How does DNA testing work?

Scientist working in crime lab

DNA testing is a powerful tool for identification and has many practical applications.

Common uses include:

  • Parental testing - to establish if someone is the biological parent of a child
  • Forensic testing - to help identify suspects or victims in a criminal investigation
  • Gene therapy - to test parents or foetuses for genetic conditions or birth defects
  • Genetic 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

DNA projection
  • DNA's double helix is like a long twisted ladder
  • The rungs of the ladder are made of pairs of four molecules called nucleotides
  • 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 pairs

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 unique.

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

Anna Anderson
  • 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 person.

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 billion.

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