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Now, when it became possible to read DNA sequences directly, it was natural to wonder, how this diversity would be reflected at the level of DNA spelling, on the DNA sequence itself?

And in fact, when it was possible to "read" DNA, one found such things as single-based changes as a C here on this chromosome and a T here on that chromosome, indicating that there were some spelling differences. In working with applications of DNA to medicine, these spelling differences are what we use to trace the inheritance of either the green or the purple chromosome in a family and see if it correlates with a disease.

How Many "Spelling Differences" Exist in Our DNA?

But in fact, I'll talk to you about a different use of all those spelling differences. How much spelling difference is there? Well, there is almost complete identity between any two human beings. Look at the neighbor to your left and to your right. You're 99.9% identical. That should make you feel very common, part of a common species. But of course, in a genome of three billion letters, even a tenth of a percent difference translates into three million separate spelling differences. And so I invite you again to look to the left and look to the right and notice how unique you are. There is no one in this audience who has the same DNA sequence as anyone else.

And indeed, your DNA sequence is unique amongst all DNA sequences of any human that has ever lived and will live for quite some time to come. Unless you have an identical twin, in which case you do have someone who has the same DNA sequence. But apart from that, your DNA sequence is yours and yours alone. Should you choose to leave your DNA sequence behind here in some form in some biological tissue, in principle, I ought to be able to look at it and by its uniqueness know whose it is.

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