Can identical twins have different genders (as in biological sex)?

I thought they should have absolutely the same genes. If they can, what caused it? I'd assume it MAY be because of errors in cellular division at conception.

Thank you.

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    $\begingroup$ Note that your question, and the accepted answer, seem to depend on the use of the term "gender" to mean "biological sex". You might consider rephrasing for clarity here, since "gender" is a rather more complex and nuanced social construct. $\endgroup$ – Ryan Pavlik Apr 18 '14 at 21:18
  • $\begingroup$ Done. Could you clarify what you mean by that? Or is it better if I post another question? $\endgroup$ – Wajih Aziza Apr 18 '14 at 21:24
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    $\begingroup$ @WajihAziza These might get you started: en.wikipedia.org/wiki/Gender en.wikipedia.org/wiki/Transgender $\endgroup$ – Mr.Mindor Apr 18 '14 at 22:30

Identical twins occur when a single egg is fertilized to form one zygote, which then divides into two separate embryos. Thus, identical twins are always the same sex unless there has been a mutation during development.

However, two identical twins expressing different sexual phenotypes does happen in some extremely rare cases due to environmental factors, deactivation of different X chromosomes in females, or aneuploidy. This is normally because an XXY Klinefelter syndrome zygote has split unevenly.

You are correct though, in that identical twins usually have hundreds of genetic differences early in fetal development which can be attributed to mutations in the DNA. But usually these differences do not result in twins with different sexual phenotypes, though it is possible.

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    $\begingroup$ There can also be many epigenetical differences between twins. $\endgroup$ – biogirl Apr 18 '14 at 16:17
  • $\begingroup$ So theoretically, it's impossible to happen unless it was caused by an "error". That was enlightening. Thank you. $\endgroup$ – Wajih Aziza Apr 18 '14 at 16:52
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    $\begingroup$ @WajihAziza Biology isn't at all my field so I may be off base here, but wouldn't the twinning event itself be considered an 'error' as that is outside of the normal? $\endgroup$ – Mr.Mindor Apr 18 '14 at 22:33
  • $\begingroup$ Perhaps you want to rethink calling everything outside the norm an error. It's needlessly pejorative. $\endgroup$ – swbarnes2 Dec 3 '14 at 22:56

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