Some background information: in this world there are races that are ageless. Meaning they stop aging at what humans consider age 30. This of course means that both men and women remain fertile, essentially indefinitely. This could mean that your great-great-great-great-grandfather could theoretically have a child with your great-great-great-granddaughter, if you aren't keeping track.

But that raises the question: outside of three to four generations, and anything outside third or fourth cousins, should one keep track of blood relations?

I am well aware of inbreeding depression, so naturally siblings and first cousins should be discouraged from producing the next generation. But with so many generations between them, blood relations so distant, is keeping track worth the effort?

  • $\begingroup$ This reminds me of my crisis when read about a situation where the brother had a child who was older than his little sister by a hundred years. Is that child still the little sister's niece or nephew? $\endgroup$
    – Skye
    Aug 31, 2016 at 13:54
  • $\begingroup$ depends, are they bothered if their great-great-great-great-grandfathers mates with their great-great-great-granddaughters or they simply don't care? $\endgroup$
    – Charon
    Aug 31, 2016 at 14:06
  • $\begingroup$ @渡し守シャロン good point. But I mean more from a gentics point of view. I'm sure there will be those that care and those that don't. $\endgroup$
    – Fayth85
    Aug 31, 2016 at 14:08
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    $\begingroup$ Heinlein explored this concept in several of his works. His basic answer was the same as what Separatrix posted, once technology reached the point of supporting it. He wrote several characters that were aware of their ancestor/descendant relationships, whom conceived children because their gene charts were in favor of it. Suggest "Time Enough for Love" as a fun read that touches on the topic. $\endgroup$
    – Rozwel
    Aug 31, 2016 at 17:57
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    $\begingroup$ It does not beg the question. It raises it. I've submitted an edit to that effect. Please approve it, whoever can do so. begthequestion.info if you need further info. $\endgroup$ Aug 31, 2016 at 20:09

4 Answers 4


You don't, what you do is take the other option: Pre-marital genetic testing.

This is common in several places and groups including Saudi Arabia and anyone with Jewish ancestry.

This is assuming your civilisation is at the technological stage where this sort of testing is possible.

In a more primitive environment, family is going to be very important to survival. I'd expect to see several generations of a family still living in the same house/compound, making keeping track of family much simpler than in our modern world of individualism.


Genetically your genetic similarity-by-descent* halves with each step removed. This applies equally to vertical relationships (i.e. grandparent->parent->child) and horizontal relationships (i.e. sibling->first cousin->second cousin). So you would share the same genetic similarity-by-descent (12.5%) with your great-grandchild as you do with your second cousin. Even the impact of first cousin relationships on rates of congenital defects is small (estimated at an increase of 1.7-2%) and so a rule of two generations would probably be sufficient or three if you want to be strict.

In practice, I would think the social limits would be more significant in the vertical than the horizontal direction - having a grandparent who you had a relationship with as a child marry your child is likely to be feel uncomfortable to those involved but the genetics are not likely to be harmful. This accords with legal frameworks in the real world which are frequently inconsistent with genetic realities, for example in my country (the UK) it's legal to marry your first cousin but illegal to marry your grandchild and adoptive relations are also counted.

*similarity-by-descent is the number of genes that are identical because they come from the same ancestor, the true genetic similarity is likely to be much higher because so much our genetic material is shared across the whole species.

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    $\begingroup$ +1 for a well written answer. I would point out though, that in defense of your county, the restrictions you mention are probably less based on "genetic realities" and more based on eliminating tax loopholes (there is no inheritance tax if the entirety of one's estate is left to a spouse), which is also why adoptive relations are counted. $\endgroup$
    – sharur
    Aug 31, 2016 at 17:34
  • $\begingroup$ I'm not convinced genetic relationships should be the basis for restrictions more pointing out that they don't answer the question on their own. $\endgroup$ Aug 31, 2016 at 17:54
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    $\begingroup$ My point is that the restrictions are to close a tax loophole. For example, if one only had one grandchild, had a bunch of money, and say had a cancer diagnosis of 3 months to live, one could marry said grandchild, and leave everything to them, thus bypassing inheritance tax. I agree it's a clumsy way of closing it, but in general there are (for social reasons, if nothing else) few who genuine users who would be harmed by it. Or to put it another way, genetics aren't the sole determining factors behind the adoption of the restrictions. $\endgroup$
    – sharur
    Aug 31, 2016 at 18:25
  • $\begingroup$ For "horizontal" relationships, the halving is the average. In the extreme cases, we have identical twins or the "identical cousins" played by Patty Duke in her eponymous TV show, who have 100% the same genes, vs. a hypothetical brother and sister who inherited from each parent exactly the chromosomes the other did not, and whose descendants would therefore also have no common inheritance other than what the pair of progenitors happened to already share. $\endgroup$ Aug 31, 2016 at 20:14
  1. First and foremost, why are these races ageless? That much implies a mechanism for cell replication notably different from that in H.Sap.
  2. Consider the microbiology you're using, and adjust the consanguinity ramifications appropriately. If these people cease to age, is it because their DNA doesn't deteriorate through repeated replication?
  3. Overall, I would think that the lack of ageing implies that these races have far fewer errors in their DNA, and a far lower incidence of error over time. Perhaps this cleanliness derives from a less forgiving reproduction process, such that mutations are almost always fatal.
  4. As Separatrix pointed out, one would expect these people to have genetic testing. If not, this also suggests that they don't have a reason to worry about their genetic health. Again, reinforcing bad genes isn't a problem for them, eh?

Keep firmly in mind that our "cousins" rules for consanguinity is merely a gross heuristic for risk of reinforcing unfavourable alleles. In order to make this a valid question in SF, it seems that you'll need to engineer a background that avoids the above objections: these people do have a problem with reinforcement, but they have yet to develop research into their own reproductive material, despite enjoying incredibly long lives.

Is there some other endeavour more important for every last member of the race?

That given, we still need the general parameters for the problem: how many chromosomes (or equivalent) do they have? What proportions of undesirable material do they observe? What level of risk is acceptable to them?

Once you decide those parameters, just do the math ... or is that the crux of your problem?

  • $\begingroup$ Some very good points, and definitely something worth thinking about. Thanks +1 $\endgroup$
    – Fayth85
    Sep 1, 2016 at 11:34

Inbreeding can be beneficial from a social and economic point of view, it strengthens the family bounds and wealth, that's why it is supported in great part of the world today and it had an important role in history.

The reason we don't inbreed is because two persons born of the same parents have 25% chance to give birth to a disabled child, and this chance decrease by 50% for every generation distance between two familiars.

Me and my father share 50% of genetic code, therefore we will have 25% chance to bear a disabled child.

Me and my cousin share 12,5% of genetic code, therefore we will have 6,25% chances to bear a disabled child and so on, the chances will split for every generation.

For example the inbreed between a Great-grandfather and a granddaughter results in 3,125% chance to have a child with disabilities.

Inbreeding a great-great-great-great-grandfather and a great-great-great-granddaughter has almost 0% chances to result in a child with health problems,technically the same as to mate with someone from the other part of the world that you never met.

As every single person ever lived and living on this planet are technically cousins, it is completely useless to keep track of the family tree after 8 generations.... 16 if you want to be really strict and precise but then anyone you meet on the streets could be your 16th grade cousin or closer anyway, probably even the entire city.

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    $\begingroup$ I have no idea where you're plucking the 25% figure from; it's certainly not grounded in genetics. It looks like you're thinking of the chance of being homozygous, but this is not the same thing as "chance of a disabled child" at all and is, in any case, a per locus chance. $\endgroup$ Aug 31, 2016 at 15:42
  • $\begingroup$ en.wikipedia.org/wiki/Inbreeding..... $\endgroup$
    – Charon
    Aug 31, 2016 at 15:54
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    $\begingroup$ Yeah, you've not understood that properly. Those percentages do not mean what you've described them as ("chance to bear a disabled child") $\endgroup$ Aug 31, 2016 at 16:10
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    $\begingroup$ -1 For the total and complete lack of understanding of high school level genetics. $\endgroup$ Aug 31, 2016 at 16:17
  • $\begingroup$ Note: “me“ is the objective form, “I” is the subjective. Something like “Me like icecream” is improper English. $\endgroup$
    – JDługosz
    Aug 31, 2016 at 16:42

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