# Why are identical twins not EXACTLY identical? **URL:** \ **Category:** Factual Questions\ **Created:** [August 14, 2022, 2:16pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641 "2022-08-14T14:16:29Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [August 14, 2022, 11:54pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/21 "2022-08-14T23:54:00Z")
Cellular replication through division is happening everywhere, and rapidly, in the developing embryo. It’s chaotic. In two embryos with identical DNA the processes are going on but are not 100% in synch necessarily. This difference in time can have an effect on the end result, as can position in the uterus, variations in specific conditions at the area of umbilical attachment, etc. Cite: in publication. ---
**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:** [August 15, 2022, 12:11am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/22 "2022-08-15T00:11:53Z")
> [@Andy\_L](#): > > Identical twins spend at least 9 months in a very similar environment, so that adds to the genetic identicalness It would be interesting to induce twinning in an IFV zygote, then implant each one in a different uterus. I’d expect the twins to differ more than the usual, but I wonder about the exact degree. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [August 15, 2022, 12:20am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/23 "2022-08-15T00:20:56Z")
Me, too. ---
**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:** [August 15, 2022, 12:22am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/24 "2022-08-15T00:22:49Z")
Is “Me 2” what twins call their sibling? ---
**Author:** ![needscoffee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/needscoffee/32/1076_2.png) [@needscoffee](https://boards.straightdope.com/u/needscoffee)\ **Post date:** [August 15, 2022, 1:30am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/25 "2022-08-15T01:30:51Z")
> [@dolphinboy](#): > > Then don’t use “silly examples” when answering a Factual Question. We’re looking for factual answers. It seemed pretty clear to me it was an example of how a theoretical environmental difference could potentially cause variability. ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [August 15, 2022, 1:42am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/26 "2022-08-15T01:42:54Z")
> [@needscoffee](#): > > It seemed pretty clear to me it was an example of how a theoretical environmental difference could potentially cause variability. Sure, and that’s all that needed to be said about it. Height differences in identical twins has nothing to do with oxygen levels on bunk beds, so that’s not true. Someone may have thought it was a serious answer and been misinformed. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [August 15, 2022, 1:44am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/27 "2022-08-15T01:44:48Z")
Of course! ---
**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 15, 2022, 1:54am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/28 "2022-08-15T01:54:25Z")
Even if the parents are feeding them the same things, they might still end up with different diets. Maybe one of them ate broccoli while over at a friend’s house, and got sick, and consequently developed a food aversion to broccoli. Maybe an attractive classmate caught the attention of one of them, and that one is deliberately dieting to try to be more appealing to the classmate. Maybe one of them got an after-school job, and so isn’t home in time for dinner any more, or maybe the job is at a fast-food place, and so they end up eating there a lot. Similarly, one might end up getting more exercise, or taking up smoking, or get pregnant early in life, or any of the myriad other ways that environment can differ. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [August 15, 2022, 1:59am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/29 "2022-08-15T01:59:05Z")
> [@dolphinboy](#): > > Height differences in identical twins has nothing to do with oxygen levels on bunk beds, so that’s not true. Someone may have thought it was a serious answer and been misinformed. I didn’t quite believe it but took it seriously enough to google a bit, and was surprised to discover that [bunk position may affect one’s risk of developing asthma](https://pubmed.ncbi.nlm.nih.gov/10400479/) (which can also impact childhood development): > [@](#): > > Subjects sleeping in the bottom bunk may be exposed to house dust particles detached from bedding of the top bunk. Our aim was to ascertain whether this exposure could influence the development of mite sensitization and/or allergic symptoms in these individuals. […] > > Mite sensitization rate and prevalence of allergic respiratory disease were similar for the top-bed and bottom-bed groups, whereas prevalence of asthma was significantly higher in the latter. […] > > **Conclusions:** Sleeping in bunks constitutes a greater risk of developing asthma for subjects sleeping in the bottom bed. ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [August 15, 2022, 2:09am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/30 "2022-08-15T02:09:21Z")
nevermind ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [August 15, 2022, 11:55am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/31 "2022-08-15T11:55:56Z")
> [@racer72](#): > > One died from uterine cancer at 25, t To be fair, that was probably from HPV, an STI, so it depends on who she was “with” … but it could also be due to inherited OR spontaneous predisposition … So unless she was taking the twin thing to the extreme and doing everything the same… its not a good example to show genetic differences. ---
**Author:** ![Tfletch1](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tfletch1/32/5340_2.png) [@Tfletch1](https://boards.straightdope.com/u/Tfletch1)\ **Post date:** [August 15, 2022, 5:11pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/32 "2022-08-15T17:11:11Z")
There’s a whole field of epigenetics where different chemical marks can be added to the DNA or the proteins coating the DNA which alters the expression of genes. These epigenetic markers can be stable through DNA replication and cell division. The epigenetic field also includes non-coding RNAs that can have major influences on gene expression. Here’s just a sample of the role of epigenetics on the phenotypes of twins. [https://learn.genetics.utah.edu/content/epigenetics/twins/](https://learn.genetics.utah.edu/content/epigenetics/twins/) ---
**Author:** ![Joeu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/joeu/32/2911_2.png) [@Joeu](https://boards.straightdope.com/u/Joeu)\ **Post date:** [August 15, 2022, 6:17pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/33 "2022-08-15T18:17:13Z")
I remember CC, the first cloned cat, and her donor, Rainbow. They were genetically identical, like identical twins, but looked nothing like one another. Here’s the Britannica article (with pictures. Both are quite cute. But you would never guess that CC is a clone of Rainbow from looking at them). [CC, The First Cloned Cat | Britannica](https://www.britannica.com/list/cc-the-first-cloned-cat) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 15, 2022, 11:16pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/34 "2022-08-15T23:16:51Z")
I just came across this article from the New Yorker, 1995: [Double Mystery | The New Yorker](https://www.newyorker.com/magazine/1995/08/07/double-mystery). Warning: It takes over an hour to read and contains more information that you probably want. One astonishing fact is that there are three known examples of MZ (monozygotis) twins of opposite sex. The explanation is that when the fertilized egg split, there was an error and one of the zygotes didn’t get the Y chromosome. ---
**Author:** ![Beckdawrek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beckdawrek/32/3647_2.png) [@Beckdawrek](https://boards.straightdope.com/u/Beckdawrek)\ **Post date:** [August 16, 2022, 12:02am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/35 "2022-08-16T00:02:15Z")
There’s a thing where one twin get more nutrition in utero. It will cause the twins to be really different in size. It’s called parasitic twinning. I have very young twin grandsons. They are mirror identical twins. They both have a mole on the earlobe. When they face each other the moles don’t face each other. My DIL says their hair has a cow lick. The hair twirls in opposite curls. . I understand they will both be left handers. Then I got confused and a headache so I told them to shut-up. ---
**Author:** ![jjakucyk](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)\ **Post date:** [August 16, 2022, 4:07am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/36 "2022-08-16T04:07:24Z")
> [@racer72](#): > > The first pair matched so close they both had moles on their upper lips in the same spot. Let me throw out an opposite situation. I know a pair of twins that look so similar I needed to identify a mole or two on their face to help differentiate, and the best I could do is find one that they both have but it’s slightly more to the side on one of them versus the other. The thing is, they’re NOT identical twins, they’re fraternal. So how does something like that fit into this conversation? My brain hurts. ---
**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 16, 2022, 10:06pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/37 "2022-08-16T22:06:54Z")
Heck, I once met a pair of sisters who were two years apart, and I still couldn’t tell the difference even when they were standing side by side. ---
**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:** [August 16, 2022, 11:15pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/38 "2022-08-16T23:15:19Z")
The Olsen twins are not identical, yet spent years playing the same character on _Full House_. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [August 17, 2022, 1:44am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/39 "2022-08-17T01:44:48Z")
> [@Tfletch1](#): > > There’s a whole field of epigenetics where different chemical marks can be added to the DNA or the proteins coating the DNA which alters the expression of genes. Is there any sense of how early monozygotic twins begin to diverge? Egg cytoplasm is not homogenous, with various actors including mRNA etc differentially spread throughout it. ISTM that from first split those may be unevenly divided with epigenetic differences in expression possible from word go. Is that a reasonable understanding? ---
**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:** [August 17, 2022, 2:06am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/40 "2022-08-17T02:06:00Z")
> [@Beckdawrek](#): > > There’s a thing where one twin get more nutrition in utero. It will cause the twins to be really different in size. It’s called parasitic twinning. Yep. I tutored two twin boys. The "donor " was 4 inches shorter than his twin. [Previous page](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641.md?page=1) [Next page](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641.md?page=3)