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restriction mapping A technique for determining the sites at which a length of DNA (e.g. from a chromosome) is cleaved by restriction enzymes. By cleaving the DNA with various such enzymes, both individually and in combination, and analysing the resultant number and size of fragments by electrophoresis, a restriction map, indicating the order of restriction sites in the original DNA, can be deduced. This can then be integrated with a classical linkage map. Restriction mapping is routinely applied to organisms undergoing genetic investigation. Gene deletions or rearrangements that alter the restriction sites can be detected as changes in the pattern of fragments obtained. This may be used, for instance, to diagnose certain genetic abnormalities in the fetus. The fragments are separated by gel electrophoresis and identified using specific gene probes, as in the Southern blotting technique. The absence of a certain fragment in a fetal DNA digest can be diagnostic of a pathological change in the fetal gene containing the corresponding restriction site.
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