Gene mapping

Gene mapping is the creation of a genetic map assigning DNA fragments to chromosomes.

When a genome is first investigated, this map is non existent. 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.

Physical Mapping
Physical Mapping is the process of determining how DNA contained in a group of clones overlap without having to sequence all the DNA in the clones. Once the map is determined, we can use the clones as a resource to efficiently contain stretches of genome in large quantity. This type of mapping is more accurate than genetic maps.

In maps based on a genetic fingerprinting of the clones, the stretches of DNA are identified according to how they are cut by a restriction enzyme. Once cut, the DNA fragments are separated by electrophoresis. The resulting pattern of DNA migration (ie. its 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.

Outlined above are alternative ways to investigate to create genetic maps (radiation hybrids, STS,...).

Genes can be mapped prior to the complete sequencing of a by independent approaches like in-situ hybridisation. Once the genome has been sequenced, in-silico approaches perform the gene finding. The such suggested genes are compared with the experimental evidence for the respective gene.

Disease-association
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.