# Why are identical twins not EXACTLY identical?

**URL:** <https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641>\
**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:** 4\
**Page:** 4

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**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 26, 2022, 10:46pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/61 "2022-08-26T22:46:02Z")

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I don’t think it can be ruled out … and not just stress, there are many potential other differences from how the world is interacted with attended to and perceived, to nutrition, and on. But the methylation differences that are correlated vary by cohort and seem inconsistent with something like stress which would be more likely to cause similar epigenetic changes across cohorts.

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**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 27, 2022, 10:26am UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/62 "2022-08-27T10:26:50Z")

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Oh. The article does discuss that the changes may not be causative but instead “downstream” with their focus on medications used as part of care causing epigenetic changes as a possibility.

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**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:** [September 9, 2022, 6:38pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/63 "2022-09-09T18:38:52Z")

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Homogenous is a term used in genetics meaning “having identical genes.” Genes represented as such in a Punnett square as AA or bb are said to be homogenous (same genes). Aa or Bb are referred to as heterogenous (different genes).

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**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:** [September 9, 2022, 7:01pm UTC](https://boards.straightdope.com/t/why-are-identical-twins-not-exactly-identical/969641/64 "2022-09-09T19:01:46Z")

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That’s not the term he was looking for, though, by his description.

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