Number of chiasmata and recombination fraction How does the

Number of chiasmata and recombination fraction? How does the number of chiasmata influence the recombination fraction? For example, in the paragraph below, why does having only 1 chiasmata mean the smallest recombination fraction can be 36%?

In one recombinantinbred population derived from the cross between the inbred John Innes Germplasm lines JI15 and JI399, the recombination fraction between the R locus (genotyped using a molecular marker assay) and Gp is 36%, resulting in an expected segregation ratio of 9.6:2.4:2.4:1.6 rather than 9:3:3:1. Mendel would have needed about 200 plants in the ‘smaller number’to have a 5% statistically significant deviation from independent assortment. Furthermore, linkage group V in pea, most likely corresponding to chromosome 3, behaves unusually in this cross because the number of chiasmata is never greater than one [12]; usually two or three occur. The recombination fraction calculated above is therefore the smallest that Mendel could have encountered, so it is unlikely that genetic linkage would have been discernable in any of the crosses that Mendel examined.

Solution

Recombination frequency/fraction () is a measure of genetic linkage used to construct a genetic linkage map in which a single chromosomal crossover (formation of chaismata) will take place between two genes during meiosis.

Chiasmata is the actual breakage of the phosphodiester bond during crossing over. The larger the number of map units between the genes,the greater will be the frequency of the crossing and when two genes are located on the same chromosome, the chance of a crossover producing recombination between the genes is related to the distance between the two genes. So obviously number of chaismata (crossovers) influences the recombination fraction.

One chiasmata means only one cross over had occured or in other words exhange of genetic material occured only one time,therefore recombination frequency/ fraction will be less.