Question

How can the concept of recombination frequency be used in genetic mapping?

Answer 1

GENETIC MAPPING

The linkage of the genes in a chromosome can be represented in the form of a genetic map or linkage map or chromosomal map.

Alfred sturtevant first constructed the genetic map by using the frequency of crossing over (i.e. recombination frequency) between two genes to prepare the first genetic map of X-chromosome of Drosophila.

Two important aspects of genetic mapping are

i) Deterimination of the linear order with which the genes are arranged with respect to one another (gene order)

ii) Deterimination of the relative distance between the genes (gene distance)

Because recombination i.e crossing over tends to occur with increasing frequency as the distance between two specific gene loci increases, the percentage of recombination (or recombination frequency) can be used to represent a measure of distance (map distance) between the two genes. The percentage of recombination (crossing over) between two loci is directly related to the physical distance between the loci. 1% crossing over or recombination is equivalent to 1 map Unit or 1cM. By analyzing the percent recombination among the progeny of parents that are heterozygous for a number of linked genes, a genetic map that places the genes in a linear array can be constructed. so,

Map distance = Number of recombinant offspring/Total number of offspring x 100

The unit of measurement for genetic map is known as a map unit or centimorgan (cM). 1 map unit or 1 cM is equal to 1% recombination. Thus, two genes that recombine with a frequency of 3.5% are said to be located 3.5 map units apart.

Example:

Two genes (A and B) are located on the same chromosome. An AABB individual is crossed to an aabb individual to produce AaBb offspring. The AaBb offsprings are then test crossed to aabb individuals. Assume total 400 offsprings produced outof which 325 offsprings are parental and 75 are recombinant. Recombination frequency and map distance are calculated as follows