DNA Fingerprinting - Forensics

DNA Fingerprinting - Forensics

At the other extreme from genome markers like restriction enzyme length polymorphisms would be genetic loci at which nearly every individual is unique and different. In such a case a child would inherit one or the other of the allelic states of each marker from each parent. Another child of the same parents would again inherit markers from the parents, but it would be a different set. Thus, some markers in the two children would be the same and others would be different. On the other hand, two unrelated individuals would possess virtually no markers in common.

After the above abstract description, let us consider a real situation. Scattered through the human genome are stretches containing repeats of a 32 base pair sequence. In any one of the stretches the sequence is repeated over and over again, sometimes reaching a length of thousands of nucleotides. Different individuals possess these repeated sequences, called minisatellites, in the same locations, but the number of repeats of the short sequence varies from person to person.

Consider the consequences of digesting the genomic DNA with a restriction enzyme that cleaves frequently, but lacks a cleavage site in the repeated sequence. After such cleavage the DNA is separated according to size by electrophoresis. The fragments containing the repeated sequence are then identified by Southern transfer using a radioactive probe containing the 32 base repeated sequence.

For a typical individual, the cleaving and probing procedure resolves about 20 fragments of size greater than several thousand base pairs. Since each individual possesses different numbers of the 32 base repeats in these long stretches, each possesses a different collection of sizes for these large fragments. In short, the sizes of these fragments are DNA inheritable “fingerprints” unique to each chromosome of the individual. These DNA fingerprints are of forensic and legal value since they provide a unique linking of an individual to the DNA that can be extracted from a small amount of skin, blood, or semen, or for the determination of familial relationships.