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Why is the recombination frequency higher if the genes are farther apart?

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up vote 6 down vote accepted

The genetic phenomenon referred to as recombination reflects the process of crossing over which occurs during meiosis. Crossing over creates an exchange of genetic information between homologous chromosomes. To a first approximation cossing over events take place at random positions along the aligned chromosomes. Consequently the further two loci are apart, the more likely that there will be a crossing over event between them. Thus the recombination frequency can be used to measure the distance between two genetic loci (or genes).

Supplement added in response to comment from OP

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An analogy

Imagine a piece of string. It has 6 marks on it (1-6 on the diagram) which divide it into 7 intervals (A-G). We are going to cut the string at one of the marks. We throw a die to decide which position to cut.

What is the probability that our cut will separate A and G? It's 100% since all single cuts will do this.

What is the probability that our cut will separate A and D? It's 50% since single cuts at 1,2 or 3 will do this.

What is the probability that our cut will separate A and B? It's 16.7% (1/6) since only a cut at position 1 will do this and the probability of throwing a 1 on our die is 1/6.

Think of the intervals (A-G) as genes and the cut sites (1-6) as possible crossing over events. If two homologous chromosomes are going to undergo a single crossover somewhere at random, then the closer together are the genes (intervals), the less likely is it that the crossover (cut) will take place between them. This assumes of course that the site of crossing over is chosen at random.

In the model presented here, if the order of the letters was scrambled and didn't know what it was but we were told the frequencies of occurrence of the separation of the intervals with random cutting we could deduce the order of the letters.

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To me the statement "Consequently the further two loci are apart, the more likely that there will be a crossing over event between them" is still not obvious. It is the core of my question. Please elaborate further. –  DurgaDatta Feb 5 at 16:06
    
Thank you very much for the effort. However, I am not yet clear, sorry for this. Crossing-over involves genes physically being 'tied up' temporarily. I fail to see how is cutting analogous to it? Why should chances of such 'tying up' increased if the genes are farther apart? –  DurgaDatta Feb 6 at 1:25
    
I don't know what you mean by being 'tied up'. Crossing over involves a physical interaction between two aligned chromosomes at a randomly chosen point. The further that two genetic loci are apart then the more likely it is that the randomly chosen point of crossing over will lie between those loci. I really can't think of any other way to explain this, so I hope someone else comes along and gives you the explanation you are looking for. –  Alan Boyd Feb 6 at 10:36
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