However, since there has never been a complete mapping of any single human’s genome (much less a set of twins), this assumption has never been proved.
It is possible that, in some cases, the replicating zygote splits apart for a reason. There are, during fetal development, a number of tests performed by the cells on themselves and each other. There are sometimes mutations and, while spontaneous abortions are common, sometimes the replicating cells will slough off cells that they have identified as ‘wrong’.
In theory, it’s possible that both bodies of cells would be viable. This presents the possiblity (though not the likelihood) that two individuals could be identical twins, derived from the same sperm and egg, yet not exactly identical, genetically.
Biology isn’t very good at following strict rules and reproduction is messy.
Also, a lesser-known fact is that non-identical twins may vary in the amount of DNA they share. There are “half-identical” twins (egg splits, is inseminated by two different sperm) who IIRC share 75% of their DNA. And still stranger, “half-fraternal” twins who have different fathers and so share only 25%. Both those types are pretty rare though.
Those numbers are theoretical averages. “Half-identical” twins have identical chromosome sequences from their mother, & somewhere in between 0% and 100% of the same sequences from their father. It’s probably not exactly half.
Of course, all humans share a goodly amount of genetic information anyway.
[digression] By the way, I think a single ovum could be fertilized by sperm from two different men, but it would be terrifically unusual.[/digression]
Actually, identical twins do have some DNA that is not identical (and Paul Cunningham may yet deserve to win his bet).
The DNA in a person’s B-lymphocytes (the type of white blood cell that makes antibodies) undergoes mutations throughout life. These so-called somatic mutations are critical in allowing the lymphocytes to make antibodies against a wide range of foreign targets.
Bottom line is that even if their DNA was identical at conception, the DNA in a pair of identical twins’ B-lymphocytes differs as they age. (As an aside, this may explain why even identical twins are not perfect donors for each other’s transplants and why diseases like type I diabetes have less than 100 percent concordance between identical twins.)
First: in monozygotic twinning, the division of the chromosomes is not always perfect. Thus it is theoretically possible, for example, for one twin to be phenotypically female (XO, Turner Syndrome) and the other to be phenotypically male (XYY, polysomy-Y). These would obviously not be identical, but they would still be monozygotic.
Second: It has been estimated that each person has one or two new mutations (mutations their parents don’t have). Some of these would occur in the egg or sperm, so monozygotic twins would share these new mutations. But presumably some of these mutations would occur after separation.
I think foolsguinea is theorizing a combination of domina’s “half-identical” and “half-fraternal” situation. A single egg splits before fertilization and is fertilized by sperm from 2 different men.
Look, if you want to get really picky about it, any individual human has TONS of very minor variations in their DNA from cell to cell. So you can’t really even say that my DNA is identical to my DNA. I think it best to say that identical twins’ DNA is as similar to each others’ as to their own.
OK, as the author of this piece, I think I need to set a few things straight. I knew when I got this question that the answer wasn’t going to be as simple as it seemed, but as a contributing writer, my primary goal is to ANSWER THE QUESTION and to do so understandably. While I knew that the answer was extremely simplified, “we’ll be here for a semester otherwise.” However, simplified was fine for the scope of the question. The question was certainly incomplete (the question failed to indicate whether the reader was a fraternal or identical twin) but it did also fail to specifically state whether the DNA from identical twins are absolutely identical, base pair to base pair.
I should also point out that, in the many responses above, I noticed that most involve qualifiers in their statements, such as “in theory,” “if,” “is not always,” “it is theoretically possible,” “it has been estimated,” and “presumably.”
As for B-lymphocyte DNA, I would have to agree with what bibliophage has stated, but Smeghead’s reply is equally correct. As a further example then, I pose this question: “Is the DNA from a tumor sufficiently similar to its host as to be considered identical?”
Hey, don’t sweat it, Hawk. The Staff Report is for anyone and everyone, the mass audience, so you’re right to leave out minor details and gloss over some points. The board here, though, is home to us nerds and nitpickers who revel in those same details. If we can worry about the Uncertainty Principle’s effects on a dropped hammer, then we can certainly worry about B-lymphocyte DNA.
In Do identical twins have different fingerprints? Cecil discusses this sort of thing with respect to (duh!) fingerprints. Saith Cecil “Presumably these minor but crucial differences arise from random local events during fetal development, the same kind of thing that makes each snowflake unique.” Your DNA doesn’t contain a down-to-the-cell blueprint of your body; it’s more of a recipe for growing a person. Two identical twins may well both go bald at about the same age, and in pretty much the same way and at the same rate, but if you took “scalp prints” of the positions of all the individual hair follicles on their heads, they wouldn’t be exactly the same.