Thanks for this cite, KG; I enjoyed reading it. Seems like it was written by a grad student interested in combining language as flowery as possible while on a soapbox, but it makes some great points about the non-identicality of twins, particularly from the perspective of (non)-histocompatibility.
The OP wants to know about DNA. The main thing that’s debunked in the article you cite is that identical (nuclear, at least) DNA does not mean identical phenotypic expression, particularly for biplacental monozygotic twins. Another remarkable point which I guess I knew from news reports but hadn’t really considered is how different even conjoined twins can be, “frequently less similar than twin pairs known to be dizygotic.” … “Dubos has glorified each individual as unprecedented, unparalleled and unrepeatable, an exaltation not denied to the members of MZ twin pairs. They form no exception to the generalization that the only invariable law of nature is variation.”
I should note the author seems equally skeptical that identical twin differences are somehow therefore purely environmental: "To explain away many a discordance between the supposedly genetically identical twins, recourse has been taken to environmental differences, cytoplasmic differences, equations of heritability versus variability, non-penetrance of the heritable mutant gene, gonadal mutation, and premutation. The cul-de-sac nature of the above explanations compels flagrant violations of Occam’s razor. To wit, the discordance exhibited by 58 per cent of MZ twins vis-a-vis congenital club-foot necessitates presupposing “special prenatal circumstances” involving “variations in the outer or the inner environment of the embryo” complicated by “a special genotype or genotypes necessary for the formation of club-foot”
The takeaway instead is that although we are our genes above all else, the way our genes are replicated is never identical nor is the phenotypic expression ever identical.
Understood. I did not mean for that to sound critical in any way.
I have exactly the same problem when researching topics for which I don’t have a properly grounded starting point.
Except that article seems to be criticizing twin studies for the assumption that identical twins are completely identical, when the whole point of twin studies is the assumption that they’re not. The whole reason for doing twin studies is to determine the extent to which something is genetic, and to the extent that it’s non-genetic, you would expect identical twins to differ.
Don’t forget about retroviruses. They insert their DNA into cells, but obviously not all somatic cells. Assuming some of those cells live and reproduce, it’s possible that not only are you different from you identical twin, but you’re not even identical to yourself! If you got a virus that targets, say, the blood, then those cells would carry a “mutation” that’s different than your skin, etc. Mutation needn’t be from radiation and carcinogens.
Because fingerprints develop as a result of random, environmental (outside) influences. Even being in the same womb, miniscule variations cause differences.
Similarly, a drop of water running down the same window pane will have a slightly different path, even with as-identical-as-possible parameters.