trade, where marriages are organized to maximise genetic diversity within an inbred extended family; the whole marriage business is called the "wreath" and is supervised by the aunts and grandmothers.

(For humans, you might have to take the Westermarck inhibition into account)

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Consanguinity is defined by a coefficient which is the probability of two genes being identical (homozygote) by random.

you can learn to compute the coefficient here. In your case, the first generation will have a coefficient of 1/8. In comparison Charles 2 the infamous king of Spain that was crippled by several and severe consequence of consanguinity had a coefficient of 1/4. A child born from a relation between first degree cousin has a coefficient of 1/16.

Its short term effects are often exaggerated but are real:

  • Increase in autosomal recessive disease. (a disease that appears only when you have both copies of gene the same and deleterious)

Other effects, pregnancy problem, risks of metabolism disease, mental disorders are hard to distinguish from other factors but seems to be more prevalent. If you want to learn more there is extensive literature on this subject on PubMed.

In your case, your small tribes may survive but have increased risks of seeing some form of rare disease. That may not appears at birth example.

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If there are no genetic issues, then there is no reason they could not continue to inbreed indefinitely.

If there are genetic problems, inbreeding will certainly manifest them over a few generations. But a genetically healthy population can be extremely inbred - as has been shown with many rare-breed domestic animals -- over many generations with no problems.

However, your society has a much bigger problem with its 'tribal code' which is likely to result in it being wiped out, by civil war or depopulation, fairly rapidly. We are all liabilities in the long run, but we don't necessarily accept that being killed is a good thing :)

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