Biomedical Engineering Reference
In-Depth Information
a particular person committed a crime or is the father of a child.
Conventional fingerprints have been used for more than 100 years
by law enforcement officials to identify a person who touched
something. The whorls and swirls of fingerprints, created when we
are still in the womb, distinguish us from one another, but they
provide little more than a “yes or no” answer—providing a match
or not for the fingerprint taken from a crime scene. DNA marker
tests used for legal purposes distinguish between all people except
identical twins. With databases of the markers found in large num-
bers of people, a DNA marker match can provide a mathematical
estimate of the chance that the blood, semen, or even a dandruff
sample at the scene of a crime could have come from someone
other than the suspect.
Similar tests are also used to determine if a particular man is a
child's biological father. DNA markers are inherited and when the
child's DNA marker pattern is compared with the mother's and the
alleged father's, it is easy to see whether the child inherited a marker
that is not present in either the mother's or the man's pattern. If so,
the man cannot have fathered the child. If the man is not excluded,
using the same mathematical methods used for crime scene inves-
tigations, the scientist can provide an estimate of the chance that
another man could be the child's father and could have provided the
DNA marker pattern the child inherited from his father.
How Are Forensic DNA Tests Done?
The target sequence for DNA profiling that is most widely used for forensic purposes has
no known function. Our genome contains many regions in which short base sequences
are repeated several times. The number of repeats is inherited. Several different short
tandem repeats (STRs) provide powerful forensic tests. To obtain STR profiles, the PCR
products generated using primers specific for each STR are clipped by restriction
enzymes that cut the DNA on either side of the run of repeats. The resulting pieces are
separated by size by exposing them to an electric field, and the size of the piece that
binds to a probe of the repeating sequence is compared with standards to indicate the
number of repeats.
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