DNA Fingerprinting

The unique set of genetic markers of an individual makes its DNA ﬁngerprint or genetic proﬁle. The presence/ absence of markers are nowadays determined by mostly PCR using speciﬁc primers for the marker (see below). These markers are called Small Tandem Repeats (STR markers) or Microsatellite DNA. Eukaryotic DNA contains some non-coding sequences where two to six base pairs are repeated tandemly (one after the other) 100 to 1000 times, so that the lengths of these repeats vary. Since they are non-coding, variations do not have an impact on phenotype. These are variable in individuals, so that can be used as makers. The advantages of using STR markers are

• they occur frequently in genome

• easily ampliﬁed by PCR

• highly variable polymorphisms and

• a large number of characterized STRs are available.

The method used earlier was probing speciﬁc sequences using labeled markers as describes earlier (see DNA Probes and hybridization).

In DNA proﬁling, a set of markers (probes or PCR primers) is used. A DNA ﬁngerprint cannot be obtained by using one marker, since there are many individuals with the same banding pattern. When more and more markers are used in combination, the probability of ﬁnding the same pattern reduced. It has been calculated that, if 13 markers are used the probability becomes between 10 billion to several trillions. Since the world population is in the region of seven billion, it is highly unlikely that two individuals to have the same genetic proﬁle/ ﬁngerprint.

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