# Can identical twins be male and female
**URL:** https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159
**Category:** Factual Questions
**Created:** [February 20, 2018, 12:58am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159 "2018-02-20T00:58:15Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@Steven\_Estes](https://boards.straightdope.com/u/Steven_Estes)
#### Post date: [February 20, 2018, 12:58am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/1 "2018-02-20T00:58:15Z")
Anther mystery I read focused on identical twins, one male and one female.
While this may seem absurd–and may in fact be–if there is sch a pair of twins I would imagine that a differing fetal environment for the genetically-identical twins would be the cause.
Has there ever been such twins?
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### Author:  [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)
#### Post date: [February 20, 2018, 1:07am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/2 "2018-02-20T01:07:36Z")
A somewhat related conversation takes place starting at post #56 on [this](https://boards.straightdope.com/sdmb/showthread.php?t=812529) thread.
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [February 20, 2018, 1:08am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/3 "2018-02-20T01:08:32Z")
By definition, it’s impossible, since identical twins have an identical genetic makeup. (At least, to start with. After separation of the fertilized egg they could start accumulating different somatic mutations.)
Identical twins are produced by the splitting of a fertilized egg in which each half goes on to develop separately. All their genetic material, including their sex chromosomes, is identical.
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### Author:  [@jtur88](https://boards.straightdope.com/u/jtur88)
#### Post date: [February 20, 2018, 1:12am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/4 "2018-02-20T01:12:44Z")
With no knowledge (and very little wisdom and no common sense at all), Im going to argue that it is technical possible, if the original zygote was a true hermaphrodite, and each of the resulting double issue ultimately met the socio-legal criteria for different sexes. Im not really sure if sex is, invariably, a biologically discernible distinction, or only one of reproductive capacity. An individual can possess the biological ability to per form either reproductive role.
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### Author:  [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)
#### Post date: [February 20, 2018, 1:14am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/5 "2018-02-20T01:14:04Z")
An outcome of a single zygote leading to twins of different genders is an extraordinarily rare freak ocurrence that involves an abnormality in the chromosomes either before or after division of the fertilized zygote. Or else you could have Semi-identical twins. The [Wikipedia entry](https://en.wikipedia.org/wiki/Twin#Human_twin_studies) says:
> [@](#):
>
> Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).[73] When monozygotic twins are born with different sexes it is because of chromosomal defects. There can be monozygotic boy/girl twins if the sex gene of the embryo has an extra x chromosome (the fertilized egg would be an xxy) then when the egg splits, one can have xx (girl) genes and one can have xy (boy) genes. This is rare, but possible. Records show there are only 10 known cases of these type twins.[citation needed] The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic. Another abnormality that can result in monozygotic twins of different genders is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X).[74] In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.
>
> Semi-identical twins
>
> Monozygotic twins can develop differently, due to different genes being activated.[75] More unusual are “semi-identical twins”. These “half-identical twins” are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.
>
> This results in a set of twins with identical genes from the mother’s side, but different genes from the father’s side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine.[76][77][78]
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [February 20, 2018, 1:19am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/6 "2018-02-20T01:19:09Z")
Now there are theoretical ways in which one of the two embryos could develop with male anatomy, and one with female. For example, testosterone is required for a male fetus to develop a penis and testicles. If the tissues that secrete testosterone in one fetus were destroyed, say by cancer, then that fetus could develop with female anatomy.
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### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [February 20, 2018, 2:17am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/7 "2018-02-20T02:17:35Z")
Considering just the DNA, rather than potential developmental differences that Colibri is dicussing:
The usual definition for identical twins is “product of one zygote” (monozygotic), rather than explicitly “completely identical DNA”, although of course the latter usually follows from the former. So we’d have to be looking at something extremely unusual happening for male and female siblings to arise from one zygote. Whether you chose to _call_ such hypothetical siblings “identical” is a matter of opinion, but let’s say monozygotic. Could it happen?
We know that XXY zygotes are possible and viable, this is Klinefelter Syndrome. It arises from nondisjunction in meiosis, where a sperm gets both X and Y, or an egg gets both X’s. So, it seems to me that it’s conceivable that a zygote could be XXY, and then a second unusual event could occur in an early mitosis to give rise to cell lineages and then fetuses with different karyotypes. The easiest way would probably be for lineage to retain XXY (male) and the other to lose the Y for XX (female).
I have never heard of this happening, but I can’t think of any reason that it’s not (in principle) possible. As I say, whether you _called_ them identical twins is a matter of choice. They would have identical DNA except for the sex chromosomes. But since the sex chromosomes influence a lot of things other than sexual characteristics, they could be phenotypically quite different.
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### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [February 20, 2018, 2:25am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/8 "2018-02-20T02:25:34Z")
ETA - I now see JRDelirious cited a description this, and it has happened.
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### Author:  [@elfkin477](https://boards.straightdope.com/u/elfkin477)
#### Post date: [February 20, 2018, 3:26am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/9 "2018-02-20T03:26:52Z")
> [@JRDelirious](#):
>
> An outcome of a single zygote leading to twins of different genders is an extraordinarily rare freak ocurrence that involves an abnormality in the chromosomes either before or after division of the fertilized zygote. Or else you could have Semi-identical twins. The [Wikipedia entry](https://en.wikipedia.org/wiki/Twin#Human_twin_studies) says:
>
> > [@](#):
> >
> > More unusual are “semi-identical twins”. These “half-identical twins” are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere.
Any idea, anyone, why the theory is that the divided egg would need to be attached? I don’t understand why it’s feasible for a zygote to divide in half healthily vs. the ova itself doing so before fertilization, which would seem to be a much more straight forward a way to end up with twins with the same maternal but different paternal DNA. I know that the ova is only a single cell, but single-celled organisms divide in half identically to reproduce…
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### Author:  [@Aji\_de\_Gallina](https://boards.straightdope.com/u/Aji_de_Gallina)
#### Post date: [February 20, 2018, 3:44am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/10 "2018-02-20T03:44:39Z")
Of course. Why are you assuming their gender or even sex?
They may both have penises and XY but one can be a female, or even both. And then go back.
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### Author:  [@Smeghead](https://boards.straightdope.com/u/Smeghead)
#### Post date: [February 20, 2018, 5:42am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/11 "2018-02-20T05:42:57Z")
You could envision a pair of XY twins, one of which picks up a mutation just after they split that confers androgen insensitivity, and therefore develops into a female.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [February 20, 2018, 2:01pm UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/12 "2018-02-20T14:01:07Z")
I was just going to ask about AIS. Can it have a non-genetic cause?
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### Author:  [@Broomstick](https://boards.straightdope.com/u/Broomstick)
#### Post date: [February 20, 2018, 2:35pm UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/13 "2018-02-20T14:35:16Z")
In a the rather unusual case of [Lori and George Schappell](https://en.wikipedia.org/wiki/Lori_and_George_Schappell) one of pair of conjoined twins announced that he identifies as male - which would make the pair a ciswoman and a transman. However, given that George has physical deformities in addition to everything else the two of them deal with, it is possible George is actually intersex but the two have, apparently, never volunteered information on their physical issues to the public. In fact, while they may share the same DNA they are not, and never have been, physically identical.
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### Author:  [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)
#### Post date: [February 21, 2018, 12:59am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/14 "2018-02-21T00:59:16Z")
Sherri Tepper wrote a novel, Sideshow, that featured a pair of conjoined twins of opposite sex. She was aware of the contradiction involved but offered no explanation. The situation described by Broomstick above shows that it is indeed just barely possible.
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### Author:  [@Broomstick](https://boards.straightdope.com/u/Broomstick)
#### Post date: [February 21, 2018, 2:59am UTC](https://boards.straightdope.com/t/can-identical-twins-be-male-and-female/809159/15 "2018-02-21T02:59:06Z")
The twins in Tepper’s novel were intersex - they underwent surgery (breast implants and electrolysis for the “female” of the pair) and employed strategies like clothing (lifts in the shoes of the “male” half of the pair) to differentiate themselves. So there was an explanation, but it wasn’t explored at length or in great detail.