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)
Probably at least five
My gf works in human remains identification. On the bottom of the DNA identification reports, they have a certainty estimate:
1 in 19,000,000,000,000 (trillion) chance of being incorrect.
So if that’s any estimate of how many variations there can be, I’d say chances of repeats are pretty low lol
Note: I didn’t believe her when she quoted 1 in 13 trillion, so she double checked the next day. They seem… confident
Correction: I talked to her again, and was completely wrong, because my memory refused to accept the real number:
1 in 200,000,000,000,000 (trillion) chance of being wrong.
And to the commenter that commented about common identifiers, you are correct. She’s fairly confident they use the SNP method.
I hope people reading this have SOME measure in their mind of how immeasurable even ONE trillion really is.
To count to a million it takes about 11.5 days, a billion roughly 31.7 years, and a trillion approximately 31,700 years, assuming one number per second without breaks. Then do it 18 more times.
Literally mind boggling. I made a correction and was wrong by a factor of 10 because I couldn’t accept it was that high. But she showed me a screenshot of the report…
Does have me wondering what the ratio looks like when applied to isolated (maybe mildly inbred) communities.
As in, there’s a lot of genetic variety for humans as a whole, but maybe less so for communities that, uhh, don’t get out much.
That’s pretty interesting. I’ve done some genotyping before, and I’m guessing that they probably check a handful of distinctive genes, not the entire genome.
For example, if they checked 45 binary markers (e.g. positive or negative) that would get you into that ballpark (assuming that they are evenly distributed and not correlated). Of course, there are also markers that are not binary but can have many different variations other than positive and negative.
That’s actually really cool to know. She was fairly confident they use the SNP method (it’s a third party provider), and described it the way you did. Sounds like full genome is possible but she didn’t think it likely, your explanation would support that.
Also, I made a correction to the numbers…I was very wrong lol
All of them
Human genetic code has maybe 0.2% variation in base pairs, the average individual gene has maybe 13 thousand base pairs, and the whole code has about 3.1 billion base pairs.
This is crappy, quick and dirty math, but this might be one of those high-complexity situations (like weather modeling) where minimal detail can be closer to the truth than an intermediate amount anyway. So, we’ll model it as a random selection of 0.2% differentiated genes, meaning ~240,000 take ~500, which comes out to a number with 1556 digits. The amount needed to get a 50/50 chance of a repeat will be lower, with 777, almost 778 digits.
If you allow weird, implausible and mostly stillborn combinations it’s closer to 2~240,000 . If you include mutations, it gets much much higher to the point of basically being undefined. It’s more about how many changes is still human, and after that how big a genome a cell can handle.
Damn dude 😳
My go-to insult from now on is going to be:
“6.213165126500310316378124236349955462389737376162847388361… x 101555 possibile human specimens, and somehow I’m unfortunate enough to have to come across you.”
Awesome answer! Thanks.
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!
Five:
Baby, Sporty, Posh, Scary and Ginger.
This is a plot point in the movie Jupiter Ascending. Mila Kunis’s character is heir to a planet or something like that because she’s genetically identical to an unrelated person from space
This is funny cause I just had a conversation with an Uber rider kinda along these lines.
I moved from a very popular area to an area not so dense that my mom’s side of the family is from. And now suddenly, against what the numbers should say, I got not one but TWO fucking doppelgangers! I said maybe my genes are overrepresented around here.
Humans have 3.2 billion base pairs. There likely has not and will never be two identically genetic humans, even identical twins are not perfectly identical due to mutations.
Now, obviously a lot of the genetic code needs to be similar, certain parts of our genetic code needs to be similar, so it isn’t literally 3.2*10^9 factorial, but even a small fraction of that number is essentially infinite. I highly doubt there has ever been two identical humans or ever will be.
3.2 billion factorial is essentially arbitrarily large. It isn’t even really useful to think of it as a number, as we couldn’t even calculate what the number is. Visualize the largest quantity you can conceptualize, then square it until you get bored.
I’m no scientician, but wouldn’t mutation mean that the possible number of gene combinations is constantly growing? And maybe even making previously non-viable permutations viable, which seems like it would really fuck with the math?
There would still be a limit as long as the species was constrained to modern homo sapien. Mutating yourself out of the species would mean you’re no longer in the set of things.
I’m also no scientitian. I believe any mutations would have to be within the existing genome or else it would no longer be a biologically modern human.
Let’s add the additional stipulation that we are not adding any meaningless genes. I know that there are a lot of them, but I’m just curious about what the math would look like and if we even have the knowledge about the genome to know which gene variations are meaningful.
I’m thinking of the doppelganger thought that experiment. Am I really the only person like me that has ever lived or what level or live? Or is it statistically possible that another genetically identical version of me could exist within the timeline of the universe (as I mentioned, with a 50% probability or better)?
To add on top of the other answers: On top of all potential genetic variation, you also have environmental and experiential variables that further differentiate people.
At least 12.
If you truly shuffle a deck of cards, you are likely to come up with a permutation that has never existed before.
The human genome is infinitely more complex than a deck of cards.
The number of possible combinations of genes is so large that the chance of a random duplication is vanishingly small. Even twins aren’t exact copies because they’ll have mutations unique to themselves.
The question isn’t whether it’s large. The question is how large is it? Right now the human population doubles every 40 years and that rate seems to be increasing.
If humans somehow expanded in population and efficiently consumed all of the energy in the universe, then that means maybe somewhere on the order of 10^65 possible people (according to some very bad napkin math just for the sake of argument).
Are there that many unique combinations of human DNA, ignoring mutations which would change the species or have no impact on biology?
