Introduction of gene mapping
The chromosome mapping or gene mapping is based on two important assumptions:(i) that genes are arranged on a chromosome in a linear fashion, and (ii) that the percentage of crossing over (recombination frequencies) between the two genes is an index of their distance apart. A chromosome map is a line on which the genes are represented points that are separated by distance proportional to the amount of crossing over. The gene mapping is based on the percentage of crossing over between genes, it is sometimes known as a crossing over map. The relationship between the cross over frequency and the distance between loci was first suggested in 1913 by A. I. Sturtevant. Thus, the chromosome map is a condensed graphic representation of relative distances between the linked genes, expressed in percentage of recombination among the genes in one linkage group. Distances between genes can be expressed in map units, where one map unit is defined as 1 per cent recombination. So, The representation in figure of relative position of genes on the chromosome is known as chromosome map in the process of identifying gene loci is called gene mapping.
Explanation of Gene Mapping
The amount of crossing over, on which the gene mapping is prepared, has been drawn from Test crosses. There is no direct microscopic examination of the chromosome. The gene mapping is based on two assumptions which are very easy to understand, those are: i) The genes are arranged in a liner fashion on the chromosome. ii) The percentage of crossing over between the two genes is an index of their distance in the chromosome.
Recombination frequency or Cross over value = Total number of recombinants in Test Cross / Total number of progeny of the Test Cross. The value is generally represented as a per cent of the total population. The variation in recombination frequency is governed by the distance between the genes. Closer the distance between the two genes, the less are the chances of crossing over. As a result there is lower frequency of recombination. The greater is the distance between the genes, the higher is the percentage of crossing over between them.
Construction of gene mapping
To construct the gene mapping of an animal or plant, its chromosomes are first represented by straight lines and then the positions of genes are determined from the percentage of crossing over data. The percentage of crossing over is governed by the distance between the genes concerned. If the two genes are closer then the chance of crossing over will be less I which will be reflected in the recombination frequency.
Technique of gene mapping
The assumption of consistent gene order along the chromosome, coupled with the fact that crossing occurs, makes it almost certain that gene loci can be determined in relation to each other. If genes are located in a consistent linear order along a chromosome, then the distance between any two genes and the amount of (Tossing over that occurs between them should be in direct proportion. Homologous chromosomes cross over each other and such cross over occasionally lead to an exchange of chromosome segments between the pairs. When such an exchange occurs, the linkage between certain genes is broken and new gamete possibilities arise. Higher the number of such gametes, greater will be the portion of the offspring showing the cross over phenotype. This percentage, therefore, gives direct measure of the amount of chromosomal crossing over. In other word, it provides a direct measure of the distance between the genes involved.
Factors Affecting Gene Mapping
(i) Double Crossing Over : This phenomenon occurs between two genes which are situated by long distance on the same chromosome. It has been observed, though there is double crossing over yet the two genes are remaining on the same chromosome. There is no apparent sign of crossing over. So, calculation of crossing over percentage may cause mistakes in the chromosome map. (ii) Interference : One chiasma may interfere to form another chiasma formation in the vicinity. As a result, one crossing over may reduces the crossing over in the vicinity. (iii) Temperature : High and low temperatures increase the frequency of crossing over. Hence, the temperature causes fluctuations in the location of genes on chromosome. (iv) X-ray : This ray increases the frequency of crossing over and disturb the location of genes on chromosome mapping. (v) Age : Experiment of Bridges shows that crossing over is more frequent in older females of Drosophila. Thus age also affects the frequency of crossing over. Hence, ageing also cause fluctuations in loci of genes on chromosome. (vi) Location : Crossing over is less frequent near centromere and near the terminal ends of chromosome. (vii) Sex : The males of many organisms show less frequency of crossing over. In male Drosophila there is no crossing over. Thus, sex may also affect the frequency of crossing over.
Utility or Importance of Gene Mapping
(i) Chromosome mapping or gene mapping are very useful in the study of genetic engineering. (ii) Chromosome maps are very helpful to find out the exact location of new mutant gene in chromosome. (iii) Chromosome maps have established the validity that genes are arranged in a linrar fashion in chromosome. (iv) Gene Mapping have established the concept that the specific genes occupy the specific loci in the specific chromosome.