I’ve heard the chances of one person sharing the same DNA with another person (who isn’t an identical twin) is 3 trillion to 1.
So does that mean there are 3 trillion possible DNA combinations that could theoretically become human beings? And if so, with 6 billion people out there isn’t the chances of some duplicate of me being out there only 1 in 500? Is my math wrong here?
So what are the chances that, out there somewhere, there are any two people in the entire world who share the same DNA who aren’t identical twins?
There are about 25,000 genes. Some come in dominant/recessive forms. To get one trillion possibilities all one needs is 40 different dominant -recessive genes , of which I am sure there are many many more. The actual number of possibilities in the human genome must be many orders of magnitude more.
However, my WAG is that typical DNA testing may only be able to distinguish about a trillion possibilities. That is a limitation of the testing and markers used, not the actual numbers of different genomes
Let me jump in here for a second and haphazardly wield my complete ignorance of human genetics:
Isn’t it that case that a great many of those 25,000 genes would be the same for most people. Like the “two eyes” gene, or the one for “five fingers”. Okay, I know it’s not as simple as that, but a lot of what makes people people would need to be mostly the same. Right?
If so, that limits the number of variations to well below 2^25000
I’d say there’s a better than average chance if two pairs of identical twins marry and have children. (So the people concerned would be cousins). Still near zero, of course.
You could get exactly the same effect much more simply by just having one set of parents who have two kids. In other words, just ordinary siblings. But even there, each kid would choose one of two different chromosones, for 46 chromosones, for a chance of a match of only 1 in 2[sup]46[/sup], or 1 in 70 trillion. And this is even assuming that the chromosones are passed down whole, without any crossovers between when the gametes for those children were produced (I’m not sure just how common chromosone crossovers are).
The human genome is on the order of 3 billion base pairs of DNA. It contains somewhere between 25,000 and 35,000 genes. The overwhelming majority of the remainig DNA (~97% or 2.91 million bp) is called junk DNA, because we haven’t figured out yet what it’s purpose is. It does have a still undetermined purpose, but that purpose is probably largely independant of the exact sequence of letters in the genetic “code” here, and mutations which replace one of these letters for another in the region of junk DNA would have no effect on the outward characteristics of a person (phenotype). You can sometimes even add or remove entire chunks of DNA, often thousands of letters long, to these regions without hurting anything.
Every cell in your body has a different sequence of DNA from each and every other cell in your body. This is because the rate of mutation at any given site in your DNA is on the order of 1x10^-5, and you have 1x10^9 possible sites in each copy of your DNA. That means that every time one of your cells divides, it makes a copy of itself that is not quite the same as the parent cell. Most of these changes are in the junk regions, simply because junk DNA comprises such a large percentage of your total DNA. But it’s quite meaningless to say that you might have DNA which is identical to another person’s - you’re not even identical to yourself.
There are probably more possible ways to arrange those 3 billion letters, and still get a human at the end, than there are atoms in the universe.
Um, yea…that’s what I was thinking! So, basically…everything is everything. I like that! Good thing I didn’t get my slide rule out.
Welcome to the boards Twhitt…awesome anwer!
Could you give some cites here please? This argues against the training I received. Training that’s very old ( >20 years) and I’d like to update myself.
As I WAG’ed it appears that common DNA tests give frequencies matches often estimated in the range 1 in 1-10 trillion
e.g. “The frequency of the DNA profile obtained from the stain on White House intern Monica Lewinsky’s dress was reported to be 1 in 7.9 trillion.”
from http://www.fathom.com/course/21701758/session3.html
I saw this murder trial on TV once – took months – where the defendent’s DNA
was at the crime scene and both vicitms DNA were inside his Ford Bronco. Took the jury only a few minutes to acquit.
So, basically…everything is everything. I like that! Good thing I didn’t get my slide rule out. 