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I read an article which says that identical twins have 100% similarity between their DNA, but they have different fingerprints. Does that mean the DNA is different in the fingers? What body features can be used to exactly identify the identical twins?

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DNA is not the only thing in biology that determines a phenotype (observable trait, e.g. fingerprint). Far from it.

Traits can be affected by the environment (a very good example, temperature-dependent sex determination), stochastic (random) phenomena - these are know to occur in transcription ("reading DNA") or the decay of mRNA (messenger molecules which instruct protein building) - and by epigenetic phenomena. The degree to which DNA is actively read, or silenced, does not wholly depend on the DNA. In fact, this depends on the exact state of the cell or nucleus. Roughly speaking, cells in your brain (e.g. neurons) and the cells that line your lungs have identical DNA, but they are shaped, look, behave and react differently to the outside world. This is due to epigenetics, think of it as the life history of the cell.

Here's an analogy: reading the exactly same book can have different effects on people. The book is the DNA, its information unchanging, but the person and their exact status are the complicated world of the environment and the cell that is using its DNA for some purposes. PS. Terrible analogy but it may help.

By no means is my list exhaustive. Hope you can see how that identical twins with identical DNA can have a different shade of skin color (tanning!), different fingerprints (subtle and complex differences during digit development), or a different sense of fashion (psychology, cognition, what have you).

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  • $\begingroup$ You hit on the most important points. However, it is also worth pointing out that identical twins don't actually have totally identical DNA. They split from the same embryo and there they have identical DNA, but during development mutations occur which makes identical twins not truly 100% identical. Very close to 100%, but not exactly. $\endgroup$ – Eff Jan 15 at 9:07
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    $\begingroup$ Very good point. I've had the discussion with peers whether or not to include this point in discussions with a layman on these kinds of questions. Bit of a thought experiment: why aren't homozygotic twins perfectly identical? My gut response is not to bring DNA mutations into the answer. I'm sure you'll agree the contribution of somatic mutations to fingerprints is insignificant. Incidence of cancer in twins is much likelier to be caused by genetic history rather than a lifetime of mutation accumulation. But you're absolutely right, it could influence many (inconspicuous) phenotypes! $\endgroup$ – S Pr Jan 15 at 12:17

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