# Are siblings more closely related to you than your parents and children are? **URL:** \ **Category:** Factual Questions\ **Created:** [March 16, 2015, 6:34am UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097 "2015-03-16T06:34:02Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Doug\_K](https://avatars.discourse-cdn.com/v4/letter/d/b5ac83/32.png) [@Doug\_K](https://boards.straightdope.com/u/Doug_K)\ **Post date:** [March 17, 2015, 2:39pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/21 "2015-03-17T14:39:54Z")
> [@naita](#): > > You can’t judge a thread by the title alone. The content of the OP makes it clear this about genetic relatedness. The OP asked if it’s about genetic relatedness. > [@protoboard](#): > > Does that mean that genetically they are closer to you than your parents and even your own kids are? The answer is no, that’s not what it means. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [March 17, 2015, 3:01pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/22 "2015-03-17T15:01:02Z")
> [@Mijin](#): > > I don’t think this works. 3 crossovers on average per chromosome is not the same thing as saying there are 3 possible arrangements for each chromosome. > > Calculating what the odds are when we allow random-sized portions of DNA to come from either parent is beyond my abilities, but I’d be happy to wager the chance of getting less than 33% commonality between siblings, or greater than 66%, is far less likely than 4 in 1000. Yes, the simplest concept - no genetic crossover - means you have 23 pairs of chromosomes; and you either got the same one as your sibling, or a different one. (most obvious - boys get dad’s Y, girls get dad’s X). The odds here are a simple coin toss, 50-50. Let’s say you get mom’s X1, sibling also gets mom’s X1. However, in duplication during your egg’s creation, some of mom’s X2 switched to X1 (and vice versa); but didn’t for sis) or vice versa. However, you are right. these switcheroo segments could be 1 or some of genetic code, of the thousands of individual genes in the segment. So you’re right, the top-level back of the envelope calculation probably seriously underestimates the odds of convergence. From a very simple, no-crossover, 23 chromosomes - 50-50 event, 23 tries, probablility that say match \<= 6 (i.e. less than 1/4 match) is 0.0173 or about 1 in 57. The odds of no match in 23 coin tosses (or all match) is 1.19209289550781E-07 or 1 in 10 million. Of course with 23 chromosomes, assuming no crossover, you are likely either 11/23 or 12/23 match but there’s no 50-50. (as others point out, an X has more data than a Y and typically do not exchange IIRC, so again, this complicates any discussion or calculation) Add in random switches of random lengths of content in each of 46 chromosomes and the odds are even wilder of a the match is extremely low or extremely high. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [March 17, 2015, 3:05pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/23 "2015-03-17T15:05:47Z")
> [@Doug\_K](#): > > The answer is no, that’s not what it means. Yeah, I expect that answer was the one protoboard was looking for when he posted the OP. ---
**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:** [March 17, 2015, 6:00pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/24 "2015-03-17T18:00:53Z")
I am a geneticist\* and would add several minor clarifications to what has been stated. On average, a person shares the same proportion of _nuclear, somatic_ DNA with his/her \*full \*siblings as with a parent, 50%. We inherit a small amount of DNA that is not in the nucleus of the cell. This mitochondrial DNA is overwhelmingly inherited from the mother. So overall we get a tiny bit more DNA from our mothers than our fathers on this count. The sex chromosomes are substantially different in size. So a male, with one of the smaller Y chromosomes, gets even less overall DNA from the father. Same number of chromosomes but fewer base pairs. On average you share 50% of your DNA with your full siblings, and obviously less with half siblings. But indeed you could, by random chance, share less or more than 50% with those full siblings. The laws of probability indicate that anything very far away from 50% would be a low probability. Then there are the truly unusual circumstances in which there are more or fewer of a particular chromosome or portion thereof) than is normal. Down Syndrome, Turner Syndrome, Kleinfelter’s, et al all have one parent donating more DNA than the other to the offspring. Beyond truly unusual, roaming into the realm of downright weird, is [Uniparental Disomy](http://en.wikipedia.org/wiki/Uniparental_disomy), where you inherit two somatic chromosomes from one parent and none from the other. Finally, there are a few areas in which under normal circumstances gene expression is different depending upon which parent donated the DNA. While each parent gave a chromosome with the same gene, only the allele from one parent is used. - well, hold a BS in genetics anyway ---
**Author:** ![RivkahChaya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rivkahchaya/32/8919_2.png) [@RivkahChaya](https://boards.straightdope.com/u/RivkahChaya)\ **Post date:** [March 17, 2015, 6:35pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/25 "2015-03-17T18:35:59Z")
> [@TriPolar](#): > > Just an anecdote, I grew up with two brothers born about 2 years apart that had a remarkable physical resemblance. I first met them when they were about 11 and 13 and looked very similar but not unusually so for brothers. When I last saw them they would have been about 18 and 20 and had for several years been taken for twins. They were more similar in appearance than some identical twins I’d seen at that age. I have another anecdote: my grandmother has an identical cousin, as in “Met Cathy who’s lived most everywhere,” identical. It’s uncanny. They are something like 17 years apart, so it’s simple to tell them apart, but they look just amazingly alike. They are built alike, their hands are alike, their faces are identical, their hair grows identically, so much, that they even wore their hair similarly, in spite of being different ages, and get this-- different nationalities. My grandmother in a naturalized American who was born in Slovakia in 1917, and came to the US as a toddler. Her cousin was born in Czechoslovakia, and now lives in the Republic of Slovakia. Her cousin has never been to the US and doesn’t speak English. I’m sure they share about the same amount of their DNA as ordinary cousins-- maybe a little more (they don’t even have parents who are identical twins like Patty and Cathy). It’s just a coincidence that the DNA they share codes for outward appearance. There’s a photographer named Francois Brunelle [who did a series of photos of unrelated people (mainly strangers) who happened to look like siblings, in some cases almost like twins.](http://www.incrediblethings.com/web/strangers-that-look-like-twins/) ---
**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:** [March 17, 2015, 6:45pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/26 "2015-03-17T18:45:01Z")
> [@Iggy](#): > > Finally, there are a few areas in which under normal circumstances gene expression is different depending upon which parent donated the DNA. While each parent gave a chromosome with the same gene, only the allele from one parent is used. Sounds interesting. How does that work? ---
**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:** [March 17, 2015, 8:32pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/27 "2015-03-17T20:32:43Z")
> [@septimus](#): > > Sounds interesting. How does that work? The DNA molecule is a often described as a ladder, with base pairs forming the rungs of the ladder in an organized manner. Adenine bonded to one rail of the ladder pairs with Thymine bonded to the other rail (A + T). Similarly Guanine pairs with Cytosine (G +C). But sometimes the bases are chemically altered. The [addition of a methyl group](http://en.wikipedia.org/wiki/DNA_methylation) to Adenine and/or Cytosine is read as a signal not to use that particular DNA. It is silenced. This methylation can be a normal part of cellular development. It may be a key component of how a stem cell becomes a specialized cell, by turning off those genes that will not be needed in its specialized role. And in some specific regions of the genome that methylation takes place very early in development and in an unusual way, by methylating DNA based upon which parent it was inherited from. This broad usage of methylation in this parent specific way is known as [genomic imprinting](http://en.wikipedia.org/wiki/Genomic_imprinting). One such area it is used is on chromosome 15. On a specific section of chromosome 15 the paternal DNA is normally methylated, and thus silenced. So long as the matching maternal section is ok, everything is fine. But if that maternal DNA is a defective allele (or deleted\*) a serious neurological condition occurs, Angelman Syndrome\*\*. The gene in question is known as _UBE3A_. A nearby section of the same chromosome 15 has the maternal DNA being methylated and thus turned off. Again, if the paternal DNA is ok, everything is fine. But if the paternal DNA is a defective allele (or deleted\*\*\*) a different condition occurs, Prader-Willi Syndrome\*\*\*\*. Several genes, including \*SNRPN \*and \*necdin \*are involved. \*deletion on the maternal chromosome is the most common reason for Angelman Syndrome. \*\* The genetic test for Angelman Syndrome actually tests for methylation of the neighboring \*SNRPN \*gene responsible for Prader-Willi Syndrome. \*\*\* deletion on the paternal chromosome is a common reason for Prader-Willi Syndrome. \*\*\*\* Actress Mayim Bialik, who plays Amy on The Big Bang Theory, completed her PhD with a dissertation on Prader–Willi syndrome, titled,“Hypothalamic regulation in relation to maladaptive, obsessive-compulsive, affiliative, and satiety behaviors in Prader-Willi syndrome.” ---
**Author:** ![Cartoonacy](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@Cartoonacy](https://boards.straightdope.com/u/Cartoonacy)\ **Post date:** [March 19, 2015, 9:24pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/28 "2015-03-19T21:24:59Z")
> [@bump](#): > > So in other words, my brother and I are each 50% mom and 50% dad, but that 50% may not overlap very much- i.e. he and I may have 10% of the genes from Dad in common, and 90% of Mom, even if in total, each parent accounts for 50% of our genes. Just as a point of order, you do get exactly 50% from each parent, totaling 100%. But the overlap with your siblings don’t necessarily have to add up to 100% as in your example. You and your brother each have 50% of your mother’s genetic material, but it could conceivably (no pun intended) be an exclusive 50%; the same goes for your father, meaning you and your brother could have no genes in common. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [March 19, 2015, 10:51pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/29 "2015-03-19T22:51:00Z")
> [@Cartoonacy](#): > > Just as a point of order, you do get exactly 50% from each parent, totaling 100%. But the overlap with your siblings don’t necessarily have to add up to 100% as in your example. > > You and your brother each have 50% of your mother’s genetic material, but it could conceivably (no pun intended) be an exclusive 50%; the same goes for your father, meaning you and your brother could have no genes in common. If they had no genes in common they’d be completely different species! 😉 ---
**Author:** ![Mighty\_Girl](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mighty\_Girl](https://boards.straightdope.com/u/Mighty_Girl)\ **Post date:** [March 20, 2015, 11:58am UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/30 "2015-03-20T11:58:27Z")
> [@naita](#): > > If they had no genes in common they’d be completely different species! 😉 That explains my family. ---
**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:** [March 20, 2015, 1:58pm UTC](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097/31 "2015-03-20T13:58:08Z")
> [@Cartoonacy](#): > > Just as a point of order, you do get exactly 50% from each parent, totaling 100%. But the overlap with your siblings don’t necessarily have to add up to 100% as in your example. > > You and your brother each have 50% of your mother’s genetic material, but it could conceivably (no pun intended) be an exclusive 50%; the same goes for your father, meaning you and your brother could have no genes in common. Let’s not forget that every human has about 99.9% of his genes in common with every other human. > [@naita](#): > > If they had no genes in common they’d be completely different species! 😉 More likely, you’d be from another planet! All life on earth has some genes in common. [Previous page](https://boards.straightdope.com/t/are-siblings-more-closely-related-to-you-than-your-parents-and-children-are/715097.md?page=1)