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 fetuses 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 color to aspects of our personalities.
Every cell in our bodies – from heart to skin, blood to bone – contains a complete set of 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 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.
- 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 accurate are DNA tests?
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. Too 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.