[ExI] Genetic recombination

[Stuart LaForge]

On 2026-10-26 03:17, Ben Zaiboc via extropy-chat wrote:
> On Saturday, 24 October 2026 at 18:16, spike at rainier66.com 
> <spike at rainier66.com> wrote:
> 
>> -----Original Message-----
>> From: extropy-chat <extropy-chat-bounces at lists.extropy.org> On Behalf 
>> Of Ben Zaiboc via extropy-chat
>> ...
>> 
>> >>... Do we have those among us who can clarify?  It might be clearer if I had said something like “this series of base pairs was expressed and its origin was from your father, but at this point, it switches over to the next series of base pairs, inherited from your mother.”  I don’t know the best way to express that idea.
>> 
>> 
>> >...This might help:
>> 
>> www.genome.gov/genetics-glossary/Crossing-Over
>> 
>> https://biologynotesonline.com/dna-recombination/
>> 
>> >...The purpose of crossing-over is to get new combinations of alleles, generating diversity. If you had two identical eggs, fertilized by two identical sperm, the result would be astronomically unlikely to produce the same combinations in each...

The purpose of crossing over in meiosis is to allow homologous 
recombination to create novel chromosomes by allowing existing 
chromosomes to trade stretches of DNA to generate genetic diversity. 
Because of crossover, your paternal chromosomes are not identical to 
your fathers, but each instead contains parts of your mother's 
homologous chromosomes. So sexual reproduction is not just mixing and 
matching whole chromosomes, but small pieces of each chromosome also.


>> --
>> Ben
>> 
>> OK cool thx Ben, that is how I understood it.  It has been a loooong 
>> time since I studied it.
>> 
>> In AncestryDNA the DNA kits detect genetic units down to the 
>> centiMorgan level.  Imagine we make a simplifying assumption (not a 
>> realistic one but a vastly simplifying assumption) that centiMorgans 
>> are not divisible (acting as little atoms of DNA kinda) and then treat 
>> the chances of crossover as 50% with each centimorgan, then the 
>> chances of two recombinations of N alleles being identical is about 
>> the same as the chances of two guys flipping a coin N times and 
>> getting the same sequence.  If N is 3, the chances of the two fellers 
>> getting the same sequence is 2^-3 or one in 8.  Ben does that sound 
>> right?

That's not quite how it works. Centimorgans are defined as a 
probabilistic approximation of distance between two genes on a 
chromosome. So each centimorgan is is defined as a distance along a 
chromosome that would result in a 1% chance of crossover. So two genes 
that are 15 centimorgans apart on a chromosome would have a 15% of 
crossing over from one chromosome of the pair pair to the other. The 50% 
you're talking about is the maximum measurable frequency of crossover. 
Basically any two genes that are 50 centimorgans apart on a single 
chromosome are far enough apart to indistinguishable from the genes 
being on separate chromosomes.

Note that cM distance measure is very rough because it is being measured 
by observed phenotypes like red eyes or curly wings. These days actual 
distances can be measured much more precisely through genomic methods 
like DNA sequencing.

Stuart LaForge



> 
> 
> I have no idea what this means. I don't know what Morgans are (even 
> after looking it up), and can't follow your maths.
> 
> 
>> Now again with the (unrealistic) simplifying assumption of treating 
>> centimorgans as atoms of DNA, we can estimate (in reality wildly 
>> underestimate) the probability that our DNA recombined the way it did.
>> 
>> AncestryDNA has a 50 dollar DNA kit which measures about 7000 
>> centiMorgans.  Recall that experiment I described before where a 
>> particular ovum and a particular sperm is recombined a skerjillion 
>> times to see if they ever recombine in exactly the same way twice.  
>> With the unrealistic but simplifying assumption of indivisible 
>> centimorgans, there are about 2^7000 different ways those two DNA 
>> strands can combine.  The hotel guy in Jason's video used the typical 
>> number of sperm in a cheerful evening.  He said 200 million, well OK 
>> that would need to be a very VERY cheerful evening.  In reality it is 
>> probably more like 50 million (but hey, I was a teenager once (I 
>> think.))
>> 
>> In any case, if we argue the odds of that cheerful evening producing 
>> the comely Nora Alvarez is one is 200 million, this is an unimaginably 
>> vast understatement.  As the video comments about the hard game, Nora 
>> is not impossible, only absurd.  The chances of that one sperm from 
>> the (very)^2 cheerful evening isn't one in 200 million, it's more like 
>> one in 10^2300.  Not impossible, absurd.  We are all a product of a 
>> one in 10^2300 event.
>> 
>> This is all a sim.  We are all in a simulation.
>> 
>> Well, OK not we.  You are.  You are the only human.  We are software.  
>> Prove "me" wrong.
> 
> 
> I don't know why you suddenly veered off into the simulation argument, 
> but in any case, sorry, I can't help with all this maths stuff. Way 
> above my head. All I can say is that this sounds to me very like the 
> anthropic principle, and I don't really see what you're getting at or 
> arguing for, or against.
> 
> We may all be a product of a one in 10^2300 event (biologically, and 
> probably much much more unlikely cosmically), but so what? We're still 
> here. If we weren't, we wouldn't be able to talk about it. I'm afraid I 
> just can't see the point of all this.
> ---
> Ben
> 
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