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When a genome is first investigated, this map is nonexistent. The map improves with the scientific progress and is perfect when the genomic DNA sequencing of the species has been completed. During this process, and for the investigation of differences in strain, the fragments are identified by small tags. These may be genetic markers (PCR products) or the unique sequence-dependent pattern of DNA-cutting enzymes. The ordering is derived from genetic observations (recombinant frequency) for these markers or in the second case from a computational integration of the fingerprinting data. The term "mapping" is used in two different but related contexts.
Two different ways of mapping are distinguished. Genetic mapping uses classical genetic techniques (e.g. pedigree analysis or breeding experiments) to determine sequence features within a genome. Using modern molecular biology techniques for the same purpose is usually referred to as physical mapping.
Physical Mapping 
In physical mapping, the DNA is cut by a restriction enzyme. Once cut, the DNA fragments are separated by electrophoresis. The resulting pattern of DNA migration (i.e., its genetic fingerprint) is used to identify what stretch of DNA is in the clone. By analysing the fingerprints, contigs are assembled by automated (FPC) or manual means (Pathfinders) into overlapping DNA stretches. Now a good choice of clones can be made to efficiently sequence the clones to determine the DNA sequence of the organism under study (seed picking).
Macrorestriction is a type of physical mapping wherein the high molecular weight DNA is digested with a restriction enzyme having a low number of restriction sites.
There are alternative ways to determine how DNA in a group of clones overlap without completely sequencing the clones. Once the map is determined, the clones can be used as a resource to efficiently contain large stretches of the genome. This type of mapping is more accurate than genetic maps.
Genes can be mapped prior to the complete sequencing by independent approaches like in situ hybridization.
The process to identify a genetic element that signs responsible for a disease is also referred to as "mapping". If the locus in which the search is performed is already considerably constrained, the search is called the "fine-mapping" of a gene. This information is derived from the investigation of disease-manifestations in large families (Genetic linkage) or from populations-based genetic association studies.
See also 
- Genome project
- Human Genome Project
- Quantitative trait locus
- Genetic fingerprinting
- Genetic linkage
- Sulston score
- Optical mapping
- Terry A. Brown (2006). Genomes 3. New York: Garland Science Publishing.
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