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NancyLebovitz comments on Open Thread - Aug 24 - Aug 30 - Less Wrong
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Is sexual relationship between two consenting adult siblings ethically okay and should it be legal? It's an interesting ethical problem because there are so many complicated dimensions and it could go either way. I think generally when it comes to social issues our society seems to head in a right direction, people are becoming more tolerant of people who choose differently and so on, but incest between siblings seems to become maybe more taboo even though there are some rational justifications you could make for it.
Some have made parallels to homosexuality as in it doesn't directly hurt either of the sides of this kind of relationship. There are social problems, I mean if I got to choose I wouldn't like any of my relatives to get together because that would evoke feelings of disgust, unnecessary drama and awkwardness, but I think I would have said the same thing about homosexuality 70 years ago. In theory inbreeding shouldn't be a problem, but in practice it probably is. Even though contraception is widespread in this day and age, if large number of siblings have sex with each other, some of them will inevitably end up having kids.
German ethics council decided that it should be legal because it's a person's fundamental right and people's right to self-determination is more important than the protection of your family.
From what I've heard, the genetic risks have a lot to do with how genetically similar the forebears of the couple are. If all the grandparents are from the same small region, it's a lot riskier than if the grandparents are from different continents.
I don’t understand why the origin of grandparents should matter.
To the best of my knowledge, the main problem with incest is recessive alleles. For example, if the grandfather’s genotype is ”aA” (where “a” is a very rare recessive allele) and his children (parents’ generation) mate with each other, then there is a relatively high chance (1/16) that the grandchildren would be of “aa” genotype (which might be extremely deleterious or even lethal). Having another grandparent from a different continent should not change this.
Why wouldn't having grandparents from different continents make rare alleles less likely to be reinforced?
There is some reinforcement, but it’s not very significant.
For example, consider an Ashkenazi Tay-Sachs carrier who marries a person from China. If their children mate, the chance that the grandchildren would have Tay-Sachs disease is (1/2)^4=1/16. If instead of a Chinese, this Ashkenazi Tay-Sachs carrier marries another Ashkenazi (who have ~0.03 chance of being a carrier), the chance that the grandchildren would have Tay-Sachs disease is almost the same, ~1/16*1.12. In absence of incest, a grandchild of a Tay-Sachs carrier would have a ~0.03/8 (i.e. ~17 times smaller) chance for getting the disease.
Ashkenazi Jews is too large a category. Try Ashkenazi Jews from a region where Tay Sachs is common for all the grandparents.
I don’t think this is possible.
Tay-Sachs allele used to slightly increase evolutionary fitness in heterozygotes (i.e. people who carry just one Tay-Sachs allele). This allowed the allele to increase in frequency until ~3% of Ashkenazis became its carriers. But once the local frequency becomes high enough the negative effects (the risk that a random couple produces children with two Tay-Sachs alleles) balance the positive effects on fitness. Thus in any region it should be impossible for Tay-Sachs to be common for all the grandparents.
I think this is true, but its pretty risky even in the best case.