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In this article we will discuss about the introduction and significance of chromosome mapping.
The diagrammatic representation of the relative distance between the linked genes of a chromosome is called linkage or genetic map. Since a linkage map is the outcome of crossing over studies, it is also called crossover map. The method of constructing genetic maps of different chromosomes is called genetic mapping or chromosome mapping. Chromosome maps were first prepared in Drosophila and maize. Now the chromosome maps have been prepared for many animals and plants.
The frequency of crossing over between two genes is directly proportional to the distance between them on the chromosome. A frequency of one per cent crossing over between two genes is taken to represent one unit of map distance between the genes and is now termed as Centimorgan or CM in honour of the famous geneticist Thomas Hunt Morgan.
The first important step before starting the genetic mapping of chromosome is to know the number of chromosomes and genes of a particular species by conducting hybridisation experiments between the wild and mutant types. After the linkage group is known, the relative distance between each linked gene is determined.
The distance between two given genes is calculated according to the percentage of crossing over. This is because crossing over is directly proportional to the distance between the genes. For example, if the percentage of crossing over between two linked genes is 1%, it means that the map distance between two linked genes is one unit or one Centimorgan.
Let us create a hypothetical map of a chromosome with three genes A, B, and C. If the cross over percentage between A and B is 10%, these genes are plotted on a linear scale as shown in the Fig. 7. Similarly, if the crossover percentage between B and C is 3%, the cross percentage between A and C will determine the position of C, which may be 3 units from B towards A or away from A. This can be found out by crossing A and C.
If the crossover percentage between A and C is 13 per cent, C will be plotted 3 Centimorgans on the right side of B.
Morgan and Sturtevant conducted experiments in Drosophila on the recombination frequencies of three genes – ‘b’ for black body, ‘vg’ for vestigial wings and ‘cn’ for cinnabar eye.
The recombination frequencies obtained are as follows:
a. Recombination frequency between ‘b’ and ‘vg’ = 18 %
b. Recombination frequency between ‘b’ and ‘cn’ = 9%
c. Recombination frequency between ‘cn’ and ‘vg’ = 9.5
If gene ‘b’ is plotted at one end of the chromosome, gene ‘vg’ will be 18 Centimorgan away from it. The gene ‘cn’ which is 9 centimorgan from ‘b’ and 9.5 centimorgan from ‘vg’ will be plotted approximately midway between the genes ‘b’ and ‘vg’. Thus, the sequence of gene will be b, cn and vg. However, it should be understood that the percentage of crossing over between two genes is only approximately proportional to the distance between them.
The recombination frequency is determined as follows:
Significance of Chromosome Mapping:
a. The chromosome map gives the exact location of genes in a chromosome.
b. The order and the approximate distance between the genes can be learnt from a map.
c. The map indicates the strength of the linkage between two genes and their chances of crossing over.
d. The map helps in confirming genetic principles such as:
i. Genes are located in a linear fashion in the chromosomes
ii. Genes are found in specific loci on chromosomes.
e. They provide accurate information for genetic manipulation.