The
number is essentially infinite. Using an estimate of mutation frequency
of around 2 x 10^-8 per base pair per replication event, we get 60
novel mutations in every living human being. There are 7 billion humans,
so we know that some 420 billion different variants are possible. And
that is just the number of new changes that arise in a single
generation. The number passed down and recombined from previous
generations is much larger.
A more interesting (if harder-to-define) question might be how many different traits
can be embodied in human DNA. After all, most human DNA has little or
no function, so mutations in these low-information sequences are
inconsequential.
The universe of genes that are actually expressed in humans - called the exome
- is comprised of about 30 million bases of DNA. Sequence changes in
this DNA are more likely to lead to an actual change in the functioning
of genes and their expressed products.
One effort to quantify exome variability
estimates that each human carries about 13,500 variants, and that
perhaps 300 of these affect gene function. Again, you can multiply this
number out by the number of persons now living (7 billion), or the
number that have ever lived (108 billion)
to get another very large number. But that is the lower limit. The
number of possible combinations is 1 x 2 x 3 … x 300 = 3 x 10^614, a
number that is so large as to be meaningless. At least to my desktop
calculator, which returns this response when I try to plug it in:
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