PCR is performed. The product (essentially a large amount of DNA covering a portion of the gene  of interest) results. We visualise ​this product by applying it to a gel which is subjected to an electric current causing the fragments to move downwards at a speed determined by their size. as shown on the right. The product has a particular size ( a set number of bases pairs) and shows up as a single band (A).

A restriction (or cutting) enzyme is then applied. If the PCR product is normal, this enzyme cuts it into two smaller pieces. On the gel, these show further down, as they are smaller (B and B).

Each sample application on the gel is called a lane and runs vertically downwards. Lane 1 shows a normal subject's DNA before cutting, and Lane2 shows his DNA after cutting.

Lane 3 shows an abnormal subject (he carries the R59W mutation for VP) before cutting. The typical PCR product is present (a). Lane 4 shows his DNA after cutting. Note the difference. Half his PCR product (made by the gene inherited from his normal parent) has behaved normally, and has cut into two smaller pieces,(b and b). However the half made by the mutated gene inherited from his affected parent has failed to cut, and remains at the start (a).

This is a quick and reliable test for the inheritance of a mutation, provided one knows just which mutation one is looking for.