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Genetics Question

Number Six

JREF Kid
Number Six
Joined
Sep 5, 2001
Messages
5,016
Maybe the premises in the question are wrong and if so please correct me.

A man and a woman have two kids. (On average) each kid has 50% of genes in common with the other kid.

Those to kids have kids, those being first cousins to each other, and they have 25% of genes in common. Etc. The next generation is 12.5%, then 6.5%, etc.

But if you go more and more you get near zero. Does that make sense? Because if you pick to unrelated humans at random they don't have 0% of genes in common do they? I guess my main question is, if you pick two unrelated humans at random, what percentage of genes do they share?
 
The siblings would share 50% of the small subset of genes which vary among humans. The rest they would share, as they share with all humans. I have no idea what the percentage of total genes that is, but I believe less than 2 or 3 percent of our total genetic code determines the differences between one person and another.
 
Also note that there aren't an infinite number of alleles of any given gene. So even if I have a different allele or two from my sister, I still likely share that allele with millions of other people. There are, of course, rare alleles. I think 20 alleles of a critical gene would be a lot.

~~ Paul
 
What defines an individual genome is the combination of allelles.
Genes have an environment - which mostly consists of other genes.
Genes which don't "work" as part of a large team of other genes will be selected against.

It's perfectly possible two totally unrelated people might have identical genomes. Just incredibly improbable... like all the air going into three corners of a room spontaneously.
What's certain is that they will have very, very similar genomes. If the difference between humans and chimps is 3% of the genome (or 5, whatever), the difference between any two humans must be a lot smaller.
 
Number Six said:
Maybe the premises in the question are wrong and if so please correct me.

A man and a woman have two kids. (On average) each kid has 50% of genes in common with the other kid.
Click to expand...

That's a slight oversimplification, because most genes don't vary among humans. Specifically, if Mommy and Daddy have the same gene, then the kid will have that gene in common with both of them, even though they only got (wolog) Mommy's copy of the gene.


Those to kids have kids, those being first cousins to each other, and they have 25% of genes in common. Etc. The next generation is 12.5%, then 6.5%, etc.
Click to expand...

Again, this is an oversimplification but largely correct.

But if you go more and more you get near zero. Does that make sense?
Click to expand...

Yes. What that means is that near zero of the genes in the individual came from some specific person. But that doesn't mean that the genes that they got from somewhere else aren't identical to the genes that person had.

I guess my main question is, if you pick two unrelated humans at random, what percentage of genes do they share?
Click to expand...

Of the total genome? Something like 99% of the genome is shared among nearly ALL humans. Of the remainder, there are usually a few common variants (think about how many people you know with the gene for left-handedness or the ability to roll the tongue).
 
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