Crossing over and the distance of alleles

[laxboi33]

The further apart the alleles are on the chromosome the more likely a cross over.

 

Can someone explain to me why this happens?

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to be more specific, why is it less likely that crossing over would occur between two alleles that are closer together?

[CharonY]

This is basically based on the assumption that each point on the chromosome has the same chance of a crossover event (which is not true near the centromer). Hence, if you look at a longer stretch, the overall chance of a crossover occurring is higher than within a shorter one.

Edited October 12, 2009 by CharonY

[laxboi33]

so it is based on the length of the chromosome?

[GDG]

so it is based on the length of the chromosome?

 

No, just the distance between the genes.

 

Imagine a chromosome:

 

----*------------------------*-------------------

 

I've flagged two genes at random (*) for purposes of illustration. Assume that crossing-over can occur at any point (-) on the chromosome. In the illustration above, there are 24 units (arbitrarily assigned) between the two flagged genes. If crossing-over happens in between the two *'s, they get separated: otherwise, they are inherited together. If crossing-over happens to the left of the first one, or to the right of the second one, they get inherited together.

 

Now consider this chromosome:

 

----&--&-----------------------------------------

 

Between these two flagged genes (&), there are only 2 units. Clearly, there are many fewer opportunities to cross-over in between these two genes -- 1/12th the number of the genes in the first chromosome. All else being equal, the two **'s will be separated 12 times as often as the two &&'s.

[Mr Skeptic]

Say if you had two pages from a book, and you tore both pages in the same place and stuck them together with the piece from the other page. The closer together two words are, the likelier they are to be on the same piece. But words farther apart are likely to end up on different pages because the tear is likely to be between them.