»  How does DNA identify someone?  (Back)

Among your DNA, there are sequences that are identical among many people but when considered as a group, the variations among several sequences are unique to you. Think of it as different physical characteristics- your identity in terms of how you look is made up of height, weight, eye color, hair color, skin color, hair styles, facial hair, etc. Many people may have a few of these in common, but very few have each physical characteristic exactly like yours. DNA can be looked at in a similar way.

There are "markers" used in human identity (HID) testing which are small regions of DNA that have well-characterized structures and locations on our chromosomes. Think of these markers as the criteria we listed above for physical identity. Good markers for HID have about 15 variants (ie. hair colors) among all people. Some variants may be present in 25% of all people, others much less. For a person's DNA being tested, they might have "variant 20" at a particular marker. 15% of all people, on average, may have that same variant 20 at that location. That doesn't match that person's DNA with a child or a sample found at a crime scene because it could be a match to any of the 15% of the people with that same variant. This is like saying I'm looking for someone with blonde hair. More markers (usually 8-15 for HID) must be used to characterize a person's DNA, just like we use many attributes to characterize a person's physical appearance.

This now allows for more specific matching, based on the probabilities of another person having the exact same set of variants for the set of markers. Variants that occur less often make the match more likely. It isn't a match if variants are different. Again going back to the example of physical appearance, if the person sought has black hair and a red shirt but the person tested against the criteria has only black hair, it is less likely you have the right person.

If this did not answer your question, please use our web questionnaire.