Friday 20 January 2006
Positional cloning involves examining DNA markers in families in which an inherited disease is transmitted to search for a chromosomal segment that is consistently handed on to the family members who are affected by the disease. This allows the underlying gene to be mapped to a chromosomal region.
Identifying mutations in one of the genes in this region reveals the gene that is responsible for the disease.
Because in many common single-gene disorders, such as cystic fibrosis, there was no clue to the nature of the defective gene product, an alternative strategy called positional cloning, or the "candidate gene," approach had to be employed.
This strategy initially ignores the biochemical clues from the phenotype and relies instead on mapping the disease phenotype to a particular chromosome location. This mapping is accomplished if the disease is associated with a distinctive cytogenetic change (e.g., fragile-X syndrome) or by linkage analysis.
In the latter, the approximate location of the gene is determined by linkage to known "marker genes" or SNPs that are in close proximity to the disease locus. Once the region in which the mutant gene lies has been localized within reasonably narrow limits, the next step is to clone several pieces of DNA from the relevant segment of the genome.