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Different Procedures for Mapping of Molecular Markers - Molecular markers available in different forms (including those representing transcribed genes) have been mapped in a variety of animals, the most extensively studied among these being humans, mouse and Drosophila. Several approaches have been used, which also vary depending upon the nature of the organism.
These approaches include the following:
(i) Segregation and linkage could be studied through conventional method by a study of parents, Fl and F2's (or back cross progenies). If at a locus, the genotypes of two parents are designated as A and B, and that of Fl is designated as H (codominant), the distribution of genotypes at different loci in an F2 or backcross segregating, population can be used, through a computer device, to obtain the genetic map. This has been done in Drosophila and mouse.

(ii) Recombinant inbred (RI) strains have been produced in mouse, through continuous sib mating in segregating populations, so that the RI strains which differ at two or more loci can be used for twoor three point crosses to generate information on recombination frequencies.

The recombinant inbreds can also be used just like F2 population along with the parents from which the population of RIs has been derived.

(iii) In the above two approaches, sometimes difficulty is encountered in identifying allelic differences. To overcome this problem, in, mouse interspecific crosses (Mus musculus x M. spretus) were used. Mus spretus is the most distantly related species and gives fertile F 1 hybrids with laboratory mouse.

The Fl hybrids can be backcrossed in either direction and the backcross progenies may be used for generating multilocus linkage maps of mouse genome.

(iv) In human beings, crosses between desired parents can not be attempted. Similarly, RI lines can not be produced and interspecific crosses are not possible. In view of this, a panel of known families are used for ordering and 'Spacing (recombination frequencies) of molecular markers. LOD scores are calculated and used for linkage analysis.

(v) Thousands of genes and other molecular markers in Drosophila have also been mapped through in situ hybridization onto polytene chromosomes.

 
 

 

 
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