Recombination and intergenic distance

[TytoAlba95]

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

 

[atyy]

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

 

[WooKi]

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).

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.

 

[jim mcnamara]

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.

 

[jim mcnamara]

Thanks to everyone who contributed. Thread is now closed