The january 2012 issue of National Geographic says that twins “share virtually the same genetic code.” I would have thought they shared an identical genetic code. Why “virtually”?
Mutations may have occurred in some cells of either twin’s embryo after separation – depending on how early that mutation happened, it could propagate to a significant percentage of one twin’s cells and not the other’s. I suspect it’s mainly weasel words to cover cases like this.
Spontaneous mutations, DNA replication errors and mutagens/carcinogens/cancers can alter DNA throughout a person’s life.
Offhand, I’m not sure that this is a significant proportion of DNA, and the way that most forensic testing is done to determine identity/relationships between people would probably not find most of these differences. Some differences really have no impact on the cell/animal anyways since the codingsystem is pretty robust- if a DNA sequence that should be UUA gets changed to UUG, the cell will still make lysine and there won’t be any biological indicator that something was altered.
I used to teach high school. I taught twin sisters who said they were identical, but didn’t quite look it. I wonder how much an X chromosome affects appearance- by the time a girl’s born, one X in each cell is shut down into a Barr body (what they look for in Olympic sex tests). Twin girls could definitely be running on different mixes of X chromosomes.
Identical twins also (we think) are more “identical” as infants than they are as senior citizens. That’s because of something called “epigenetic drift”. Basically, chemical markers (“epigenes”) switch genes on or off based on things like smoking, body weight, activity level and stress (the familiar “lifestyle choices”), as well as things like amount of nurturing done and style of parenting when you’re young. Since even identical twins don’t have identical experiences, they have different epigenetic markers activated. The theory is that this is why one identical twin might develop diabetes or cancer while the other doesn’t, or they may develop it years apart.
Somewhat related but really weird.
As was said–mutations. The DNA of the cells in your left hand is only* virtually *identical to that in your right hand, for the same reason.
IIRC from when my wife was pregnant with twins, identical twins share around 95% of the same genetic code. There are also environmental factors such as placement in the uterus, amount of nutrients that flow through the umbilical chord, which twin is born first, how much of a time lag for twin B to be born, and then after birth differences in the environment.
My twins are likely not identical but we haven’t done the genetic test for a definitive answer. My twin B is autistic, and likely that way because of hypoxic damage at birth.
I’d have to see a cite for that to believe it.
Those are not genetic.
That’s why “environmental factors” was used.
According to Wiki, I mis-remembered. Here is the wiki quote: Twins are a valuable source for observation due to their genotypes and family environments tending to be similar. More specifically, monozygotic (MZ) or “identical” twins, share nearly 100% of their genetic polymorphisms, which means that most variation in pairs’ traits (measured height, susceptibility to boredom, intelligence, depression, etc.) is due to their unique experiences. Dizygotic (DZ) or “fraternal” twins share only about 50% of their polymorphisms. DZ twins are helpful to study because they tend to share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) by virtue of being born in the same time and place.
They probably differ genetically if you include mitochondria; those are descended from a population that comes along with the egg, and can vary genetically even from one body part to another inside one person much less between two people.
Some previous threads on this issue:
Do identical twins have identical DNA?
Do (identical) twins have identical DNA?
A question about identical twins.
Be alert for the postings of one KarlGauss in the above threads. He claims to know what he’s talking about.
Thanks for the great, informative responses. I am enlightened!
Yes, but that’s a difference in phenotype, not genotype.
If we think of genes as a book, the parts which are being read change with time; for each twin, different parts will be getting read at a given time and the difference will be bigger with age. But the book isn’t changed by which parts are being read and which aren’t.
The probably don’t. Although possible, it would be an extremely rare event for twins (identical or otherwise) to have different mtDNA from the mother. Mutations in mtDNA only happen on the scale of thousands of years.
I saw a PBS special on this and what others are saying is correct.
How they explained it was that babies share almost all the same DNA. During people’s lives, gene get turned on and some get turned off.
This is how your body knows how to go through changes especially at puberty. Some genes seem to be turned on by hormonal changes, other simply seemed to be on a timer.
Environment effects whether or not those genes get turned on at the correct time. Like if one twin, eats poorly and the other eats well.
The program showed identical twins at different ages and indeed the older identical twins shared less and less DNA. That said, it still was a very small differences.
A standard test for paternity for instance, would not be able to determine which twin fathered a baby. You’d need more complex tests to determine that.
No. As has been said, epigenetics is NOT genetics. It’s a misconception I see more and more online lately. The DNA does not change over time. What changes is how it’s read. The code itself remains identical, barring mutations and the like.
This is the key issue - mitochrondria are rings of genetic material free-floating in the cell, rather than part of the nucleus of chromosomes. since it comes from the egg, all mitochrondrial material is from the mother. When the cells divide, the nucleus basically duplicates and splits - but that’s not what happens in the rest of the cell contents; part of the content of the cell goes one side, and part the other. It is possible that the cell split that results in the two individuals means that each could end up with a copy some of the mother’s mitochrondria rings that the other did not get.
I have seen some articles on scientific studies that use mitochrondrial DNA to establish maternal lines. It’s one way to tell identical twins apart. But it’s certanly not 5% IIRC.
Also, there’s a random probability of copy errors happening whenever a chromosome duplicates. Portions of the DNA can migrate between the pairs of genes, etc. It’s not usually a perfect copy.
True, which is why I began with “also”. Sorry, lazy shortcut on my part. That “also” was supposed to take the place of, “Well, this question’s been answered quite well, and here’s this other neat thing that makes identical twins not as identical to each other as elderly people as they were as infants.” 
I am currently reading Inside of a Dog, a book about what dogs perceive and communicate and what their world is like, and the author mentions that dogs have been able to distinguish between identical twins by scent in controlled studies (where one assumes that factors like clothing, perfume choice and bathing have been controlled for).