# Why aren't all my Sperm identical?
**URL:** https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316
**Category:** Factual Questions
**Created:** [January 17, 2007, 1:48pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316 "2007-01-17T13:48:39Z")
**Posts on this page:** 8
**Page:** 2
### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [January 17, 2007, 5:00pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/21 "2007-01-17T17:00:37Z")
[QUOTE=John Mace]
But not as high as a purely random process would predict, right? Genes that are further located apart on the chromosome are more likely to end up on different chromosomes during the cross-over process than are genes that are closer together. Genes that are very close together and tend not to separate from each other are said to be linked.
[/QUOTE]
Correct.
[QUOTE=Nava]
Sorry, that’s the information I was taught. I stand corrected (although the majority of the sources I’ve googled still don’t even talk about hemophilic women).
[/QUOTE]
Most likely because they are very rare compared to male hemophiliacs.
---
### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [January 17, 2007, 5:03pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/22 "2007-01-17T17:03:21Z")
[QUOTE=Colibri]
Most likely because they are very rare compared to male hemophiliacs.
[/QUOTE]
This is not only because the recessive allele is itself rare, but also because a female hemophiliac has to have a father who is himself hemophiliac. Before treatments existed, male hemophiliacs often died in childhood.
---
### Author:  [@John\_Mace](https://boards.straightdope.com/u/John_Mace)
#### Post date: [January 17, 2007, 5:40pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/23 "2007-01-17T17:40:58Z")
[QUOTE=Colibri]
Correct.
[/QUOTE]
To take this further, and I’ve always been a bit hazy on the details…
Is it because the chromosomes are not simply swapping genes (or more correctly alleles) willy-nilly, but the cross over process is literally a physically crossing over of the chromosomes and whole chunks are passed between the two determined by the cross over points? So, is it a physical limitation of the chromosome itself to bend in space and cross over so that having two cross over points with only one gene inbetween would be a very rare occurance? Does the location of the centromere have any significance in this process?
Also: On a given chromosome pair do the number of cross overs follow some statistical distribution (maybe a Poisson distribution) so that we could say that X number of cross overs would occur in Y number of cases? And would that be determined by the length of the chromosome and/or some other parameters?
If that’s too big a subject to deal with in this thread, just let me know. I’m really only asking about what you know off the top of your head. Thanks in advance, as always!
---
### Author:  [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)
#### Post date: [January 17, 2007, 5:59pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/24 "2007-01-17T17:59:38Z")
[QUOTE=hawthorne]
I recall seeing suggestions that there were “helper sperm” that attempt to assist their brethren in the face of competition. Is my memory correct?
[/QUOTE]
If I remember right, a single sperm is unable to penetrate the ovum on its own. There needs to be hundreds or thousands of sperm pecking away at the ovum cell wall in order to “soften it up” enough that some random sperm can force its way through. I think it works because each sperm carries a tiny bit of enzyme or other “solvent” that individually is too little to do much. But when they arrive in the thousands, it adds up enough to prep the ovum for fertizilation. This is why a low sperm count can be bad for fertility. Not enough sperm reach the egg to soften its tough outer wall.
---
### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [January 17, 2007, 6:13pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/25 "2007-01-17T18:13:57Z")
[QUOTE=John Mace]
To take this further, and I’ve always been a bit hazy on the details…
Is it because the chromosomes are not simply swapping genes (or more correctly alleles) willy-nilly, but the cross over process is literally a physically crossing over of the chromosomes and whole chunks are passed between the two determined by the cross over points? So, is it a physical limitation of the chromosome itself to bend in space and cross over so that having two cross over points with only one gene inbetween would be a very rare occurance? Does the location of the centromere have any significance in this process?
Also: On a given chromosome pair do the number of cross overs follow some statistical distribution (maybe a Poisson distribution) so that we could say that X number of cross overs would occur in Y number of cases? And would that be determined by the length of the chromosome and/or some other parameters?
If that’s too big a subject to deal with in this thread, just let me know. I’m really only asking about what you know off the top of your head. Thanks in advance, as always!
[/QUOTE]
Yes, there is a physical breakage and re-linkage going on between the two chromosomes. Double crossovers occur, but the fact that they do not often just affect one gene may have more to do with the statistical probability of where breakages take place than physical limitations of the chromosome. Offhand, I don’t know what the effect of the centromere might be, if any.
Wiki article here:
> **[Chromosomal crossover](https://en.wikipedia.org/wiki/Chromosomal_crossover)**
>
> Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis is usually initiated before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually oc...
There is a statistical distribution of crossovers between genes on the same chromosome, which is used to calculate recombinant frequency and was also used to map genes before DNA could be sequenced directly.
Wiki article here:
> **[Genetic linkage](https://en.wikipedia.org/wiki/Recombinant_frequency)**
>
> Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart. In other words, the nearer two genes are on a chromosome, the lower the chance of recombination between them, and the mo...
---
### Author:  [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)
#### Post date: [January 17, 2007, 8:24pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/26 "2007-01-17T20:24:05Z")
[QUOTE=Colibri]
Yes, there is a physical breakage and re-linkage going on between the two chromosomes. Double crossovers occur, but the fact that they do not often just affect one gene may have more to do with the statistical probability of where breakages take place than physical limitations of the chromosome. Offhand, I don’t know what the effect of the centromere might be, if any.
[/QUOTE]
A centromere tends to repress crossing over, so recombination is more frequent at the ends of the chromosome. In addition, a crossover event will itself tend to repress other crossovers in its vicinity, so double crossover events, when they do occur, will cause entire blocks of genes to recombine rather than a single gene.
---
### Author:  [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)
#### Post date: [January 18, 2007, 2:03pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/27 "2007-01-18T14:03:32Z")
[QUOTE=Terminus Est]
A centromere tends to repress crossing over, so recombination is more frequent at the ends of the chromosome. In addition, a crossover event will itself tend to repress other crossovers in its vicinity, so double crossover events, when they do occur, will cause entire blocks of genes to recombine rather than a single gene.
[/QUOTE]
Are there identifiable traits that are known to only occur in the end sections of the chromosome? What I’m wondering is: does this result in a greater likelihood of variation in certain traits than other?
---
### Author:  [@John\_Mace](https://boards.straightdope.com/u/John_Mace)
#### Post date: [January 18, 2007, 4:51pm UTC](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316/28 "2007-01-18T16:51:02Z")
[QUOTE=Hypno-Toad]
Are there identifiable traits that are known to only occur in the end sections of the chromosome? What I’m wondering is: does this result in a greater likelihood of variation in certain traits than other?
[/QUOTE]
Since you have a 50/50 chance of inheriting either chromosome, what difference does it make which one an allele is located on (either before or after meiosis)?
Imagine shuffling a deck. What’s the odds of drawing the ace of spades? Now, shuffle the deck twice as long. Do the odds change?
You will find certain traits have a tendancy to be inhereted together more than others, but that’s a slightly different matter (see my note about linked genes, above). Additionally, there are regulatory genes (genes that turn other genes on or off), and where they are positions realative to othere genes on the chromosome can have some affect, but there is no requirement that the genes they regulate be located on the same chromosome in the first place.
I would say that the effect you are asking about would be tiny, if it exists at all.
[Previous page](https://boards.straightdope.com/t/why-arent-all-my-sperm-identical/388316.md?page=1)