Large-Scale Mapping of a Gene
Classical genetic linkage maps assign particular gene loci to particular chromosomes based on phenotypic markers. Cytogenetic studies can further localize genes to particular chromosome bands, for example by associating a modified phenotype with the deletion of a particular band. Large regions of the chromosome can then be mapped molecular markers , for example with special restriction endonucleases that recognize sites only at infrequent intervals (Not I has a nine-base recognition site). Sub-regions within these fragments can be cloned and mapped with conventional endonucleases. Contig mapping and localization of a gene of interest to a particular fragment allows the gene to be mapped precisely. The smallest cloned fragments may still be hundreds of thousands of nucleotides in length.