# can co-joined twins be fraternal
**URL:** https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173
**Category:** Factual Questions
**Created:** [March 12, 2008, 11:11pm UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173 "2008-03-12T23:11:18Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@letter\_after\_rho](https://boards.straightdope.com/u/letter_after_rho)
#### Post date: [March 12, 2008, 11:11pm UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/1 "2008-03-12T23:11:18Z")
or are they always identical?
---
### Author:  [@jasonh300](https://boards.straightdope.com/u/jasonh300)
#### Post date: [March 12, 2008, 11:17pm UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/2 "2008-03-12T23:17:37Z")
Co_n_joined twins are always identical. They come from a single egg that splits. In this case, the egg doesn’t split all the way.
Fraternal twins come from two separate eggs and are never joined to begin with.
---
### Author:  [@chorpler](https://boards.straightdope.com/u/chorpler)
#### Post date: [March 12, 2008, 11:42pm UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/3 "2008-03-12T23:42:05Z")
When I read this, I wondered if fraternal twins ever implant so close as fertilized eggs that their cells grow up against each other and commingle as they divide, thus creating conjoined fraternal twins. But then I thought “If conjoined twins’ DNA wasn’t identical, wouldn’t their immune systems start attacking each other and eventually kill one or both of them?”
---
### Author:  [@ouryL](https://boards.straightdope.com/u/ouryL)
#### Post date: [March 12, 2008, 11:46pm UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/4 "2008-03-12T23:46:18Z")
It can be supposed that there could be chimeric conjoined twins.
---
### Author:  [@panache45](https://boards.straightdope.com/u/panache45)
#### Post date: [March 13, 2008, 12:10am UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/5 "2008-03-13T00:10:15Z")
[QUOTE=jasonh300]
Fraternal twins come from two separate eggs and are never joined to begin with.
[/QUOTE]
Right, they are never joined \*\*\*to begin with, \*\*\*but there have been cases of the joining of fraternal embryos. I’m looking for a cite.
---
### Author:  [@jasonh300](https://boards.straightdope.com/u/jasonh300)
#### Post date: [March 13, 2008, 12:17am UTC](https://boards.straightdope.com/t/can-co-joined-twins-be-fraternal/441173/6 "2008-03-13T00:17:29Z")
[QUOTE=panache45]
Right, they are never joined \*\*\*to begin with, \*\*\*but there have been cases of the joining of fraternal embryos. I’m looking for a cite.
[/QUOTE]
> **[Chimera (genetics)](https://en.wikipedia.org/wiki/Chimera_%28genetics%29)**
>
> A genetic chimerism or chimera (en-US), also chimaerism or chimaera (en-UK), (/kaɪˈmɪərə, kɪ-/ ky-MEER-ə, kih-) is a single organism composed of cells of different genotypes. animal chimeras can be produced by the fusion of two (or more) embryos. In plants and some animal chimeras, mosaicism involves
> distinct types of tissue that originated from the same zygote, but differ due to mutation during ordinary cell division.
> Normally, genetic chimerism is not visible on casual inspection; however, it ...
This pretty much covers it, but it is rare among humans (they cite 40 reported cases) and they don’t mention any where there were two distinct beings partially joined together. Just a single person with the genetics of two people. I can’t find anything about two partially joined embryos…where the second embryo wasn’t completely absorbed.