Recombination and intergenic distance

In summary, the chance of crossing over between two closely placed genes is considered low compared to those that are distantly placed on a chromosome because the farther apart the genes are, the more likely a crossover will occur and the higher the proportion of recombinant products will be. This is due to the fact that the probability of recombination is not uniform across the genome and can depend on the distance between two gene loci. This is evident from experimental evidence and the concept of a Holliday junction, which shows that a crossover can occur at one specific point, rather than two separate points.
  • #1
TytoAlba95
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Why is the chance of crossing over between two closely placed genes is considered low compared to those that are distantly placed on a chromosome?

'The frequency of recombinants produced by crossing over is the key to chromosome
mapping. Fungal tetrad analysis has shown that, for any two specific linked
genes, crossovers take place between them in some, but not all, meiocytes

(Figure 4-7). The farther apart the genes are, the more likely that a crossover will
take place and the higher the proportion of recombinant products will be.


Thus, the
proportion of recombinants is a clue to the distance separating two gene loci on a
chromosome map.'

-An introduction to genetic analysis by Griffith
 
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  • #2
In a simple model (overly simple, but it can give you the main idea), if a crossover can take place anywhere with equal probability, then considering any length of chromosome, the longer the length considered, the higher the chance of a crossover occurring within the length. There is a longer length between genes that are far apart, and thus more chances for a crossover to occur between genes that are far apart.

The argument is not quite correct, because the probability of recombination is not uniform across the genome. For example, crossovers are infrequent near centromeres.

https://www.fredhutch.org/en/news/spotlight/2018/09/bs_nambiar_molecularcell.html
 
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  • #3
If probability of crossover had been independent of location completely, the chance of crossover wouldn't depend upon distance. Rather that probability of recombination depends upon distance is experimental fact. Initially recombination probability was evaluated relative to markers with no known location. But probability of recombination of one gene relative to marker was observed to be different from another gene. So it was said that distance between one gene (with higher recombination chance) is lower, and the distance was measured in Morgans.

Now to the statement that 'probability of recombination does not depend' upon location. It's kinda true, if your initial data is number of crossovers and you imply independence of location, thus postulating that, say, in genome of 100 units 100 crossovers are observed with mean length of 1 unit. But it does not mean that on molecular level for single given recombination event there is no dependence upon distance. If you observe a chiasma at given location the probability of 2nd chiasmata depends upon distance from the first. Basically, make diffrence between population level (many cells) and molecular level (a single recombination event).

atyy said:
if a crossover can take place anywhere with equal probability, then considering any length of chromosome, the longer the length considered, the higher the chance of a crossover occurring within the length
This statement is false. If probability is equal the probability that given distance between two random points is observed is also equal, i.e. independent of distance. Consider interval from 0 to 1. Considere the uniform distribution of this interval. Choose two points. It is quite obvious that distribution of distances between these points is also uniform.
 
  • #4
Do you know what a Holliday junction is? I think that concept violates your point of view. It means that the two ends are swapped at one point. One crossover, one point, not two.

300px-Holliday_Junction.svg.png
 
  • #5
Thanks to everyone who contributed. Thread is now closed
 

1. What is recombination and why is it important?

Recombination is the process by which genetic material is exchanged between two chromosomes during cell division. It is important because it increases genetic diversity, which is essential for the survival and adaptation of a species.

2. How does recombination occur?

Recombination occurs during meiosis, the process of cell division that produces gametes (sex cells). During meiosis, homologous chromosomes pair up and exchange genetic material through a process called crossing over.

3. What is intergenic distance?

Intergenic distance is the physical distance between two genes on a chromosome. It is measured in base pairs (bp) and can vary greatly depending on the organism and chromosome.

4. How does intergenic distance affect recombination?

A shorter intergenic distance means that two genes are physically closer together on a chromosome, increasing the likelihood of them being inherited together. This can decrease the frequency of recombination between those two genes.

5. Can recombination and intergenic distance be manipulated?

Yes, recombination and intergenic distance can be influenced by various factors such as environmental conditions, genetic mutations, and artificial selection. However, these processes are complex and not fully understood, and manipulating them can have unpredictable consequences.

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