Identical twins are not quite identical

[TheVat]

https://apnews.com/article/identical-tw ... 278117e8ef

[charon]

...

Well, forgive me if I'm being too literal, but if two human beings were precisely, exactly, indistinguishably, identical it would be a miracle.

_{And I'm not talking about Jesus!}

[Forest_Dump]

Since I am not often inclined to chase down web sites I would opine based on a line of thought I was engaged with not too long ago...

Probably nothing could be more genetically identical than cells resulting from mitosis of a single fertilized egg. Every subsequent division produces cells that are genetically identical and yet at some point something external causes some cells to become brain cells and others to be toenails. This has to be external or environmental because all the cells have the same genome right from the beginning as the genome is copied exactly during mitosis (ignoring mutations that can occur such as happens with Down's Syndrome). So if environment influences cell differentiation to that extent even within the womb, it only makes sense that environment would influence the development of twins even if monozygotic.

[TheVat]

I found reading the article helpful. I had not realized that several mutations happen so early in the development of those two initially identical copies of the original zygote. These point errors happen during replication, so that process is not as error-proof as previously though. An average of 5, but sometimes as many as 100. While point errors often don't matter or are harmless, apparently that is enough to cause differences in health outcomes, even height, later in life.

[Serpent]

My first thought was: Of course, https://www.cbc.ca/news/technology/dna-ancestry-kits-twins-marketplace-1.4980976
But how do you prove it? They seem to have done a pretty job of answering that one.

My second: Where did they find that many volunteers? Surely not on barely-habitable Iceland!

We normally assume that the standard DNA test wouldn't detect every tiny difference, but apparently, some variations are picked up, even in the superficial ancestry tracing procedures.
In a criminal case where the sample collected from a crime scene is compared to a data-base, the wrong twin might be identified - but that evidence wouldn't stand up if it were known that the suspect is a twin, but the other twin is unavailable for a sample. (I don't suppose this happens very often.) Similarly, they may have some of the same pattern of whorls, so that a partial finger- or hand-print might be misidentified in the 16-point comparison, but a close examination of a full print would show differences. (Could happen that a pair of twins collude in a crime, counting on mistaken identity. )

[Serpent]

Afterthought: the NOVA program I watched a little while might be of interest.
More on it here:
https://www.pbslearningmedia.org/resource/biot09.biotech.concpt.prsnldna/personal-dna-testing/

[charon]

...

Well, forgive me if I'm being too literal, but if two human beings were precisely, exactly, indistinguishably, identical it would be a miracle.

_{And I'm not talking about Jesus!}

Oh, I forgot to mention - it would also be impossible because they'd be two different people.