What is Genetics? : Frequent Questions & Answers

How does DNA testing on TV determine if two people are related?

Humans share 99.9% of our DNA with each other. That means that only 0.1% of your DNA is different from a complete stranger! However, when people are closely related, they share even more of their DNA with each other than the 99.9%. For example, identical twins share all of their DNA with each other. If the DNA of two unrelated people is compared, you would expect that the 0.1% of DNA that is unique to each of them would be very different. However, people who are closely related, for example mother and child, would share about half of that 0.1% of their DNA with each other. In other words, the DNA of a mother and her child is much more the same than the DNA of people who are not related to each other. You also share more DNA with your more distant relatives than with people you aren’t related to, but as relatives get further away in the family tree, you share less and less DNA with them.

Paternity testing looks at the DNA of two people (often a father and a child) to determine how much of their DNA is the same and how much is different. If the two people share more DNA with each other than would be expected, the test will be positive: they are highly likely to be father and son. This type of test is over 99.9% accurate and it is extremely unlikely that unrelated people would share enough DNA to cause the test to be falsely positive. DNA paternity testing is so highly accurate because the variations in human DNA are very diverse among people that are not related but are very similar among people who are related. Paternity testing is different than other types of genetic testing in that it is only looking at common variations in the DNA that don’t cause diseases. This means that paternity testing cannot be used to diagnose or predict if someone is going to get a disease.

Learn more about other types of genetic tests here: What is genetic testing?