Archaeologists believe they have found the lost grave of
King Richard III and have been testing the bones for DNA against known
descendants of Richard's family. But how does DNA testing actually work?
Common uses include:
- Parental testing - to establish if someone is the biological parent of a
- Forensic testing - to help identify suspects or victims in a criminal
- Gene therapy - to test parents or foetuses for genetic conditions or birth
genealogy - to find out more about someone's ancestry
What is DNA?
At the heart of DNA testing is the molecule DNA. It carries our genetic code
and determines traits from eye colour to aspects of our personalities.
Every cell in our bodies - from heart to skin, blood to bone - contains a
complete set of our DNA.
What is the DNA test looking for?
- DNA's double helix is like a long twisted ladder
- The rungs of the ladder are made of pairs of four molecules called
- Adenine (A) and thymine (T) always bond together as a pair, as do cytosine
(C) and guanine (G)
- Our genetic code is made of 3 billion
99.9% of the DNA from two people will be identical. The
0.1% of DNA code sequences that vary from person to person are what make us
These sequences are called genetic markers, and are the part of the code that
forensic scientists use when doing a DNA test.
Identical twins are the only people who have identical genetic markers.
However, the more closely related two people are, the more likely it is that
some of their genetic markers will be similar.
The key to DNA testing is knowing where to look in the billions of letters of
genetic code to find the genetic markers that will identify the important
similarities or differences between people.
How does the DNA test work?
Cases cracked by DNA
- In the 1950s, Anna
Anderson famously claimed to be the Grand Duchess Anastasia, the only
survivor of the Russian royal family. After Anderson died, DNA tests proved she
was no relation to the Imperial family.
- Serial killer Timothy Wilson Spencer was the first US man to be sentenced to
death from DNA evidence. David Vasquez, wrongly convicted of one of Spencer's
crimes, was the first man in the USA to be cleared by DNA testing.
- In 1912, four-year-old Bobby Dunbar disappeared during a fishing trip but
was found eight months later. Bobby was returned to his family, but another
woman claimed he was her son. In 2004, DNA testing revealed the boy was not
Bobby Dunbar - whose fate remains unknown.
Parental, forensic and genetic testing look for
similarities in the genetic markers between two biological samples.
Because all cells in the body contain exactly the same DNA, samples can be
taken from almost anywhere in the body, including skin, hair follicles, blood
and other bodily fluids.
A forensic scientist might be asked to compare DNA from skin cells found
underneath the fingernails of an attack victim, with the DNA from a blood sample
taken from a potential suspect.
First of all, the DNA is isolated from the cells and millions of copies are
made, using a method called 'polymerase chain reaction', or PCR.
PCR uses a naturally occurring enzyme to copy a specific stretch of DNA over
and over again. Having lots of DNA makes the genetic code easier to analyse.
The DNA molecules are then split at particular locations to separate them
into known 'chunks' and the code at those specific points is analysed to create
a DNA fingerprint. The fingerprints from the two different samples are then
compared to see if they match.
How accurate are DNA tests?
Steve Jones explains the power of DNA testing
The accuracy of DNA tests has big implications. DNA tests are sometimes the
only evidence to prove that a suspect was involved in a crime, or free someone
who has been wrongly convicted.
It is easy to tell if DNA from two biological samples does not match. But a
match doesn't make you totally certain that the two samples come from the same
There is always a small chance that two different people's genetic markers
could be the same, especially if they are related.
To reduce the chance of error, scientists test more than one genetic marker.
The more identical markers there are in two samples, the more accurate the test.
However, testing more markers takes more time and is more expensive. Forensic
DNA tests usually examine six to ten markers. The chances that two unrelated
people have identical profiles is less than one in one