Looking only at genetic code, is it possible that there have ever been two genetically identical people who are not twins (or clones)? How many medically distinct human beings can their actually be?
I’m assuming that we’re only talking about biologically modern human beings. So the genes that make us human cannot be eligible for variation.
If we don’t include environmental factors and non-DNA genetic material, what is the actual number of genes that can vary from one person to another? Do we even understand the human genome well enough to make this kind of calculation?
I’m assuming from combinatorial math that it’s more humans than can ever exist through the course of the entire universe. But what is the actual number? If those genes are varied at random, how many people will it take before they say a 50% chance that two of them are identical? For example, it only takes 23 people to have a 50% chance that at least two of them have the same birthday.
Edit: I found an interesting article about the complications with trying to calculate this number. The number seems to be on the order of 10^(tens or hundreds of thousands)
A conservative estimate for the number of ‘people combinations’ is about 10^4,515,450.
There are 10^19 grains of sand on earth.
There are 10^50 atoms on earth.
There are 10^68 possible orderings for a deck of cards.
There are 10^80 atoms in the observable universe.
There are 10^120 possible chess games.
Imagine one new chess game is played every second. It would take until the heat death of the universe to finish all possible games (more than 10^100 years). That’s just chess. Human genome is magnitudes more advanced.
What is that number based on and does it exclude mutations that would change the species or be biologically meaningless?
There are about 15 million single-nucleotide polymorphisms (i.e. ‘spots in the genome that can change’) that commonly change in the human genome. For each of those spots, there are 2 possible alleles. So 2^15,000,000 ~= 10^4,515,450.
And yes, this number is excluding “batshit” mutations that don’t make sense. This is a conservative estimate because it doesn’t include rare SNPs or structural variants (i.e. when lots of nucleotides get removed, inserted, or flipped).
This is also a decent answer, using a different approximation. If someone asks again I’ll just say “a few orders of magnitude of orders of magnitude”, since it covers both. The discrepancy comes from me implicitly assuming more correlation between variations, of which there is some IRL.
Keep in mind some of them do nothing. If you just go by appearance, unrelated dead ringers already happen!