# 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:** 1
**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:** [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/773532/1 "2016-12-01T15:00:44Z")
Since you get half your DNA from each parent, could two siblings have no DNA connection whatsoever? And how little DNA would they have to share to safely have children? ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [December 1, 2016, 3:03pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/2 "2016-12-01T15:03:20Z")
This isn’t my area of expertise, but I don’t think it’s possible to get DNA from only one parent. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [December 1, 2016, 3:04pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/3 "2016-12-01T15:04:39Z")
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. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 1, 2016, 3:08pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/4 "2016-12-01T15:08:27Z")
Too bad we’re not like lady rabbits and can carry fetuses from two different male donors at the same time. a) I just learned that and I thank OP for giving me the opportunity to force it into a conversation. b) It might not even be true, but somebody just told me that. ---
**Author:** ![CandidGamera](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/candidgamera/32/2878_2.png) [@CandidGamera](https://boards.straightdope.com/u/CandidGamera)\ **Post date:** [December 1, 2016, 3:08pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/5 "2016-12-01T15:08:34Z")
I’m going to assume she means that, for instance, the daughter gets all of her DNA from the mother’s ‘A’ side and the father’s ‘B’ side, and the son gets all his from the mother’s ‘B’ side and the father’s ‘A’ side. Or to simplify and look at one chromosome to illustrate : Mom has XX. Dad has XY. Daughter has XX. Son has XY. They clearly got different DNA from Dad. So as long as they each got copies of different X Chromosomes from Mom, they are genetically distinct for that pair. ---
**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:** [December 1, 2016, 3:09pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/6 "2016-12-01T15:09:25Z")
Fraternal twins can have different fathers, if the mother released two eggs and had sex with two men in a very short period of time. ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [December 1, 2016, 3:14pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/7 "2016-12-01T15:14:12Z")
It is theoretically possible that full siblings could not share any DNA, other than mitochondrial DNA. But the odds are so astronomical as to make it impossible. As eggs and sperm are produced there is a process called crossing over that effectively shuffles the alleles between each of the pairs of homologous chromosomes. To have two full siblings not share any DNA then all of the crossing over would have to happen at precisely the same locations during the production of eggs or sperm. Then the chromosomes would have to assort in a suitable manner. And the right egg and sperm would have to meet up to create the “opposite” sibling. Not going to happen. Ever. ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [December 1, 2016, 3:16pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/8 "2016-12-01T15:16:13Z")
> [@Leo\_Bloom](#): > > Too bad we’re not like lady rabbits and can carry fetuses from two different male donors at the same time. > > a) I just learned that and I thank OP for giving me the opportunity to force it into a conversation. > b) It might not even be true, but somebody just told me that. It’s not true. It is possible for a woman to carry fraternal twins who are half-siblings genetically. Just takes input (pardon the pun) from two male sources, each fertilizing a different egg. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [December 1, 2016, 3:17pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/9 "2016-12-01T15:17:27Z")
Okay, going back to the original question and assuming you mean full siblings… Generally, as I understand it, a child receives 1 of each chromosome pair from each parent. Therefore, if we ignore genetics crossovers and such, the chance that two non-identical siblings receive exactly “opposite” chromosomes from both parents would seem to be the same chance that they get exactly the same chromosomes for both parents… 1 chance in 2[sup]46[/sup] (Since there’s two chromosomes in each pair, and they’re getting 46 chromosomes total from both parents.) This includes accounting for the fact that one sibling would have to get the X chromosome from the father, and the other the Y chromosome, thus they will be different genders. That’s something like 1 chance in 64 trillion, as I make it. ---
**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 1, 2016, 3:19pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/10 "2016-12-01T15:19:19Z")
In theory, sure, just like you can in theory flip a perfectly balanced coin as “heads” a million times in a row. But it is very, very, very, very, very, very, very, very, very, very, very, unlikely. If no [crossing over](https://en.wikipedia.org/wiki/Chromosomal_crossover) took place, the odds would be one chance in 8,388,608 (if I’m doing my math right.) With crossing over, you can add an undetermined number of zeros to that number. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 1, 2016, 3:34pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/11 "2016-12-01T15:34:20Z")
> [@Iggy](#): > > It’s not true. > > It is possible for a woman to carry fraternal twins who are half-siblings genetically. Just takes input (pardon the pun) from two male sources, each fertilizing a different egg. Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)? Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity. Speaking of hermaphroditism, let alone parthogenesis, I myself would be famously fecund could phalanx fascia be fertilized. ---
**Author:** ![Anaglyph](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@Anaglyph](https://boards.straightdope.com/u/Anaglyph)\ **Post date:** [December 1, 2016, 3:34pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/12 "2016-12-01T15:34:46Z")
> [@Darren\_Garrison](#): > > In theory, sure, just like you can in theory flip a perfectly balanced coin as “heads” a million times in a row. But it is very, very, very, very, very, very, very, very, very, very, very, unlikely. If no [crossing over](https://en.wikipedia.org/wiki/Chromosomal_crossover) took place, the odds would be one chance in 8,388,608 (if I’m doing my math right.) With crossing over, you can add an undetermined number of zeros to that number. However, you will alway have crossovers, since these keep the homologous chromosome pairs together. Theoretically, if you could isolate the two cells generated in the first meiotic division in both oogenesis and spermatogenesis, you could generate complementary twins. Since this division is very asymmetric for the egg cell, producing a large egg cell and a small pole body, you would probably have to transfer the nucleus of the pole body to a poper egg cell. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [December 1, 2016, 3:38pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/13 "2016-12-01T15:38:26Z")
> [@Leo\_Bloom](#): > > Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)? > > Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity. It’s just that the way you phrased your original statement you made it sound like it was impossible for humans to do it. That’s all. It may be super common in rabbits and very rare in humans, but both can do it. ---
**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 1, 2016, 3:39pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/14 "2016-12-01T15:39:13Z")
Two things get passed down from parent to child almost unchanged: mitochondria and Y-chromosomes. Mitochondria are passed from mother to all; her children. There is a minute amount of mutation in each generation, so it changes gradually over centuries, but any two siblings will have mitochondrial DNA as near identical as makes no odds. Y-chromosomes get passed from father to son. At each generation there is some gene exchange during meosis, and some mutation, but a pair of (non twin) brothers will have Y-chromosomes that are about 99% the same. ---
**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 1, 2016, 3:44pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/15 "2016-12-01T15:44:48Z")
> [@Leo\_Bloom](#): > > Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)? > > Before I go look it up, … [Here it is](http://www.snopes.com/photos/people/mixedtwins.asp) ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 1, 2016, 3:47pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/16 "2016-12-01T15:47:30Z")
Thanks. I owe you one (1) look up. ---
**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 1, 2016, 3:47pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/17 "2016-12-01T15:47:37Z")
> [@Anaglyph](#): > > However, you will alway have crossovers, since these keep the homologous chromosome pairs together. But you could (in theory) have two siblings formed where the exact same crossover locations happened for every chromosome, but each sibling got the opposite set of segments. Probably not enough time between now and the heat-death of the universe to make that likely to happen, but it _could_. ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [December 1, 2016, 3:48pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/18 "2016-12-01T15:48:21Z")
> [@Leo\_Bloom](#): > > Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)? > > Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity. > … Pretty much as you describe. How common it occurs is not known since in many cases it could go undetected absent a reason to do a DNA test. But the phenomenon is common enough to have a scientific term, [heteropaternal superfecundation](https://en.wikipedia.org/wiki/Superfecundation#Heteropaternal_superfecundation). Wikipedia references a [study](https://www.ncbi.nlm.nih.gov/pubmed/1488855) involving paternity suits in which 2.4% of fraternal twins tested were found to have different fathers. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 1, 2016, 3:53pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/19 "2016-12-01T15:53:03Z")
Oh, how I wish I could remember who told me about the rabbits, go find him, and casually mention that! ---
**Author:** ![Isosleepy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/isosleepy/32/479_2.png) [@Isosleepy](https://boards.straightdope.com/u/Isosleepy)\ **Post date:** [December 1, 2016, 3:53pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/20 "2016-12-01T15:53:49Z")
Since Humans share DNA with Chimps and bananas, having two brothers sharing no DNA would mean one of them is some sort of alien experiment. Having 2 brothers have no origin of genes in common is possible, but vanishingly unlikely. Imagine you have a very long board with 20,000 numbered holes. You have 40,000 ping pong balls, 20,000 white, 20,000 black. All numbered. ( there’s one black number 1, and one white -etc) Now you start filling the holes, either a black #1 or a white, a black #2 or a white - till all holes are filled. Do this without looking! Now put all the balls back and do it again. What are the odds that the 2 arrays are completely different? My calculator rounds it to 0. If we were only doing this 2000 times, my calculator still rounds to zero, but i think the odds would be 1 to some number with about 91,000 zeroes in it, if it was 4000 times it’d have 8,281,000,000 zeroes in it, and after that my head starts hurting and we’re not even to a fraction of what 20,000 times would be… People with actual expertise in genetics and/or exponential math please correct the assuredly many errors above, but I hope I gave some idea as to how astronomically unlikely this is. [Next page](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532.md?page=2)