I have given my high school biology students the thought experiment of "What would happen if a researcher induced twinning of a female zygote and then replaced one of the X Chromosomes with a copy of the father's Y chromosome". In theory you should get what would be about as close as possible to male/female identical (monozygotic twins). However, I don't believe it would be possible for this to occur in nature (feel free to correct this if I am wrong). The closest I am familiar with is "Semi-identical twins in which the egg is fertilized by two sperm and then twinning occurs. However this isn't really the same. However, I realized that Androgen Insensitivity Syndrome, in which a mutation leads to the body not responding to masculinizing hormones, leads to a male developing as female. Is anyone aware of this ever happening? I realize it would need to occur after the zyote/morula split.

Similarly if the SRY gene (essentially the master control switch for masculinizing an embryo) has ever been transferred on to an early embryo of a pair of female monozygotic twins, essentially inducing one twin to develop male.

In essence I'm asking for evidence of physical manifestations of (near)identical monozygotic twins. Has any of this ever been done in research on other species?


1 Answer 1


This is an important question to geneticists because monozygotic twins are the only ethically acceptable way to research genetically identical humans.

I'm sure the sort of non-symmetrical fission where specific DNA coding for genes are not present or are modified to completely different phenotype in the formation of the twins probably has happened, but it incredibly rare. In stem cells, asymmetric division has been observed frequently (warning: google doesn't like PlosOne right now). In development, unless its actively been activated as an asymmetric division, this would be incredibly rare- this would be akin to a birth defect in a single fetus - how many cases of mixed male and female cells in a single baby. I can't even find a reference for such a thing. Much more common are semi-identical twins, where two different sperm fertilize a single egg which may have split. Semi-twins are still vanishingly rare - the first report was in 2007).

Such twins are probably very rare. Their existence and discovery relies on three unusual, and possibly unlinked, events: first, that an egg fertilized by two sperm develops into a viable embryo; second, that this embryo splits to form twins; and third, that the children come to the attention of science.

This might be the case you are thinking of? In one case reported there was a gender difference.

Souter and her colleagues investigated the twins' genetic makeup because one was born with ambiguous genitalia. This twin turned out to be a 'true hermaphrodite', with both ovarian and testicular tissue. The other twin is anatomically male.

They weren't monozygotic though. Just others from the same mother ;)

Monozygotic twins are not completely identical, epigenetic fingerprinting - including DNA methylation and chromatin DNA binding patterns (pnas - paywall) change while the fetus develops and as the child and adult age. This can cause quite significant differences as the twins age. Epigenetic markers change the dynamics of brain development and can make a difference in the occurance of leukemia.

There are also modifications to the DNA sequences of cells, copy number variations (CNVs) are multiple (sometimes hundreds or thousands of times over) repeats of short (2+ base pairs) DNA sequences. These also occur as the cells differentiate and divide to form tissues and organs. Twins show significant differences there too as this is not a deterministic mechanism in development.

I can't find any references to twins showing asymmetric AIS. but the number of variants requiring this makes it sounds like its possible. If you have a reference post it and I can have a look?

note added: this paper shows that methylation profiles appear identical at least in utero.

  • $\begingroup$ I am aware of the semi-identical twins (which to me is a bit of a misnomer). Also, while I share your sentiment that monozygotic twins vary (quite significantly over time) in their epigenomes, let's remember that, by definition, this is not genetic variation. My students got me to thinking a bit more about it simply because of the fascinating idea of being able to see what one would look like if they had developed as the other gender. I'm sure that it would be vanishingly rare, if it's happened, but that doesn't mean it hasn't happened...:) $\endgroup$ Commented Jan 3, 2013 at 3:35
  • $\begingroup$ @single_digit just doing my best to be comprehensive. (The CNVs are genetic variations tho.) It probably has happened - with 8 billion people on earth, the odds are not bad. The article notes that such oddities are usually not reported, esp if they don't result in some defect of some kind.. cheers. $\endgroup$
    – shigeta
    Commented Jan 3, 2013 at 5:55

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