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I've gleaned a potentially flawed impression over time that a more diverse gene pool should, in theory, result in healthier progeny. Something about dominant genes promoted over recessive genes..

As a result of the above, I always assumed I should favor a mate from another region if I want to have children, thus hopefully ensuring the children have a diverse mix of DNA bits to assemble out of.

I'm an American male (almost 38), and by a curious chance I find myself dating a Russian female (almost 29) who is in a US PhD program. We've been together a little over a year, and we've discussed marriage/children. So far so good.

However there's an observational wrinkle.. She sometimes associates with a local group of Russians and I tag along. Not long ago at a gathering with several young families, of which I think many are mixed-national (Russian and American), I was discouraged to observe what I thought was a disproportionate number of children with various disorders. If memory serves, there were 2 or 3 autistic and one down syndrome child out of about 11 or 12 kids of the same age.

[It might be worth noting that this is a church group. Church might provide a kind of social safe-haven that allows these families, particularly the children, to socialize with fewer concerns. As such it might make sense to see them in greater numbers.]

Anyway, suddenly I'm doubting my prevailing assumption about genetic diversity and wondering if there is a correlation between autism and the children of parents of mixed nationalities.

In this article, I read:

In the studies conducted in Nordic countries, a statistically significant 58% increased risk of autism was observed among the offspring of mothers born abroad.

So I find myself vexed. Is there a propensity for genetic challenges in children born to parents of mixed nationalities?

(FWIW I realize our ages dip into another risk factor, but I'm specifically asking about regional genetics.)

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Did you read the rest of that article? The authors suggest many alternative possibilities for the effects they observe:

Several hypotheses have been postulated, including the idea that fathers with social disability potentially due to a genetic mechanism associated with autism may be less able to find a spouse from their own country and may therefore find a wife from a foreign country with whom to have children91. More likely, Gillberg et al.91 suggested that women born in another country may not be immunized against the common infectious agents in the country in which she gives birth and may therefore be more susceptible to relatively innocuous infections which may increase the risk for autism. Other possible explanations include a potential role of maternal stress due to the demands of residing in a new country, particularly with limited social support, or stress resulting from the experience of emigrating, perhaps due to economic or social factors. These hypotheses do not explain the relationship with maternal place of birth seen in a cohort study of children born in California between 1989-199416, which showed a 40% decreased risk of autism among the children of women born in Mexico as compared to California. The association between maternal immigration and autism risk requires further examination in other areas of the world to examine whether the relationship can truly be generalized. Gardener et al., 2009

(emphasis mine)

In addition to these factors, note that p-value for that trend was only .06, and although there are a lot of problems with basing conclusions on fairly arbitrary p-values, a p-value of .06 suggests that differences of the magnitudes observed would often occur by chance. I'd say it's an area of open study, but it is quite a leap to jump to a conclusion that mixed nationalities result in higher incidences of things like autism from 1) the observation of a very small cohort of 12 children and 2) this other study which suggests many alternative possibilities for what is a small and possibly insignificant effect.

There are two main benefits for populations to avoid becoming inbred. The first is that there may be many deleterious alleles with low-incidence across a whole population, but you have a very high likelihood of producing a homozygous recessive offspring with a close relative. The second is overall maintenance of genetic diversity across a population, which could be important if suddenly exposed to a new disease, for example.

However, it is also possible for outbreeding depression to occur, which is a reduction in fitness after mating genetically distant individuals. However, outbreeding depression usually occurs either when fitness of some variable is low at intermediate values: for example, high fitness for small or large animals, but not for medium-sized animals, perhaps due to slightly different niches within the same species. Outbreeding depression can also occur when there is sufficient genetic separation between two individuals such that their whole protein milieu has evolved on a different background, such that alleles that cause no problems in either population alone are incompatible with alleles in the other population.

This previous answer here on Biology.SE talks about the lack of evidence for outbreeding depression in humans and a bit about why. Briefly, there is simply not the type of disparity in fitness conditions or sufficient genetic variability within humans for either of those situations to occur.

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  • $\begingroup$ The site has apparently queued my upvote, but you got me--I skimmed the article and missed those alternate theories. However, would it not be true that statistic (58% increase) describes the mere condition that the mother was an immigrant...and the correlation does not regard why, which the alt theories then consider? Maybe it can be immunization and/or maybe it can be stress...or even something else, but the fact is, being a female immigrant was still the common denominator...no? $\endgroup$ – user31840 Apr 24 '17 at 21:26
  • $\begingroup$ @user31840 That's my understanding - the point, though, is that being a female immigrant is likely to be correlated with all of those other factors, and based on our understanding of not only the unlikelihood of outbreeding causing autism (for the reasons I state later) as well as the poor relationship between autism and purely genetic causes, those are much better explanations than outbreeding, especially because, like the other answer I linked notes, there isn't really any evidence for outbreeding depression in humans in any circumstance. $\endgroup$ – Bryan Krause Apr 24 '17 at 21:58
  • $\begingroup$ Given a couple of centuries of European immigration to the US (and presumably a good deal of intermingling of European peoples in previous millenia), could mating between a (mosty caucasian) American and a Russian really be consideed outbreeding? $\endgroup$ – jamesqf Apr 25 '17 at 1:11
  • $\begingroup$ @jamesqf Yes I'd agree that intermingling and immigration means there are very few groups of humans that are truly isolated from each other over an appreciable number of generations, although I don't think there is a specific definition for outbreeding, or inbreeding for that matter, it's all relative. $\endgroup$ – Bryan Krause Apr 25 '17 at 1:22
  • $\begingroup$ @Bryan Krause: I think you possibly could find such groups, depending on how much isolation is enough. Say Japanese and the !Kung people of southern Africa, or Inuit and Australian aborigines. But any two populations of mostly European descent, no. $\endgroup$ – jamesqf Apr 26 '17 at 3:36

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