The Crossover Percentage Between Two Different Genes is __________

Recombination and Estimating the Distance Betwixt Genes

Concrete crossing over during meiosis I is a normal effect. The effect of this event is to rearrange heterozygous homologous chromsomes into new combinations. The term used for crossing over is
recombination. Recombination can occur between any two genes on a chromosome, the amount of crossing over is a function of how shut the genes are to each other on the chromosome. If two genes are far apart, for example at opposite ends of the chromosome, crossover and non-crossover events will occur in equal frequency. Genes that are closer together undergo fewer crossing over events and non-crossover gametes volition exceed than the number of crossover gametes. The figure below shows this concept.

Finally, for ii genes are correct side by side to each other on the chromosome crossing over will be a very rare consequence.

Two types of gametes are possible when post-obit genes on the same chromosomes. If crossing over does not occur, the products are
parental gametes. If crossing over occurs, the products are
recombinant gametes. The allelic composition of parental and recombinant gametes depends upon whether the original cantankerous involved genes in coupling or repulsion phase. The figure below depicts the gamete composition for linked genes from coupling and repulsion crosses.

It is usually a simple affair to determine which of the gametes are recombinant. These are the gametes that are constitute in the lowest frequency. This is the directly result of the reduced recombination that occurs between two genes that are located close to each other on the same chromosome. Also by looking at the gametes that are most abundant you will be able to determine if the original cross was a coupling or repulsion phase cross. For a coupling phase cross, the nearly prevalent gametes will exist those with two dominant alleles or those with ii recessive alleles. For repulsion stage crosses, gametes containing one dominant and ane recessive allele volition exist most arable. Understanding this fact will be important when you lot actually calculate a linkage distance guess from your data.

The important question is how many recombinant chromosomes will be produced. If the genes are far autonomously on the chromosome a cross over will occur every time that pairing occurs and an equal number of parental and recombinant chromosomes volition be produced. Test cross data will then generate a i:1:1:1 ratio. But as two genes are closer and closer on the chromosome, fewer cross over events will occur between them and thus fewer recombinant chromosomes volition be derived. We then encounter a difference from the expected 1:1:1:one ratio.

How tin can we decide how close two genes are on a chromosome? Because fewer crossover events are seen betwixt two genes physically close togehter on a chromosome, the lower the percentage of recombinant phenotypes will be seen in the testcross information.
Past definition, one map unit of measurement (m.u.) is equal to ane percent recombinant phenotypes.
In honor of the work performed past Morgan,
ane yard.u. is also chosen one centimorgan (cM).

At present permit’southward determine the linkage distance between the genes
pr
and
vg. Nosotros can actually make two estimates because we have the results from coupling and repulsion phases crosses. The coupling phase analyzed a total of 2839 gametes, and of these gametes 305 (151
pr⁺
vg+ 154
pr vg⁺
) gametes were recombinant. To decide the linkage distance only split up the number of recombinant gametes into the full gametes analyzed. And so the linkage distance is equal to x.7 cM [(305/2839)*100)].

We can also perform the same calculations with the results from the repulsion phase cantankerous. For this experiment, a total of 2335 gametes were analyzed, and 303 (151
pr⁺
vg⁺
+ 154
pr vg) of these were the result of recombination. The gauge of the linkage distance betwixt
pr
and
vgfrom these experiments is xiii.0 cM [(303/2335)*100].

Obviously, we can conclude that the two genes are linked on the same chromosome. But what is the true linkage distance, the 10.7 cM value from the coupling experiment or the 13.0 value from the repulsion experiment? Really neither is correct or wrong. These once more are two estimates. Only by repeating this experiments many times using a number of different independent crosses tin can we settle on a value.

Once nosotros have settled on a value, these genes can and so be graphically displayed. Let’s say that the true altitude between the
pr
and
vg
genes is 11.8 cM, that is the average of our two estimates. We tin next display them along a chromosome in the style shown below. (Note that information technology is customary to utilize the allelic designantions of the mutant phenotype when drawing these maps.)

The final betoken that we demand to brand regards the maximum altitude that we can measure out. Considering of the fashion in which the calculations are performed, we can never take more that 50% recombinant gametes. Therefore the maxmimum distance that two genes tin exist apart and still measure that distance is but less that 50 cM. If 2 genes are greater than l cM autonomously, then we can not determine if they reside on the same chromosome or are on dissimilar chromosomes. In practice though, when experimental error is considered, as distances arroyo l cM it is hard to determine if 2 genes are linked on the same chromosome. Therefore, other mapping techniques must be used to determine thelinkage relationship among distantly associated genes. One method that allows us to deal with distantly related genes and to order genes is the three-bespeak cross.

Copyright © 1997. Phillip McClean