Question

what is the order of three genes (x, y, and z) that are located on the same chromosome, based on the following information? genes x and y have a recombination frequency of 17%. genes y and z have a recombination frequency of 9%. genes x and z have a recombination frequency of 8%.

Answer 1

The recombination frequency between genes is useful to determine the distance between genes and their order in the chromosome. The correct order of genes in this chromosome is X-Z-Y.

What is the recombination frequency?

Recombination frequency refers to the probability of crossing-over events occurrence between two genes. There farther genes are from each other, the greater recombination frequency there will be.

Recombination frequencies are also used to determine the distance between genes, and if they are linked or not.

So, if we know the recombination frequencies, we can calculate the distances between the genes and figure the genes' order out.

The map unit is the distance between the pair of genes for which one of the 100 meiotic products results in a recombinant one.

1% of recombination frequency, RF = 1 Map Unit, MU = 1 centiMorgan, cM

Now let us analyze the data

• Three genes ⇒ X, Y, Z.
• Distance between X-Y = 17% = 17 MU
• Distance between Y-Z = 9% = 9 MU
• Distance between X-Z = 8% = 8 MU

When the RF is given, we already have the distance between genes.

Now, to put these genes in order, we need to look for the most distant genes. These are probably the ones placed at the extremes of the chromosomal fragment.

In this case, the most distant genes are X-Y. So,

---------X-----------------Y---------

                  17 MU

Then, we need to place the remaining genes, which are probably between the two extremes genes. To be sure of this position, the addition of these middle genes must equal the distance between the extreme genes.

In our example, we only have one remaining gene Z.

• The distance from X to Z = 8 MU
• The distance from Y to Z = 9 MU
• 9 MU + 8 MU = 17 MU which is the distance between X and Y (extreme genes)

-----------X-------------Z----------------------Y---------

           ║----8MU---║------- 9 MU -----║

           ║-------------17 MU ---------------║

So, according to this reasoning, the correct order of genes in this chromosome is X-Z-Y.

You can learn more about the recombination frequency at

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