Genome Maps,Genetic Linkage Maps, Cytogenetic Maps, Physical Maps, Molecular Markers, Cytogenetic Map

Back to Home

Home >> Animal Biotechnology >> Genome Maps

Back to Home

	Genome Maps A genome map may be defined as a detailed schematic description of the structural and functional organisation of all the chromosomes in the genome of an organism. The genome, in turn, may be looked at in two different ways. To a cytogeneticist, it represents the haploid set of chromosomes of a diploid organism. Therefore, a polyploid organism will have more than two genomes, which may be identical (autopolyploids) or distinct (allopolyploids).
But to a molecular biological genome consists of the total genetic information present in the organism. At present, we have mainly three types of maps: (i) genetic or, linkage maps, (ii) cytogenetic maps, and (iii) physical maps. A genetic map is prepared on the basis of recombination data between carefully selected genetic markers usually ordered into suitable crosses. In such maps, the distances between genes are shown in term of map units or centriMorgans (eMs).
But in case of humans, linkage maps have to be prepared using family pedigree data; in addition genes could be assigned to specific chromosomes/chromosome arms using somatic cell hybridization. Linkage groups have been assigned to specific chromosomes in the genome by using appropriate cytogenetic techniques in conjunction with linkage studies. The chief problem of linkage mapping is the non availability of a sufficient number of genetic markers to cover the entire genome. Therefore, search for more abundant markers continued and resulted in various molecular markers, e.g., RFLP (restriction fragment length polymorphism), RAPD (random amplified polymorphic DNAs), VNTR (variable number of tandem repeats), and STR (short tandem repeats), etc.
A cytogenetic map depicts the locations of various genes in a chromosome relative to specific microscopically visible landmarks in the chromosomes, e.g., bands of salivary gland chromosomes. But in a physical map, the distances between genes are shown in terms of number of base pairs separating them. In contrast, a map unit (centiMorgan) represents that distance that allows one per cent recombination between the linked genes; 1 cM may represent 200 kb (in Arbidopsis) to 1000 (in humans) or even 1600 kb (in mice). In several species, genetic maps have been/are being integrated with the physical maps to yield a highly useful genomic map, using which genes of interest can be detected and/or isolated with the help of more convenient and highly reliable molecular markers.