[ExI] Genetic recombination

[Ben Zaiboc]

On 24/10/2026 17:33, spike wrote:
>
> From: extropy-chat <extropy-chat-bounces at lists.extropy.org> On Behalf Of Jason Resch via extropy-chat
> …
>
> >…Could you clarify this? My understanding is that if you already had two generically identical sperm and egg pairs, their combination would yield identical twins. Is there something more involved in deciding which and to what extent different genes get expressed that could vary beyond the genetic information carried by each gamete?

> 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: https://www.genome.gov/genetics-glossary/Crossing-Over
Or this: 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. It has nothing to do with expression, it happens almost immediately after the sperm DNA reaches the egg's nucleus, and is pretty much a random process regarding exactly where the crossovers occur.

Identical twins (monozygotic) happen after this stage, and are a result of a fertilized egg splitting into two at an early enough stage that each copy can continue to develop normally. Their genomes are identical because the crossing-over process happens before the split.
-- 
Ben
"The world will not be destroyed by those who do evil, but by those who watch them without doing anything." - Pablo Casals, via Albert Einstein