Cellular replication through division is happening everywhere, and rapidly, in the developing embryo. It’s chaotic. In two embryos with identical DNA the processes are going on but are not 100% in synch necessarily. This difference in time can have an effect on the end result, as can position in the uterus, variations in specific conditions at the area of umbilical attachment, etc.
Cite: in publication.
It would be interesting to induce twinning in an IFV zygote, then implant each one in a different uterus. I’d expect the twins to differ more than the usual, but I wonder about the exact degree.
Me, too.
Is “Me 2” what twins call their sibling?
It seemed pretty clear to me it was an example of how a theoretical environmental difference could potentially cause variability.
Sure, and that’s all that needed to be said about it. Height differences in identical twins has nothing to do with oxygen levels on bunk beds, so that’s not true. Someone may have thought it was a serious answer and been misinformed.
Of course!
Even if the parents are feeding them the same things, they might still end up with different diets. Maybe one of them ate broccoli while over at a friend’s house, and got sick, and consequently developed a food aversion to broccoli. Maybe an attractive classmate caught the attention of one of them, and that one is deliberately dieting to try to be more appealing to the classmate. Maybe one of them got an after-school job, and so isn’t home in time for dinner any more, or maybe the job is at a fast-food place, and so they end up eating there a lot. Similarly, one might end up getting more exercise, or taking up smoking, or get pregnant early in life, or any of the myriad other ways that environment can differ.
I didn’t quite believe it but took it seriously enough to google a bit, and was surprised to discover that bunk position may affect one’s risk of developing asthma (which can also impact childhood development):
nevermind
To be fair, that was probably from HPV, an STI, so it depends on who she was “with” … but it could also be due to inherited OR spontaneous predisposition … So unless she was taking the twin thing to the extreme and doing everything the same… its not a good example to show genetic differences.
There’s a whole field of epigenetics where different chemical marks can be added to the DNA or the proteins coating the DNA which alters the expression of genes. These epigenetic markers can be stable through DNA replication and cell division. The epigenetic field also includes non-coding RNAs that can have major influences on gene expression.
Here’s just a sample of the role of epigenetics on the phenotypes of twins.
https://learn.genetics.utah.edu/content/epigenetics/twins/
I remember CC, the first cloned cat, and her donor, Rainbow. They were genetically identical, like identical twins, but looked nothing like one another.
Here’s the Britannica article (with pictures. Both are quite cute. But you would never guess that CC is a clone of Rainbow from looking at them). CC, The First Cloned Cat | Britannica
I just came across this article from the New Yorker, 1995:
Double Mystery | The New Yorker. Warning: It takes over an hour to read and contains more information that you probably want. One astonishing fact is that there are three known examples of MZ (monozygotis) twins of opposite sex. The explanation is that when the fertilized egg split, there was an error and one of the zygotes didn’t get the Y chromosome.
There’s a thing where one twin get more nutrition in utero. It will cause the twins to be really different in size. It’s called parasitic twinning.
I have very young twin grandsons. They are mirror identical twins. They both have a mole on the earlobe. When they face each other the moles don’t face each other. My DIL says their hair has a cow lick. The hair twirls in opposite curls.
.
I understand they will both be left handers. Then I got confused and a headache so I told them to shut-up.
Let me throw out an opposite situation. I know a pair of twins that look so similar I needed to identify a mole or two on their face to help differentiate, and the best I could do is find one that they both have but it’s slightly more to the side on one of them versus the other. The thing is, they’re NOT identical twins, they’re fraternal. So how does something like that fit into this conversation? My brain hurts.
Heck, I once met a pair of sisters who were two years apart, and I still couldn’t tell the difference even when they were standing side by side.
The Olsen twins are not identical, yet spent years playing the same character on Full House.
Is there any sense of how early monozygotic twins begin to diverge?
Egg cytoplasm is not homogenous, with various actors including mRNA etc differentially spread throughout it. ISTM that from first split those may be unevenly divided with epigenetic differences in expression possible from word go.
Is that a reasonable understanding?
Yep. I tutored two twin boys. The "donor " was 4 inches shorter than his twin.