# Could two siblings share no DNA whatsoever? Is so, could they safely have children? **URL:** \ **Category:** Factual Questions\ **Created:** [December 1, 2016, 3:00pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532 "2016-12-01T15:00:44Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 1, 2016, 9:33pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/41 "2016-12-01T21:33:24Z")
Just thought I’d throw in the comment about the most recent notorious case of interbreeding, Mr. Fritzl of Austria - he kept his daughter locked in the cellar, had 7 children. As I recall, 1 died at birth and one began having seizures at age 19 after living in a locked cellar for all her life. Otherwise, the children, especially the two raised upstairs as “abandoned by mother”, were not bad, no obvious genetic abnormalities, considering the circumstances. A father-daughter cross would be equivalent to a brother-sister cross, the child would have about 1/4 of chromosomes matched up. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 1, 2016, 10:01pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/42 "2016-12-01T22:01:23Z")
> [@md2000](#): > > No, it only has to happen once. It does not matter what the first child is, just that the second one has to specifically pick 46 chromosomes that complement. Er, well I’m not sure where you were starting from. The probability based on chromosome assortment is (1/2)^46 = 10^-14, are we agreed on that? I was not double counting the two children, but previously I was not counting the fact that it has to occur in both the sperm and the egg, so previously I was thinking only (1/2)^23, that’s what I meant when I said it has to happen twice. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 2, 2016, 12:37am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/43 "2016-12-02T00:37:14Z")
Something just occurred to me. During meiosis, 2 cells are made, each containing half the parent’s DNA. These two cells are complimentary. So, in the case of a pair of fraternal twins, could two complimentary sperm fertilize two complimentary eggs? Is that possible? If so, it might happen once every hundred million twins, or so. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 2, 2016, 1:01am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/44 "2016-12-02T01:01:57Z")
> [@Peter\_Morris](#): > > Something just occurred to me. …without reading the thread! 🙂 See my post #30 and several subsequent comments, incl **Darren** re polar bodies. Where are you getting “hundred million” from? ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [December 2, 2016, 1:03am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/45 "2016-12-02T01:03:07Z")
I’m surprised after 44 posts … no one has noted the sister would be effectively a clone of the mother … the brother of the father … was it safe for the parents to breed? Maybe not … ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 2, 2016, 1:09am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/46 "2016-12-02T01:09:35Z")
> [@watchwolf49](#): > > I’m surprised after 44 posts … no one has noted the sister would be effectively a clone of the mother … the brother of the father… Not so. Siblings always get half each from mother and father. The requirement is that two siblings get precisely complementary halves. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 2, 2016, 1:23am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/47 "2016-12-02T01:23:48Z")
> [@Riemann](#): > > …without reading the thread! 🙂 I skimmed it. I somehow missedc #30. My bad. > [@](#): > > Where are you getting “hundred million” from? I think I heard somewhere that there are a hundred million sperm in an ejaculation. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 2, 2016, 1:30am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/48 "2016-12-02T01:30:21Z")
> [@Derleth](#): > > Two siblings could be fraternal twins and safely have children if they didn’t have any dangerous recessive traits. In-breeding doesn’t create genes, it just shuffles them around, just like sex between two unrelated people; the difference is that in-breeding has a higher chance of making otherwise-unusual combinations. What is special about fraternal twins? How is that different genetically than any other pair of siblings? ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 2, 2016, 1:30am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/49 "2016-12-02T01:30:32Z")
> [@Peter\_Morris](#): > > I think I heard somewhere that there are a hundred million sperm in an ejaculation. Ah, right. I got around that by having two just stick together! But then you’re missing the random chromosome assortment in the 2N\>1N reduction at the end of meiosis, there is still a (1/2)^23 factor from that. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 2, 2016, 1:33am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/50 "2016-12-02T01:33:29Z")
> [@CookingWithGas](#): > > What is special about fraternal twins? How is that different genetically than any other pair of siblings? There is no difference, I think Derleth just mis-phrased what he was saying. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [December 2, 2016, 1:55am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/51 "2016-12-02T01:55:46Z")
> [@Riemann](#): > > There is no difference, I think Derleth just mis-phrased what he was saying. Yeah, that’s probably the case. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 2, 2016, 4:50am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/52 "2016-12-02T04:50:46Z")
> [@CookingWithGas](#): > > What is special about fraternal twins? How is that different genetically than any other pair of siblings? Yes, fraternal twins are just two different eggs fertilized by two different sperm at the same time - genetically no more significant than any other pair of siblings. It’s identical twins that have exactly the same genome set and should not try to reproduce with each other… ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 2, 2016, 5:08am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/53 "2016-12-02T05:08:06Z")
> [@md2000](#): > > It’s identical twins that have exactly the same genome set and should not try to reproduce with each other… The worst outcome is expected if you go fuck “yourself”, but of course it’s not recommended for non-identical siblings either. In the former, half the defective recessive alleles in the genome are homozygous in the offspring, in the latter one-quarter. ---
**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:** [December 2, 2016, 12:22pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/54 "2016-12-02T12:22:05Z")
Of course, identical twins would have a very difficult time reproducing with each other, just because they’re the same sex. Though modern technology might someday make it possible. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 2, 2016, 1:37pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/55 "2016-12-02T13:37:59Z")
> [@md2000](#): > > Yes, fraternal twins are just two different eggs fertilized by two different sperm at the same time - genetically no more significant than any other pair of siblings. > > It’s identical twins that have exactly the same genome set and should not try to reproduce with each other… 😃 ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [December 2, 2016, 2:24pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/56 "2016-12-02T14:24:16Z")
> [@Chronos](#): > > Of course, identical twins would have a very difficult time reproducing with each other, just because they’re the same sex. Though modern technology might someday make it possible. During my search for information on polar body fertilization, I came across a link to the case of a set of aborted conjoined fetuses that had male and female genitalia. The parents didn’t allow anyone to study them, so there is only speculation on the cause. One of the speculations is that a single embryo partially split, and that one half lost it’s Y chromosome. If they had been separate, and lived, the female would still have been sterile in that situation ([Turner Syndrome](https://en.wikipedia.org/wiki/Turner_syndrome)) but if something that weird can happen, I’m not convinced that there can’t be some weird combination of genetic and developmental mistakes that could–very, very rarely–end up with “identical” twins being opposite sex and both fertile. I believe that you are supposed to double-spoiler potentially squicky stuff, so: [link](https://sonoworld.com/TheFetus/Case.aspx?CaseId=1252&answer=1) ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 2, 2016, 8:26pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/57 "2016-12-02T20:26:00Z")
> [@Darren\_Garrison](#): > > During my search for information on polar body fertilization, I came across a link to the case of a set of aborted conjoined fetuses that had male and female genitalia. The parents didn’t allow anyone to study them, so there is only speculation on the cause. One of the speculations is that a single embryo partially split, and that one half lost it’s Y chromosome. If they had been separate, and lived, the female would still have been sterile in that situation ([Turner Syndrome](https://en.wikipedia.org/wiki/Turner_syndrome)) but if something that weird can happen, I’m not convinced that there can’t be some weird combination of genetic and developmental mistakes that could–very, very rarely–end up with “identical” twins being opposite sex and both fertile. > > I believe that you are supposed to double-spoiler potentially squicky stuff, so: > > [link](https://sonoworld.com/TheFetus/Case.aspx?CaseId=1252&answer=1) As I read once, the reason for sex tests in the Olympics, before the Soviets and their amazing potions, was due to a developmental anomaly. If something interrupts the development process at just the right point, an XY fetus could fail to complete the emergence of male genitalia; the resultant child would appear to be female externally, but the vagina would be very short, no uterus, and the undescended testes would be in the position the ovaries should be and not produce male levels of testosterone. They would grow up to all appearances female, except infertile. They would have a tendency to better physical performance with slightly more testosterone than a typical female, hence over-represented in competitive sports. ---
**Author:** ![nightshadea](https://avatars.discourse-cdn.com/v4/letter/n/7ba0ec/32.png) [@nightshadea](https://boards.straightdope.com/u/nightshadea)\ **Post date:** [December 2, 2016, 9:38pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/58 "2016-12-02T21:38:02Z")
> [@md2000](#): > > As I read once, the reason for sex tests in the Olympics, before the Soviets and their amazing potions, was due to a developmental anomaly. If something interrupts the development process at just the right point, an XY fetus could fail to complete the emergence of male genitalia; the resultant child would appear to be female externally, but the vagina would be very short, no uterus, and the undescended testes would be in the position the ovaries should be and not produce male levels of testosterone. They would grow up to all appearances female, except infertile. They would have a tendency to better physical performance with slightly more testosterone than a typical female, hence over-represented in competitive sports. there was an episode of house that had a case like that but the illness of the episode was the underdeveloped testes were cancerous and going to be removed … which was stated as a well known effect of course the boy/girl was a model and supposedly soo alluring that no one had bothered to do a test at birth and just said "its a girl " What made the plot even more squickier was the teen was sleeping with who ever could get what he or she wanted or needed (including the father) and was trying to get the fact that it was just a really cute and delicate boy and not the hot girl that everyone thought it was legally suppressed so the modeling could continue ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 2, 2016, 10:45pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/59 "2016-12-02T22:45:57Z")
> [@nightshadea](#): > > there was an episode of house that had a case like that but the illness of the episode was the underdeveloped testes were cancerous and going to be removed … which was stated as a well known effect of course the boy/girl was a model and supposedly soo alluring that no one had bothered to do a test at birth and just said "its a girl " > > What made the plot even more squickier was the teen was sleeping with who ever could get what he or she wanted or needed (including the father) and was trying to get the fact that it was just a really cute and delicate boy and not the hot girl that everyone thought it was legally suppressed so the modeling could continue Jared Diamond discussed this problem when talking about sex and gender differences in “The Third Chimpanzee” and said the same thing - the person as adult is very obviously feminine, with extremely feminized features. The only interesting difference was a usually very short vagina, about 2 inches. Presumably, any sex partners when bottoming out would think “that’s because of me”… 😃 This is CAIS complete Androgen Insensitivity syndrome. There’s also Swyer syndrome, where the entire female organs develop except the ovaries; again, usually diagnosed from delayed puberty. I suppose that it’s not really weird, the person is in a respects physically female except for a lack of ovaries and possibly other internal female organs. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [December 3, 2016, 6:45pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/60 "2016-12-03T18:45:58Z")
> [@Leo\_Bloom](#): > > Speaking of hermaphroditism, let alone parthogenesis, I myself would be famously fecund could phalanx fascia be fertilized. This is kinda gross, but it made me laugh. Excellent alliteration. [Previous page](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532.md?page=2) [Next page](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532.md?page=4)