Chromosome Mapping

It has been noted that in the dihybrid genetic crosses, a recombination frequency of less than 50% indicates linkage of the two genes involved. Morgan and Sturtevant found that the recombination frequency varies for different mutant gene pairs. The difference in the recombination frequency was found to be associated with the distance between the genes.

The frequency of crossing over between two linked genes is directly proportional to the distance between them in the chromosome. This is called Morgan and Sturtevants hypothesis.

Definition

A chromosome map is a graphic representation of a linkage group in the form of a line which shows by points the sequence of the genes and the relative distance between the genes it contains. A chromosomes map is also called a linkage map or genetic map.

Since the frequency of crossing over is used in preparing chromosome maps, the latter are also called cross-over maps.

Chromosome Map Unit

A frequency of one percent crossing over between two genes is taken to represent one unit of map distance between these genes, and is termed as centimorgan (cM) in honour of Thomas Hunt Morgan the famous geneticist.

fig. 19.12 Diagram Showing the Technique of Plotting

Gene Loci in a Chromosome

fig. 19.13 A map of Drosophila Chromosome II

Why did Mendel miss linkage?

Mendel could not note linkage because of the seven characters be studied in garden pea, most of them are located in different (non-homologous) chromosomes seed colour and flower colour are in the same chromosome i.e, they are linked. However, they lie so far apart that they get separated by crossing over, and assort independently in all cases.