PAST CRIMESside of its ancestry.
Forensic analysis involves extracting DNA from a subject, then copying
and increasing it to enable it to be studied. The sample is then separated out
into parts, and scanned for areas of the sequences that repeat themselves.
These repeating sections are the ones that are unique to each individual, as
opposed to those shared within a species. They are made up of genetic
markers inherited from parents and are very unlikely to be repeated in any
other entity. This makes them an extremely useful tool for identifying the
victims or perpetrators of crimes if they can be compared with known samples
of that person’s DNA–from, for example, hairs from a hairbrush, or blood
samples already on record.
DNA can also help to identify the sex of a victim, even if the remains are
no more than undiagnostic fragments of bone, as the sequences reflect the
presence of X and Y chromosomes.
The use of mtDNA can identify members of the same family if they are
related through their mothers, and because there is much more mtDNA than
nDNA in a sample, it is particularly useful if there is little material to analyse,
or if the sample is badly degraded. Like everything else, DNA starts to decay
after death, but it is sometimes possible to recover usable DNA even after
many centuries, from inside bone or tooth structures. However, great care has
to be taken to avoid contamination–a speck of modern skin dust could
invalidate the whole sample, so this is work that has to be undertaken in sterile
laboratory conditions. Mitochondrial DNA was used to identify the bodies of
the Russian imperial family found in 1991 and 2007. They had been killed
along with some of their servants by the Bolsheviks in 1918. Using
comparisons with known surviving relatives, including Prince Philip, it was
possible to establish the identity of each member of the family, even though
two of the bodies had been subjected to an attempt to destroy the evidence by
burning.^7
In modern forensic studies, DNA samples can identify victims and
criminals, with just a one in 60 billion or so chance of being wrong. In more
ancient cases, where DNA survives in bones or teeth, it can be used to identify
the gender and ethnicity of an individual, their relationship to other bodies
found nearby, and more rarely, their actual identity.
This has proved very useful for investigating‘cold cases’ –providing new
sources of evidence and allowing investigators to reopen old files to identify
victims of crimes or disasters, and bring answers to their families.
Archaeological crime scenes are, of course, the coldest of‘cold cases’! But
even for very ancient cases, DNA is helping to provide clues.