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?
A somewhat related conversation takes place starting at post #56 on this thread.
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.
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.
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 says:
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.
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.
ETA - I now see JRDelirious cited a description this, and it has happened.
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…
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.
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.
I was just going to ask about AIS. Can it have a non-genetic cause?
In a the rather unusual case of 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.
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.
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.