Little Details

A Fact-Checking Community for Writers

Previous Entry Share Next Entry
Sibling incest, birth defects, genetic counseling
Bulbasaur: Confused
kjorteo wrote in little_details
Okay, there's really no way to ask such bizarre questions without you guys wondering what in the name of all that's holy I'm up to, so I'll just admit it: I have a story setting/universe/continuity/whatever you want to call it that involves a brother/sister pair in an incestuous relationship. At one point, they consider having a child. They're not stupid, so it's safe to assume that the whole risk of birth defects thing is something they'd at least be aware of and be concerned about. They'd probably attempt to research and see just how bad it is in order to evaluate how safe it is for them to go through with it. Of course, to know what they find out, I'd have to know the answer to that myself, and that's where things get frustrating.

Attempting to find answers on Wikipedia and Google has proven futile because, of all the search results, it would appear that 95% of them are links to or repostings of Patrick Stübing/Susan Karolewski news articles and completely uneducated wild guess figures from the peanut gallery that posts comments on said articles. Most of my searching seems to result in either people claiming (based on nothing but sheer common "well, everyone knows that") that incest = defects, or people claiming (also based on nothing but sheer common "well, everyone knows that") that "actually, it's no greater than/about the same as/actually significantly less than other factors, like age." I'd like to complain that no one bothered to back their points up with numbers, but the real problem is that everyone backed up their claim with numbers, and everyone's numbers are apparently completely made up and no one's numbers ever seem to agree. One comment I saw claimed that brother/sister incest raises the odds of birth defects "from 0.01% to 0.06%." Another claimed the odds were "ten to fifteen times" greater. The whole thing reminds me of this Freefall strip, with possibly the most noble quote ever for anyone who's been frustrated by a search.

So, would anyone happen to know if there are reliable figures regarding by exactly how much brother/sister incest increases the odds of birth defects? Bonus points if how much other factors (age of the parents, etc.) contribute are also quantified, so they can be compared.

Or, does such data even exist? The most professional-looking (or at least the least "some random guy making a comment"-like) result I found was this Straight Dope article about cousin incest, which threw in this line about siblings near the end almost as an afterthought:

"Let me emphasize we're talking strictly about cousin marriage here. The incest taboo regarding parent-child and sibling unions is still strong in Europe and most other places. Setting aside the issue of exploitation where minor children are concerned, such unions have a much higher risk of "adverse medical outcome"--7 to 31 percent, according to Bennett et al."

Forgive me for being greedy, but despite finally finding a somewhat respectable site with an answer that actually appears to cite a study, I still have unresolved issues with that statement. For one thing, "7 to 31 percent" is a hell of a spread. Were they unable to narrow it down further than that, or does the figure depend on other factors? If the latter, what are the other factors? Also, is this "Bennett et al" study the only one of its kind out there? If there are others, do their figures agree? Basically, is this information reliable?

Finally, to sort of change the subject, a lot of the results I found mention that birth defects can be controlled via proper genetic consultation and counseling, and this Wikipedia article has some pretty good information on exactly what Genetic Counseling is. However, would such a service be available to these characters? Most of my research here seems to indicate that that's something that generally requires a referral, and I'm assuming referrals aren't the easiest thing in the world to get for no reason other than "we haven't done anything yet but we're thinking about it and want to know if it's a good idea." (Maybe I'm wrong?) Also, there's the fact that, to my immediate knowledge, France is currently the only country in the world in which consensual adult incest among immediate relatives like siblings is not illegal, and this whole thing isn't set in France. Is it safe to assume that basically intending to commit a crime would make it harder to actually come forward and admit the details of the situation to the Counselor (or even get the referral?) that I think about it, that last point would probably lead to all sorts of fun trying to not get caught with the sheer amount of paperwork involved in everything these days, even if the birth defect thing turns out to not be an issue. Even getting the birth certificate without raising suspicions would probably be a trick, and good luck trying to not have the kid in the system...but that's a separate issue I'll try to work out later.

Birth defects don't just arise from nowhere to punish sinners. If they do not carry harmful recessive genes, their child will be just fine. Or, if they carry harmful recessive genes, but their child only inherits one (or no) copies of said gene, the child will be just fine.

Dominant traits are a slightly different kettle of fish, but again, it has nothing to do with whom they're screwing - if you carry a harmful trait, you can pass it on to your kids. If you don't, you can't.

They might find it easier for her to claim she doesn't know the father, but would like to see if there are any problems in her genes - if there are, there might be in his as well, and if there aren't, ditto.

They might find it easier for her to claim she doesn't know the father, but would like to see if there are any problems in her genes - if there are, there might be in his as well, and if there aren't, ditto.

Whoa. That is some serious quick thinking there. That is a fantastic idea. Thank you!

Maybe they can get a whole bunch of contradictory information and not have a clear answer... just like you did.

Hehe. I was actually thinking about that when the research was at its most frustrating. I'm sure that, at the very least, they would have some trouble with it. However, I did ultimately realize they'd probably see this as enough of a big and important deal to be worth finding out.

Generally speaking a one-off full sibling pairing carries only a very minor enhanced risk. There was a recent case in Germany that attracted a great deal of controversy and the completely inaccurate ramblings of one 'expert' were widely reported in the European press. The real risks come with systematic incestuous pairings over several generations. Try googling for incest in the Eurpopean royal families or the Bush family and you shoudl find some interesting stuff.

I can't link to anything for you because of my firewall (I'm allowed LJ but most of the links from LJ get blocked) but - at ther risk of seeming like I'm pimping out my journal - there is a link to the German case at the top of this entry of mine.

Note this part of my post:

"Attempting to find answers on Wikipedia and Google has proven futile because, of all the search results, it would appear that 95% of them are links to or repostings of Patrick Stübing/Susan Karolewski news articles and completely uneducated wild guess figures from the peanut gallery that posts comments on said articles."

You referred to them as just "the recent case in Germany" while I actually named them, but I believe we're talking about the same thing. :)

I tried my hands on a pubmed search( which gave me something that might help. My search string was "incest birth defects" and I got a few articles linking special genetic defects (fragile X syndrome, mental retardation and so on), but to read most of those articles you need to get help from a library or university. Unfortunately, my university is closed for the summer, so I'm afraid I can't be any more help.

The only other "sources" I have is my mother's claim that a lot of the children she works with (she works with severe mental retardation in young children) have related parents. But no numbers.

As for birth defects, taken from my classes in molecular biology:
Everyone has some "problems" in their DNA. Either with genes programming for weaker proteins than what's ideal or severe diseases. Problem is that when two people are related, the chances that a recessive gene that the family has will be in both chromosomes, which will lead to a disease someone with one healthy and one bad chromosome would not get (which can be anything from diabetes to severe diseases).

Most of the serious recessive diseases will just "travel" through generations without ever blooming, or just blooming once in a while when someone loses the genetic card game and both mother and father carries the same mutation/flawed gene.

I don't know where your story is set, but as for my country (Norway), you have to either have severe genetic diseases in your family, or already given birth to a sick child to get through the needle eye and be "allowed" genetic screening. Of cause, it's possible to do it on a private lab, but then you have to have both the money and a plausible reason why you skip the system.

If you want to know the details of genetic counseling, I have had a lecture by a lady who leads that department on the local hospital, so just comment with any questions you might have.

The reason why you're not finding figures which agree with each other is because, as conuly says above, genetic conditions don't come out of nowhere. (Okay, if it's a spontaneous mutation it technically would, but in that case the incest angle wouldn't be a compounding factor.)

In effect, BOTH of the arguments you were finding are correct- in most cases, the odds of getting two copies of the same gene, whether it's for a disease or something harmless like eye color or blood type, will indeed be dramatically higher with inbreeding, but may still be effectively negligible if the gene in question is very rare within the population. The "hell of a spread" you're observing is a direct consequence of the fact that there's just as much of a spread in the frequencies of harmful genes within the general population.

Googling for "Harvey-Weinberg equilibrium" and "inbreeding coefficient" will give you the math involved and ought to turn up at least a few pages with useful examples.

...and damnation, I misstyped the name of the equation you need. It's Hardy-Weinberg, not Harvey-.

In any case, using 'inbreeding' as a keyword rather than 'incest' will probably help in finding useful information. While there's only a handful of case studies in humans, for obvious ethical reasons, the subject has been studied extensively in domesticated, endangered, and laboratory-strain animals.

Robert Heinleins 'Time Enough for Love' has a story, 'The Tale of the Twins Who Weren't' which has a long and scientifically correct discussion of the odds of recessives being expressed in the child of sibling incest. You might wish to make sure your characters have clean genes (non harmful recessives only) or put them in a world with advanced gene therapy.

Well, scrolling up a bit and looking at eelpot's reply, it sounds like good genetic screening would be hard to secure as a mere precaution, without there being something that actually really went wrong in order to necessitate the referral. How would the characters check if they have the "clean genes" of which you speak? Or did you mean that I, the writer, need to ensure that somehow (like setting it in a future sci-fi world where genewashing is 300 credits or something?) I hope you meant the former, since I am kind of trying to keep this contemporary.

I actually watched a documentary on this subject a while ago and the 0.01% chance in non related parents and 0.06% in related parents was the statistic they used. According to them the chance of a genetic defect in any random couple of siblings having a child v any random non related couple having a child is not really much higher than the difference between a couple in their twenties v a couple in their late thirties.

The trouble comes when you're calculating not in a random chance but in specific parents. Because then you have to add in what specific recessive genes those parents might be carrying and the odds of Sibling A who has recessive gene B giving birth to a child with genetic defect B go up dramatically when comparing a potential relationship between Sibling A and Random Stranger X with a potential relationship between Sibling A and Sibling Z.

I believe Spain either has or is considering making sibling relationships legal btw thanks to a specific case. (It might have been the region of Spain they live in not the whole country I'm not sure)

Would you happen to know what documentary this was? Now I'm curious. It sounds interesting.

I could be wrong on this, but I believe it's not uncommon for adopted children to get genetic counseling when they get around to having children themselves, since there's really no way of telling whether or not they have a family risk of cancer, high blood pressure, etc, etc.

The reliable figures thing varies depending on the population pool you're working with. As other people have remarked, people don't just get smote by God for having a baby with their brother--it has to do with harmful recessives, but the rate of harmful recessives themselves varies pretty dramatically depending on the population you're looking at. For instance, let's say that your brother and sister come from a Ashkenazi Jewish family. Since that ethnic group has a much higher rate of harmful recessives, their kids would be much, much, much more likely to have X genetic disorder. However, if their family had never had any incest in the past and their parents were just random strangers who met, then the odds of the incest producing a harmful disease will be higher than normal, but it also depends on how rare that genetic trait is in the first place.

Hardy-Weinberg equilibrium should be able to give you specific numbers for a given disease, but you need to know the rate of that genetic trait in the general population. (Let me know if you'd like help with the math involved, it's pretty early in the morning and I'm too asleep to type out an explanation of how it works at the moment, but it's mostly just algebra.)

If you're from a specific population like that (your example of Afro-Caribbean families have a higher chance of Sickle Cell etc) the OP's couple might not be able to get full screening but just a screening for those specific genetic markers maybe.

I've been wondering about this for awhile and have had about as much luck as you have. Never asked here since it was just for an informal RP, though, but I'm glad someone else did so I can hear the answer. :)

In Australia, genetic counselling is available on a referral from a GP. I can't see any reason why they can't be referred separately, by separate doctors - they might want to cook up a reason like "my nephew has been diagnosed with cystic fibrosis".

I can't contribute much that wouldn't reiterate some of the useful information in the lovely, informative comments above... What I did was do a direct article search at my uni library. I'm afraid searching there directly as opposed to through pubmed didn't return as many relevant hits as it could have, but I did find a few articles that may prove to be an interesting read. I only skimmed the abstracts, as I'm supposed to be studying at the moment and therefore strapped for time, but I uploaded the articles for you here.

Just something you might want to clarify in the top post: Although the sibling pair is (obviously) related, is there a history of incest in the family? Or even are they from very similar gene pools? Say, if they grew up in a small town where everyone knows and is distantly related to half the town, vs. their parents are from opposite ends of the world. That would affect the likelihood of their parents having harmful recessives to pass down.

Actually, it wouldn't effect the likelihood of their parents having harmful recessives, it would effect the chance of their parents having the SAME harmful recessives.

A single harmful recessive doesn't do anything. It's when you get two of the same one that there is a problem.

Since a parent is 50% likely to pass on a harmful recessive (assuming, of course, the parent doesn't manifest the recessive), there's a 25% chance that both siblings will have the recessive, and if that's the case, there's also a 25% chance a child of a sibling union will manifest the harmful recessive.

Assuming it's a simple single gene recessive.

If you want to see the results of close family incest, take a look at both European and Egyptian royal families. The Egyptian royal families had a habit of marrying siblings, and when royals were marrying in Europe, they'd marry cousins all the time. Granted, those two are extreme cases of traditional incest, but it might help with seeing the probability of certain diseases and such.

**yawns, gets ready for work**

Actually, before I head off to work, check out this link: Cousin Marriage, the Facts It talks about the chances of birth defects in addition to legal prohibitions against marriage and where the taboo is present and such.

(Deleted comment)
(Deleted comment)
This is right out of my college-level biology text book I used last year:

"Although it is relatively unlikely that two carriers of the same rare harmful allele will meet and mate, the probability increases greatly if the man and woman are close relatives (for example, siblings or first cousins). These are called consanguineous ("same blood") matings... Beause people with recent common ancestors are more likely to carry the same recessive alleles than are unrelated people, it is more likely that a mating of close relatives will produce offspring homozygous for recessive traits - including harmful ones. Such effects can be observed in many types of domesticated and zoo animals that have become inbred.

"There is debate among geneticists about the extent to which human consanguinity increases the risk of inherited diseases. Many deleterious alleles have such severe effects that a homozygous embryo spontaneously aborts long before birth. Most societies and cultures have laws or taboos forbidding marriages between close relatives. These rules may have evolved out of empirical observation that in most populations, stillbirths and birth defects are more common when parents are closely related. But social and economic factors have also influenced the development of customs and laws against consanguineous marriages."

So...there probably is no real magic number that will serve as your percent chance of the fetus having a genetic defect, because there are too many different genetic diseases out there to just settle on one number. The chance of them having a child with a genetic defect depends upon them, more than anything. They would have to know if either of them had a genetic defect and then they could evaluate the risk. If a disease runs in their family, the child may have some problems. If the normal chance of a mutation occurring somewhere during fertilization or development, the child probably has just as good a chance as any of being perfectly normal. It depends a lot on the genetic makeup of the parents and the population the parents came from.

Awesome. That's very helpful. Thank you. :D

And are your characters thinking about their own potential child, or thinking about their own potential child and their entire downstream genetic line? Because that would change their perspective some, I would think.

Money is also a great easer of realistic requirements. If they happen to have a great boatload of cash on hand, they can buy as complete a genetic counseling session as they need, separately, or anonymously, not even necessarily in their home country.

They're not wealthy, so that probably isn't a viable route to take.

I'm assuming they'd be thinking of it in terms that their child can him- or herself reproduce, but it would be with someone unrelated, and thus there wouldn't be a series of inbreeding. He or she would be an only child (so no brother or sister to repeat this action,) so as long as the parents can refrain from sleeping with their child (which...uh...yeah, that won't be a problem,) most of the immediate roads to the child also committing incest would be closed.

All that, of course, assumes that the child will want to have direct (non-adopted) children of his or her own. I know there are exceptions, but that usually assumes he or she is heterosexual and doesn't have too active/busy a lifestyle to support settling down and child-raising, among other who knows. :)

Instead of answering your question I'm going to suggest a way to find the answer.
Stop searching human genetics and search "Line breeding" or "litter breeding" in animals.

One thing to remember, the same thing that needs to be remembered in the case of older women worried about children with birth defects: if you multiply a very small number by 10 or 20, it's generally still going to be a very small number.
Afaik, most serious birth defects are rare enough that you could make them 100x more likely, and they'd *still* happen in less than 1% of cases.

birth defects


2010-06-21 08:08 am (UTC)

my first wife and i are as far as we know not related in any way shape or form. we have had two children 1 girl and 1 boy both having Pearsons syndrome (15 cases world wide that we know of) to varied degrees of affect. my daughter died aged 7mths and my son nearer 18mths. does anyone have a rough idea what the odds are on that happening. i have now divorced my wife and re married, my new wife and i have 2 kids again 1 son 3yrs and 1 daughter 11years both with full health and life goes on.