# Brother and Sister Have Sex With Each Other; Odds of Birth Defects? **URL:** \ **Category:** Factual Questions\ **Created:** [September 20, 2012, 3:24pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272 "2012-09-20T15:24:32Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 20, 2012, 5:56pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/21 "2012-09-20T17:56:20Z")
> [@Smeghead](#): > > On the other hand, they’re usually inbred for many generations, unlike the human situation in the OP. I mentioned this before. We don’t know if that’s different from the situation in the OP. Incest may span generations. Doesn’t this increase the occurence of genetic disorders at any generation? ETA: Sorry, didn’t mean the situation in the OP. Meant post #2. ---
**Author:** ![HeyHomie](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/heyhomie/32/207_2.png) [@HeyHomie](https://boards.straightdope.com/u/HeyHomie)\ **Post date:** [September 20, 2012, 5:56pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/22 "2012-09-20T17:56:45Z")
Yes, I meant to say same father as well. Sorry I wasn’t more clear. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [September 20, 2012, 5:57pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/23 "2012-09-20T17:57:51Z")
I came within a year or so of unwittingly meeting my half-birth-sister at the office of the graduate advisor we both ended up working under as TA’s (she had left before I signed on)-thankfully soon after moving to said state (NJ) I found my birth mother and her family anyway via other means (at which point I discovered the above fact). ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [September 20, 2012, 6:07pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/24 "2012-09-20T18:07:07Z")
Each parent passes 1/2 set of genetic material to their child -totally randomly, one of each pair of chromosomes. So - a pair of siblings other than identical twins has about 50% of genes in common. If they become a breeding pair, their offspring then would have about 25% of genes duplicated. of course, then the $64,000 question is, as Lazybratsche says - what proportion of genes are defective and simply masked by a corresponding functional gene? One theory says a lot of miscarriages are the result of defective genes, that a lot of defects simply won’t allow the fetus to develop normally. So there may be some weeding-out effect there. Plus I’m sure you know of couples who have no end of miscarriages, while others seem to pop them out like clockwork until they decide enough is enough. I infer from this that the proportion of “bad genes” is probably pretty variable from person to person, and this is also a “weeding out” process to limit the number of copies of bad genes. In the (limited sample) case of the girl locked in the basement of the house in Austria, she had 7 children with her father. A father-daughter match will ahve the same 25% genes in common\*. One child died at birth (allegedly a heart defect or something) and of the others, no indication that there are any problems except perhaps environmental ones (although the convulsions the one girl had, that brought this case to light, may be indicative); the imprisoned children seem to have some problems, but the two “found abandoned” and raised upstairs seem to be normal. The trouble with breeding animals, is that also selection is for some characteristics at the price of others. Purebreds often have a reputation of being high-strung. Some dog breeds have problems with hips or other issues. One article I read discussed how modern pigs are so stupid that the mothers have to be restrained so they don’t roll over on the piglets. However, this is selection from a limited pool of genes to emphasize the ones that give the desired characteristics at the expense of others. If you dog is stupid, how would you know and how badly would you care? * * * - An individual’s full sibling or a parent would both have 50% the same genetic material. First cousins, assuming no other family connection but the 2 common grandparents - that’s 1/8 genes in common, meaning 1/8th overlap in genes and so 6.25% (1 in 16) for their offspring. In countries where such matches are allowed, the degree of genetic problems seems to be negigible. From Wikipedia: > [@](#): > > In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.7–2.8% over an average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40.[181] In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%,[182] While another study published in 2009 suggests the rate may be closer to 3.5%.[2] Put differently, first-cousin marriage entails a similar increased risk of birth defects and mortality as a woman faces when she gives birth at age 41 rather than at 30.[183] ---
**Author:** ![Malthus](https://avatars.discourse-cdn.com/v4/letter/m/85f322/32.png) [@Malthus](https://boards.straightdope.com/u/Malthus)\ **Post date:** [September 20, 2012, 6:08pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/25 "2012-09-20T18:08:52Z")
> [@TriPolar](#): > > I mentioned this before. We don’t know if that’s different from the situation in the OP. Incest may span generations. Doesn’t this increase the occurence of genetic disorders at any generation? > > ETA: Sorry, didn’t mean the situation in the OP. Meant post #2. I suppose that would depend on whether those with expressed genetic problems continue to breed - a la the Hapsburgs. 😉 In livestock, inbred stock is culled to eliminate those with undesireable traits. With people, whether those with undesireable traits get to survive and breed depends a lot on circumstances - social, medical, etc. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 20, 2012, 6:19pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/26 "2012-09-20T18:19:24Z")
> [@Malthus](#): > > I suppose that would depend on whether those with expressed genetic problems continue to breed - a la the Hapsburgs. 😉 > > In livestock, inbred stock is culled to eliminate those with undesireable traits. With people, whether those with undesireable traits get to survive and breed depends a lot on circumstances - social, medical, etc. Here in RI we have a high incidence of some disorders. This occurs in the aptly misnamed South County. For hundreds of years this was an isolated rural region with a lot of intermarriage between ‘Swamp Yankee’ families. Searching for info just gave me a long list of geneticists and genetic disorder resources, so I don’t have any details yet. I recall one of the problems was disorders popping up generations after the initial inbreeding between people who would have been considered only remotely related. Interestingly brother-sister incest is not illegal in RI, although it was at some time in the past. These weren’t all incidences of close incest, there would have been many offspring off cousins, but the past did contain some rather unseemly relationships, and not just in the backwoods. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [September 20, 2012, 8:27pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/27 "2012-09-20T20:27:12Z")
> [@TriPolar](#): > > I mentioned this before. We don’t know if that’s different from the situation in the OP. Incest may span generations. Doesn’t this increase the occurence of genetic disorders at any generation? Yes, if incest continues for many generations, that would tend to make the problem worse. I called the situations “different” simply because I was assuming we were talking about one isolated incident of incest, rather than, if you’ll excuse me, a situation where incest runs in the family. ---
**Author:** ![Annie-Xmas](https://avatars.discourse-cdn.com/v4/letter/a/ecc23a/32.png) [@Annie-Xmas](https://boards.straightdope.com/u/Annie-Xmas)\ **Post date:** [September 20, 2012, 8:37pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/28 "2012-09-20T20:37:37Z")
One anti-abortion book I read claimed that incest “only” increases the threat of birth defects from 2% to 4%. But it didn’t mention that figure includes all types of incest (i.e. uncle-niece). Of course, the book stated that “abortion is the incestuous couple’s friend, as it coers up the evidence of their crime.” ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [September 20, 2012, 11:47pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/29 "2012-09-20T23:47:25Z")
> [@Kimstu](#): > > Lesseenow, full siblings have all four grandparents in common, while half-siblings have only two grandparents in common.. Full siblings could have only two grandparents total if their parents were also full siblings, and you can get into all sort of weird situations with incest. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [September 20, 2012, 11:50pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/30 "2012-09-20T23:50:29Z")
oop, never mind ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [September 21, 2012, 5:54am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/31 "2012-09-21T05:54:11Z")
> [@md2000](#): > > of course, then the $64,000 question is, as Lazybratsche says - what proportion of genes are defective and simply masked by a corresponding functional gene? Yes, recessive defects are irrelevant unless they’re _duplicated_ … but that’s what inbreeding leads to: _duplicated alleles._ The parents of [Carlos II ‘El Hechizado’](http://blogs.discovermagazine.com/gnxp/2009/04/inbreeding-the-downfall-of-the-spanish-hapsburgs/), King of Spain, were “only” uncle and niece, but because of prior inbreeding in the Habsburg family, their genetic closeness was slightly more than that of brother and sister. Of El Hechizado’s sixteen gt-gt-grandparents, nine had Habsburg surname. > [@](#): > > The Habsburg King Carlos II of Spain was sadly degenerated with an enormous misshapen head. His Habsburg jaw stood so much out that his two rows of teeth could not meet; he was unable to chew. His tongue was so large that he was barely able to speak. His intellect was similarly disabled. His brief life consisted chiefly of a passage from prolonged infancy to premature senility. Inbreeding actually drove the Habsburg Dynasty to extinction; Habsburg Emperors after 1740 used the surname via a genealogical fiction. ---
**Author:** ![HeyHomie](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/heyhomie/32/207_2.png) [@HeyHomie](https://boards.straightdope.com/u/HeyHomie)\ **Post date:** [August 7, 2014, 11:40pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/32 "2014-08-07T23:40:55Z")
I’m bringing up this old thread to provide some new information: the situation described in my OP has happened pretty much exactly. > **[This Married Couple Apparently Found Out They Have The Same Mother](https://www.buzzfeed.com/richardhjames/this-married-couple-apparently-found-out-they-have-the-same)** > > The half-siblings were reportedly abandoned as children. ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [August 8, 2014, 1:47am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/33 "2014-08-08T01:47:19Z")
Did you need the answer fast? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 8, 2014, 2:02am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/34 "2014-08-08T02:02:10Z")
> [@](#): > > Quoth **Malthus** ’ link: > Twenty-nine children of brother-sister or father-daughter matings were studied. Twenty-one were ascertained because of the history of incest, eight because of signs or symptoms in the child. In the first group of 21 children, 12 had abnormalities, which were severe in nine (43%). In one of these the disorder was autosomal recessive. All eight of the group referred with signs or symptoms had abnormalities, three from recessive disorders. The high empiric risk for severe problems in the children of such close consanguineous matings should be borne in mind, as most of these infants are relinquished for adoption. So, of the 17 cases which had severe abnormalities, four were from autosomal recessives. What would the other 13 be from? Is there some other mechanism by which incest screws up kids? ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [August 8, 2014, 3:19am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/35 "2014-08-08T03:19:25Z")
> [@md2000](#): > > The trouble with breeding animals, is that also selection is for some characteristics at the price of others. Purebreds often have a reputation of being high-strung. Some dog breeds have problems with hips or other issues. Or to put it another way, breeding primarily or exclusively for appearance or other specific physical attributes that are valued in the breed, at the expense of health and genetic integrity. Needless to say, that kind of unbalanced priority is considered very unethical but there’s been altogether too much of it over the years. There’s nothing intrinsically wrong with purebreds, of course, but responsible buyers screen breeders just as responsible breeders screen buyers. ETA: Talking here about dogs, not really familiar with cats and other pets. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 8, 2014, 9:27am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/36 "2014-08-08T09:27:51Z")
> [@Chronos](#): > > Is there some other mechanism by which incest screws up kids? Oh dear… ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 8, 2014, 9:30am UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/37 "2014-08-08T09:30:10Z")
One problem which would certainly screw up all the studies, is that "_According to current estimates, the levels of “non-paternity” (when a child turns out to have been fathered by someone other than the ostensible father) lie somewhere between 1% and 30%, with the “best estimate” being in the region of 10%. So one child in ten is not the child of the man assumed to be their father."_ [http://www.moreintelligentlife.co.uk/content/issues-ideas/catherine-nixey/whos-daddy](http://www.moreintelligentlife.co.uk/content/issues-ideas/catherine-nixey/whos-daddy) > [@](#): > > His papa shook his head and to this, he did say > “You can’t marry that girl, I have to say no > 'Cause the girl is your sister, but your mama don’t know” > > He went to his mama, he covered his head > He told his mum what his father had said > His mother, she laughed, she said, “Go man, go > Your daddy ain’t your daddy, but your daddy don’t know” > > Read more: Madness - Shame And Scandal Lyrics | MetroLyrics ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [August 8, 2014, 12:48pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/38 "2014-08-08T12:48:07Z")
Yeah, the story about the “pedigree error” rate a few years ago related how a science teacher in London many years before had his class go home and compare their blood type to their parents’ to demonstrate genetic inheritance - only to find that 10% of the class had a blood type that did not follow from their parents. I remember trying to Google any primary sources (rather than current writings repeating the same story) only to find none. Plus I had to wonder if the whole was made up - because even when the child is the result of a hidden “pedigree error” there could be a good chance that the blood types match, so the number was simply a much lower bound on the actual result. Plus, how many lower-class Londoners in the 60’s or 70’s knew their blood type? However, family “mismatches” are apparently not uncommon, they usually come out when someone in the family needs a donor. Hence the estimates in the single digits. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [August 8, 2014, 1:20pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/39 "2014-08-08T13:20:55Z")
> [@septimus](#): > > The parents of [Carlos II ‘El Hechizado’](http://blogs.discovermagazine.com/gnxp/2009/04/inbreeding-the-downfall-of-the-spanish-hapsburgs/), King of Spain, were “only” uncle and niece, but because of prior inbreeding in the Habsburg family, their genetic closeness was slightly more than that of brother and sister. Of El Hechizado’s sixteen gt-gt-grandparents, nine had Habsburg surname. This … and by extension a good part of all the royal families of Europe in the Middle Ages. It was the technique of marrying your cousin … then if a better prospect came along later … you could ask the Pope to annual the first marriage because of “recently discovered” close kinship. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 8, 2014, 3:31pm UTC](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272/40 "2014-08-08T15:31:00Z")
Maybe worth mentioning that it is perfectly legal for first cousins to marry in the EU, and it seems in most states in the US. So long as it is not too frequent in a family tree, it causes no problems; however some Asian cultures, especially Pakistani, positively encourage it, and that has caused problems: _" recent report on births in a British-Pakistani community (where first cousin marriage is very common) demonstrated that first cousin children there were twice as likely to be born with “potentially life threatening birth defects” as compared with the children of unrelated parents."_, however _“his resulted in only a 6% chance for the children in the study, as compared with a 3% chance for the population as a whole.”_. For anyone interested in an academic study of incest, [References](http://web.stanford.edu/~scheidel/incest.htm) seems to be a good start. There is an extensive bibliography at the bottom of the page. > [@](#): > > ~ the dangers of close inbreeding have now been amply documented and analysed with respect to both human and animal populations (e.g., Bittles & Neel 1994; Ralls, Ballou & Templeton 1988). [Previous page](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272.md?page=1) [Next page](https://boards.straightdope.com/t/brother-and-sister-have-sex-with-each-other-odds-of-birth-defects/635272.md?page=3)