Chromosome walking is a method of positional cloning used to find, isolate, and clone a particular allele in a gene library. An allele is a gene for a particular genetic trait passed on from adults to their offspring, such as the allele for brown eyes in a gene for eye color. Sometimes, the approximate location of a single allele in a string of deoxyribonucleic acid (DNA) may be known. To isolate a particular allele for a genetically transmitted disease, chromosome walking may need to explore for the desired specimen in an unmapped DNA sequence outside of previously mapped sequences.
To locate a particular disease gene, the walking starts at the closest gene that has already been identified, known as a marker gene. Each successive gene in the sequence is tested repeatedly for what are known as overlap restrictions and mapped for their precise location in the sequence. Eventually, walking through the genes reaches the mutant gene in an unmapped sequence that binds to a fragment of a gene of that particular disease. Once the gene is cloned, its function can be fully identified. Throughout this process, tests are done to fully identify the properties of each successive clone, to map their locations for future use.
There are nearly half a dozen positional cloning tests that are done prior to a chromosome walk, to narrow down as much as possible the particular genetic sequences that may contain the desired mutant gene for a disease. Once the markers on either side of an unmapped probable sequence are found, the chromosome walk can begin from one of the markers. The testing on each successive clone is complex, time-consuming, and varied by species. There are different tests for genes related to plant diseases than for genes involved in a human gene library.
Positional cloning uses genetic markers that are known to inhabit the chromosomes of individuals who have specific diseases. These databases of known common traits allow testing that can be used to identify individuals who may or may not have certain recessive genes for a disease that has not presented as yet. Chromosome walking, along with all of the other tests done prior to chromosome walking, requires exceptionally well equipped laboratories within which to perform all of the cloning phases, examinations, and analysis of mutations found.