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DNA testing Information Resources
DNA testing, also known as
genotyping, looks for similarities between two samples of DNA. It has become
used extensively to establish uncertain paternity, in forensic analysis at
crime scenes and to establish inheritance in domesticated animals.
Individuals’ genes contain genetic information in the form of a four-letter
code, the sequence of which is unique to each individual (excluding
monozygotic siblings). The DNA sequence is constructed using half of the
fathers DNA and half of the mothers – from the sperm and ovum. DNA can be
extracted from any cell within the body (blood, hair or even from a cheek
swab).
To establish a family link between a child and suspected father, the DNA
sequence is decoded into a graph that looks similar to a bar code. If the
child and suspected father are related, exactly 50% of the child’s DNA
sequence will line up with the sequence of his fathers (with the remaining
50% matching with his mother’s sequence).
Forensic DNA typing is a process of comparing DNA samples from a crime scene
against those from potential suspects. The forensic scientists will examine
four to five small sections of the DNA sequence (called markers), in both
the crime scene sample and those of suspects. If the four or five samples
from the crime scene match the four or five samples provided by a suspect,
the authorities can be certain that the individual was at the scene, and
possibly contributed to the crime.
Controversially, some nations have begun
creating national DNA databases. For example, in the UK, anyone arrested of
a crime (whether found guilty or not) are made to provide a DNA sample for
the database. This has been criticised since members of ethnic minorities
are over represented in the database in comparison to their numbers in the
nation as a whole.
Of course, DNA evidence can also be used to disprove involvement. Recently,
Steven Phillips, who has served 25 years for a rape which he has always
denied involvement with, was told he could be exonerated because of new DNA
testing that clears him of any involvement in the crime. DNA can even be
obtained from deceased people.
If an autopsy was carried out, samples of DNA
may be in storage. Similarly, if tissue or blood samples were collected
prior to death by a hospital, these may still be in storage as samples are
kept for a considerable period of time. If neither of these are viable
options, the body can either be exhumed, or a DNA sample can be collected
from a relative of the deceased, which will contain common family traits.
A novel use for DNA testing is to trace family ancestry. Such tests can
provide links to relatives via a common ancestor, or can link branches of
families who have the same surname. Ethnicity can also be established using
DNA testing, so ancestral background can be mapped, and a geography of
extended kin can be drawn.
Analysis of mitochrondrial DNA, passed down from
the mother, can provide information on general relatedness between people,
and is also useful for establishing a link between two branches of families
with the same surname. Y chromosome testing (passed from father to son) is
used to establish the degree of relatedness to a common male antecedent. DNA
data can be compared with databases of existing samples to ascertain
ethnicity percentages. |
