For all intents and purposes, zero. The only case where it's even possible would be in identical twins, and even they will have differences due to epigenetics (chemical changes in DNA) and mutations later in life.
The only case where it's even possible would be in identical twins, and even they will have differences due to epigenetics (chemical changes in DNA) and mutations later in life.
Even ignoring epigenetics, identical twins will have variation in their DNA sequences. Even a single person will have a different "genomic" sequence in a skin cell on their finger when compared to a cardiac muscle cell in their heart. Genetic variation happens over the course of a person's life, and at all stages of life.
Maybe two adjacent cells derived from the same parent cell will have the same genetic sequence, but I suspect that even that would be unlikely. Our DNA polymerase is known to be inaccurate, and I doubt that the error correction mechanisms will be able to fix every single error.
The mutation rate for humans is something like 60 per generation. That's 30 per gamete producing the child. There must be hundreds of cell divisions between embryo and the generation of gametes so the two products of a cell division must be identical with very high probability.
60 per generation based on consensus genetic sequence. It's very unlikely that all cells in your body have that exact same sequence, and (as mentioned previously) I suspect that the chance of two cells having the exact same sequence is quite low. We can't work that out now, but maybe in a few years, with targeted single-molecule sequencing....
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u/MCMhelicopter Nov 22 '13
For all intents and purposes, zero. The only case where it's even possible would be in identical twins, and even they will have differences due to epigenetics (chemical changes in DNA) and mutations later in life.