# Identical Twins DON'T have identical genetic codes? **URL:** \ **Category:** Factual Questions\ **Created:** [January 2, 2012, 9:13pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175 "2012-01-02T21:13:57Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Bellwood1030](https://avatars.discourse-cdn.com/v4/letter/b/ee59a6/32.png) [@Bellwood1030](https://boards.straightdope.com/u/Bellwood1030)\ **Post date:** [January 2, 2012, 9:13pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/1 "2012-01-02T21:13:57Z")
The january 2012 issue of _National Geographic_ says that twins “share virtually the same genetic code.” I would have thought they shared an _identical_ genetic code. Why “virtually”? ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [January 2, 2012, 10:17pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/2 "2012-01-02T22:17:37Z")
Mutations may have occurred in some cells of either twin’s embryo after separation – depending on how early that mutation happened, it could propagate to a significant percentage of one twin’s cells and not the other’s. I suspect it’s mainly weasel words to cover cases like this. ---
**Author:** ![mnemosyne](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@mnemosyne](https://boards.straightdope.com/u/mnemosyne)\ **Post date:** [January 2, 2012, 10:22pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/3 "2012-01-02T22:22:28Z")
Spontaneous mutations, DNA replication errors and mutagens/carcinogens/cancers can alter DNA throughout a person’s life. Offhand, I’m not sure that this is a significant proportion of DNA, and the way that most forensic testing is done to determine identity/relationships between people would probably not find most of these differences. Some differences really have no impact on the cell/animal anyways since the [coding](http://en.wikipedia.org/wiki/Genetic_code#RNA_codon_table)system is pretty robust- if a DNA sequence that should be UUA gets changed to UUG, the cell will still make lysine and there won’t be any biological indicator that something was altered. ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [January 2, 2012, 10:31pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/4 "2012-01-02T22:31:59Z")
I used to teach high school. I taught twin sisters who said they were identical, but didn’t quite look it. I wonder how much an X chromosome affects appearance- by the time a girl’s born, one X in each cell is shut down into a Barr body (what they look for in Olympic sex tests). Twin girls could definitely be running on different mixes of X chromosomes. ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [January 2, 2012, 10:47pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/5 "2012-01-02T22:47:48Z")
Identical twins also (we think) are more “identical” as infants than they are as senior citizens. That’s because of something called [“epigenetic drift”](http://www.pnas.org/content/102/30/10413.full). Basically, chemical markers (“epigenes”) switch genes on or off based on things like smoking, body weight, activity level and stress (the familiar “lifestyle choices”), as well as things like amount of nurturing done and style of parenting when you’re young. Since even identical twins don’t have identical experiences, they have different epigenetic markers activated. The theory is that this is why one identical twin might develop diabetes or cancer while the other doesn’t, or they may develop it years apart. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [January 2, 2012, 10:58pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/6 "2012-01-02T22:58:42Z")
Somewhat related but really weird. > **[Chimera (genetics)](https://en.wikipedia.org/wiki/Chimera_(genetics))** > > A genetic chimerism or chimera (/kaɪˈmɪərə, kə-/ ky-MEER-ə, kə-) is a single organism composed of cells with more than one distinct genotype. In animals and human chimeras, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types, subtle variations in form (phenotype) and, if the zygotes were of differing sexes, then even the possession of both female and male sex organs. Animal chimeras are produced by the merger of two (or m An... ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 2, 2012, 11:42pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/7 "2012-01-02T23:42:51Z")
> [@Bellwood1030](#): > > The january 2012 issue of _National Geographic_ says that twins “share virtually the same genetic code.” I would have thought they shared an _identical_ genetic code. Why “virtually”? As was said–mutations. The DNA of the cells in your left hand is only\* virtually \*identical to that in your right hand, for the same reason. ---
**Author:** ![China\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/779978/32.png) [@China\_Guy](https://boards.straightdope.com/u/China_Guy)\ **Post date:** [January 3, 2012, 1:17am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/8 "2012-01-03T01:17:08Z")
IIRC from when my wife was pregnant with twins, identical twins share around 95% of the same genetic code. There are also environmental factors such as placement in the uterus, amount of nutrients that flow through the umbilical chord, which twin is born first, how much of a time lag for twin B to be born, and then after birth differences in the environment. My twins are likely not identical but we haven’t done the genetic test for a definitive answer. My twin B is autistic, and likely that way because of hypoxic damage at birth. ---
**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:** [January 3, 2012, 1:19am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/9 "2012-01-03T01:19:08Z")
> [@China\_Guy](#): > > IIRC from when my wife was pregnant with twins, identical twins share around 95% of the same genetic code. I’d have to see a cite for that to believe it. > [@](#): > > There are also environmental factors such as placement in the uterus, amount of nutrients that flow through the umbilical chord, which twin is born first, how much of a time lag for twin B to be born, and then after birth differences in the environment. Those are not genetic. ---
**Author:** ![China\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/779978/32.png) [@China\_Guy](https://boards.straightdope.com/u/China_Guy)\ **Post date:** [January 3, 2012, 1:44am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/10 "2012-01-03T01:44:51Z")
> [@John\_Mace](#): > > Those are not genetic. That’s why “environmental factors” was used. According to Wiki, I mis-remembered. Here is the [wiki quote](http://en.wikipedia.org/wiki/Twin_study): Twins are a valuable source for observation due to their genotypes and family environments tending to be similar. More specifically, monozygotic (MZ) or “identical” twins, share nearly 100% of their genetic polymorphisms, which means that most variation in pairs’ traits (measured height, susceptibility to boredom, intelligence, depression, etc.) is due to their unique experiences. Dizygotic (DZ) or “fraternal” twins share only about 50% of their polymorphisms. DZ twins are helpful to study because they tend to share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) by virtue of being born in the same time and place. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 3, 2012, 3:37am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/11 "2012-01-03T03:37:46Z")
They probably differ genetically if you include mitochondria; those are descended from a population that comes along with the egg, and can vary genetically even from one body part to another inside one person much less between two people. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 3, 2012, 4:04am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/12 "2012-01-03T04:04:52Z")
Some previous threads on this issue: [Do identical twins have identical DNA?](http://boards.straightdope.com/sdmb/showthread.php?t=587150) [Do (identical) twins have identical DNA?](http://boards.straightdope.com/sdmb/showthread.php?t=80286) [A question about identical twins.](http://boards.straightdope.com/sdmb/showthread.php?t=559032) Be alert for the postings of one KarlGauss in the above threads. He claims to know what he’s talking about. ---
**Author:** ![Bellwood1030](https://avatars.discourse-cdn.com/v4/letter/b/ee59a6/32.png) [@Bellwood1030](https://boards.straightdope.com/u/Bellwood1030)\ **Post date:** [January 3, 2012, 4:20am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/13 "2012-01-03T04:20:13Z")
Thanks for the great, informative responses. I am enlightened! ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [January 3, 2012, 5:38am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/14 "2012-01-03T05:38:28Z")
> [@WhyNot](#): > > Identical twins also (we think) are more “identical” as infants than they are as senior citizens. That’s because of something called [“epigenetic drift”](http://www.pnas.org/content/102/30/10413.full). Basically, chemical markers (“epigenes”) switch genes on or off based on things like smoking, body weight, activity level and stress (the familiar “lifestyle choices”), as well as things like amount of nurturing done and style of parenting when you’re young. Since even identical twins don’t have identical experiences, they have different epigenetic markers activated. The theory is that this is why one identical twin might develop diabetes or cancer while the other doesn’t, or they may develop it years apart. Yes, but that’s a difference in phenotype, not genotype. If we think of genes as a book, the parts which are being read change with time; for each twin, different parts will be getting read at a given time and the difference will be bigger with age. But the book isn’t changed by which parts are being read and which aren’t. ---
**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:** [January 3, 2012, 6:37am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/15 "2012-01-03T06:37:14Z")
> [@Der\_Trihs](#): > > They probably differ genetically if you include mitochondria; those are descended from a population that comes along with the egg, and can vary genetically even from one body part to another inside one person much less between two people. The probably don’t. Although possible, it would be an extremely rare event for twins (identical or otherwise) to have different mtDNA from the mother. Mutations in mtDNA only happen on the scale of thousands of years. ---
**Author:** ![BeaMyra](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@BeaMyra](https://boards.straightdope.com/u/BeaMyra)\ **Post date:** [January 3, 2012, 7:11am UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/16 "2012-01-03T07:11:01Z")
I saw a PBS special on this and what others are saying is correct. How they explained it was that babies share almost all the same DNA. During people’s lives, gene get turned on and some get turned off. This is how your body knows how to go through changes especially at puberty. Some genes seem to be turned on by hormonal changes, other simply seemed to be on a timer. Environment effects whether or not those genes get turned on at the correct time. Like if one twin, eats poorly and the other eats well. The program showed identical twins at different ages and indeed the older identical twins shared less and less DNA. That said, it still was a very small differences. A standard test for paternity for instance, would not be able to determine which twin fathered a baby. You’d need more complex tests to determine that. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [January 3, 2012, 4:59pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/17 "2012-01-03T16:59:11Z")
> [@BeaMyra](#): > > The program showed identical twins at different ages and indeed the older identical twins shared less and less DNA. That said, it still was a very small differences. No. As has been said, epigenetics is NOT genetics. It’s a misconception I see more and more online lately. The DNA does not change over time. What changes is how it’s read. The code itself remains identical, barring mutations and the like. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 3, 2012, 7:37pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/18 "2012-01-03T19:37:13Z")
> [@Der\_Trihs](#): > > They probably differ genetically if you include mitochondria; those are descended from a population that comes along with the egg, and can vary genetically even from one body part to another inside one person much less between two people. This is the key issue - mitochrondria are rings of genetic material free-floating in the cell, rather than part of the nucleus of chromosomes. since it comes from the egg, all mitochrondrial material is from the mother. When the cells divide, the nucleus basically duplicates and splits - but that’s not what happens in the rest of the cell contents; part of the content of the cell goes one side, and part the other. It is possible that the cell split that results in the two individuals means that each could end up with a copy some of the mother’s mitochrondria rings that the other did not get. I have seen some articles on scientific studies that use mitochrondrial DNA to establish maternal lines. It’s one way to tell identical twins apart. But it’s certanly not 5% IIRC. Also, there’s a random probability of copy errors happening whenever a chromosome duplicates. Portions of the DNA can migrate between the pairs of genes, etc. It’s not usually a perfect copy. ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [January 3, 2012, 7:49pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/19 "2012-01-03T19:49:42Z")
> [@Nava](#): > > Yes, but that’s a difference in phenotype, not genotype. True, which is why I began with “also”. Sorry, lazy shortcut on my part. That “also” was supposed to take the place of, “Well, this question’s been answered quite well, and here’s this other neat thing that makes identical twins not as identical to each other as elderly people as they were as infants.” 😉 ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [January 3, 2012, 7:54pm UTC](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175/20 "2012-01-03T19:54:24Z")
I am currently reading [Inside of a Dog](http://insideofadog.com/), a book about what dogs perceive and communicate and what their world is like, and the author mentions that dogs have been able to distinguish between identical twins by scent in controlled studies (where one assumes that factors like clothing, perfume choice and bathing have been controlled for). [Next page](https://boards.straightdope.com/t/identical-twins-dont-have-identical-genetic-codes/608175.md?page=2)