I’ve heard that identical twins, even though they have the same DNA, will have different fingerprints. Are there any other physiological differences that occur ( or could possibly occur)? Not talking about a scar or something acquired in life, but an actual inherent difference.
For that matter, what about cognitive differences? Could one twin be an introvert and the other an extrovert? One schizophrenic while the other is normal? One night owl and one early bird?
DNA is not a roadmap for your entire life. Extensive development happens in the womb, and in all stages of life.
I’ve known identical twins that are not the same height, that have very different aptitudes and strengths, both physical and mental.
I read the op and then looked it up, and Telemark already posted a link to the very same article I was going to. So here’s a link to a different cnn article from February about semi-identical twins that I found 
If one twin is homosexual, the other has a 50 percent chance (roughly) of also being homosexual, so it’s not that uncommon for one twin to be straight and the other gay.
The OP mentions introversion and extroversion, and this also has a strong genetic component but can also differ, so yes, one twin can be an introvert and the other an extrovert.
According to The Google (I had no idea so I had to look this one up), in identical twins, schizophrenia affects both twins roughly 40 to 60 percent of the time. So yes, easily possible for one twin to be schizophrenic and the other not.
In about 20 percent of identical twins, one twin is right handed and the other is either left handed or ambidextrous. Left-handedness in general is also significantly more common in twins than in the general population. Not my area of expertise but the last I heard no one really knew why.
I think that they just don’t come out at the same time. I know three sets of twins both of whom are gay, and none of them came out together. One pair came out at 16 & 22, one came out at, IIRC, 20 & 26, and the other had a huge lag-- one came out in her mid-20s; the other, an ex-husband and two kids later, came out at 41.
There were stretches of time when you could have interviewed any of those pairs, and they would have reported being a gay/straight pair.
This article from The Atlantic Monthly talks about some studies that have been done of identical twins who were raised apart. (Some were adopted separately and didn’t even know for a long time that they had a twin.)
I’ve known two sets of identical twins, one set being my cousins and in both cases, the easiest way to differentiate them was from their personalities. One being more extroverted than the other. Whether this is nature or nurture is probably open to debate. The ‘good twin’ vs the ‘bad twin’ being a common movie theme.
There are twins who act and speak alike, but with observation, it’s eventually clear that one is more dominant and one more passive.
Physiologically, the fact that their fingerprints are identical, points to there must also be other physical differences that aren’t necessarily immediately distinguishable. My sisters would point the the physical differences between my female cousins, but I could never pick them out when they were side by side.
I knew a pair of identical twins in high school that had dramatically different personalities. One was very soft-spoken and more introverted, the other very gregarious. Doesn’t say anything about a generalization, but identical DNA doesn’t explain everything.
Oh, yes to this. I saw a pair like this on an episode of, IIRC, Nova. Additionally, the twin brother of David Reimer was schizophrenic, and David himself, for all he had endured, was not.
There are lots of diseases that can affect a person that are not genetic in origin: I know an identical twin who has Addison’s disease. She is thinner than her sister now, and her complexion is darker, which is a symptom of Addison’s. I’ve seen pictures of them when they were younger, and they really were totally identical.
I also know a couple who has boys who are identical, except they are mirror images of one another-- one has a cowlick on the right side, the other on the left, and so forth.
I knew a pair of identical twins. One majored in math (I supervised a summer research project), the other in physics. They both won Rhodes scholarships and went off to Cambridge. They got degrees there. The physicist got a PhD, became a professor and is now a well-respected physicist at a major university. The other simply lost interest; wrote for an alternative newspaper, became a translator, but his life never really gelled. I have no idea what he is doing now. But they were clearly very different in many ways. They looked identical.
One of the three reunited identical triples from “Three Identical Strangers” had depression and eventually committed suicide. The surviving two don’t appear to be affected by depression.
One pair of my relatives are identical twins. One is gay, the other straight. (at 50, the straight one has several children) Both are very smart professionals, happily married. One is thinner than the other which makes them easy to tell apart now - unlike when they were young and I had to remember which one was wearing what when I visited. One was into figure skating and one into rifle shooting as kids. But, they both had the same sardonic sense of humor like their dad.
Another difference is that the repertoire of antibodies possessed by identical twins is quite distinct for each. This comes about by the somatic (hyper) mutations of the cells of the immune system. Without this mechanism, all of us would have a fixed spectrum of antibodies and might well encounter a “foreign protein” for which we had no antibodies and were not able to generate the new antibodies either. In any case, the antibody producing cells of identical twins are definitely not identical. If nothing else, this sheds light on why only one of two twins (or triplets) may get a particular autoimmune disease
Identical twins do not have precisely the same DNA. Once the egg splits, each subsequent egg is subject to the same incidents of genetic mutation as any other cell. Mostly these differences are minuscule and don’t affect development.
But in extremely rare circumstances it’s possible for identical twins to actually be different sexes. This can occur when a male (XY) egg splits but the Y chromosome becomes lost or damaged in one twin. That twin only has an X chromosome and develops biologically female but often with a number of developmental abnormalities.
That’s called “Turner syndrome,” and women who have it are very often Deaf, so I actually know like, 5 people with this. All but one are intellectually normal, and the one who isn’t has learning disabilities, but comes across as “normal,” just to chat with. They all have the same fairly minor-appearing skeletal abnormalities (I don’t know how serious these might be for long-term health), mostly around the head and neck, and they are all very short. The tallest one is maybe 5’2. I’ve never inquired about any of their reproductive statuses, but I know with absolute certainty that at least one has periods, because she asked for Tylenol once for cramps. Don’t know one way or the other about the others.
None is a twin. Still, thought I’d note that.
They all look like normal women, other than the sort of odd shape to their necks, but it doesn’t look in any way masculine.
It’s probably possible to have this and not be Deaf, but my experience skews toward people who are Deaf.
23andME can give quite different results.
Interesting article about twins: https://www.nationalgeographic.com/magazine/2012/01/identical-twins-science-dna-portraits/
This has been my experience.
It’s remarkable how easy it is to tell them apart once you get to know them.
It also makes you realize the how laughable the old Hollywood trope of twins switching out on their significant others is. That would never happen in a million years.
Question on this point. Each individual’s fingerprints are formed by random events during gestation, so even twins won’t have the same fingerprints. But are the fingerprints similar? There are three common types of fingerprint, loop, whorl and arch, and several uncommon or rare types. Loops may be radial or ulnar. Will twins have the same type of print on the same finger?
The correlation on a trait within identical twins is an estimate of the maximum heritability of that trait. So for some of the examples given above, if 60% of the time one twin has schizophrenia the other one also does, then that would be a correlation of 0.60. That suggests that the maximum heritability for schizophrenia is 0.60. The actually heritability could be lower, though.
This is fairly intuitive when thinking about identical twins. If two individuals have the same genetic makeup (excluding a few de novo mutations), yet they differ on a trait, then something more than just genetics is influencing that trait.