Crossover Probability

[Physics Guy1421]

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I read in a textbook that the probability of gene crossover occuring on homologous chromosomes increases when the genes are farther apart, but it didn't give an explanation for this.

I'm a little confused here and was wondering if anyone could offer an explanation as to why this is the case?

Thanks!

 

[bigstix808]

if there were two genes that were 100 units apart, there would be 98 points of possible crossover space between them.

now if there were two genes that were 12 units apart there would only be 10 possible points of crossing over.

-*-----------------*------O------------------- (chromosome 1)
|all this space for crossover|
---*-----------------*------O----------------- (chromosome 2)
OR
-*--*--------------------O------------------- (chromosome 1)
^^|only that space for crossover|
---*--*--------------------O----------------- (chromosome 2)


does that help?

 

[Physics Guy1421]

if there were two genes that were 100 units apart, there would be 98 points of possible crossover space between them.

now if there were two genes that were 12 units apart there would only be 10 possible points of crossing over.

-*-----------------*------O------------------- (chromosome 1)
|all this space for crossover|
---*-----------------*------O----------------- (chromosome 2)
OR
-*--*--------------------O------------------- (chromosome 1)
^^|only that space for crossover|
---*--*--------------------O----------------- (chromosome 2)


does that help?

Hey thanks for the response I appreciate it. I'm still kinda confused though...I see that if there are 100 units between genes vs. 12 units there would be more available units for crossover. But these units of crossover are not part of the gene. Is crossover then just exchange of segments of DNA that are in between genes? If so then I don't see how crossing over of DNA between genes would lead to genetic diversity, since your not recombining any of the gene sequence.

Any input? Thanks in advance!

 

[bigstix808]

yea, the "units" between the genes have nothing to do with the genes in question (gene A and gene B - gene C will have nothing to do with A or B but is still present on the chromosomes as a separate gene for reference). remember, for homologous chromosomes there are similar genes (A, B, C, etc...), just different alleles (A,a; B,b; C,c).

SET 1
----A-------------c----B-------O----------- (chromosome 1)
----a-------------C----b-------O----------- (chromosome 2)


SET 2
----A---B---------c------------O----------- (chromosome 1)
----a---b---------C------------O----------- (chromosome 2)

now if you were to randomly pick one point for crossing over to take place in the first set of chromosomes you might get something like this...

----A---/-----------C---b------O----------- (chromosome 1)
----a---/-----------c---B------O----------- (chromosome 2)

now you can see that on chromosome 1 there is A and b, and on chromosome 2 you have a with B. if these two chromosomes were to split, you now have independent assortment.
--------------------------------------------------------------------------

now for the second scenerio...
if a random crossover were to take place on the second set (genes A and B are closer now) we might get something like this...

-----A----B------/-------C------O-------------- (chromosome 1)
-----a----b------/-------c------O-------------- (chromosome 2)

if you were to just look at genes A and B, you would think there had not been any crossing over...but looking at gene C you can see that there has. the fact that gene A and gene B were so close together on the chromosome made the chance of the crossing over to take place is that small space between them less likely.

that's all i got 😀

 

[Physics Guy1421]

Thanks for the great explanation I got it now!