Answering this question is extremely complicated and in some criminal cases quite controversial.

In the simplest terms, a so called “dna match” is simply the “random match probability”. The random match probability reveals the probability that a person picked at random would match the crime scene sample. Put in a less confusing way, it answers the question “if I picked a random person walking down Main Street, what would be the likelihood that he or she would have the same genetic profile as this crime scene sample.”

In contrast to this definition, many prosecutors, judges, and even criminal defense attorneys mistakenly believe that a so called “dna match” provides the probability that a person is the source of the sample.

While the difference between these two points may appear not to matter much, in a death penalty case, the distinction can be life or death. It’s important to elaborate a little bit.

Let’s say witnesses described the suspect as having red hair and the accused also had red hair. The random match probability would tell you how likely it is that a person picked at random would have red hair. In this way, it would help you determine how much weight to assign to the fact that the accused is red headed also.

Now, imagine that the witnesses described the perpetrator as having six toes on one foot. Again, the Random Match Probability would tell you how common an occurrence we would expect that to be. If you had no other information about a suspect, you could easily conclude that redheadedness gives you almost no useful information about whether a specific redheaded defendant was the perpetrator; simultaneously, you know that six toes is fairly rare, and so even without any other information, you may have strong suspicions about a defendant with six toes. In neither case could you say with total confidence the defendant was the perpetrator because neither redheadedness, nor six toed-ness, is wholly unique.

In order to produce a source probability, you would need additional information, and a new calculation. In the case of redheadedness, you may want the suspect’s sex, height, age, geographic location, etc. in the case of six toed-ness, however, you might be comfortable with much less. Mistaking the source probability for the random match probability is at times known as the prosecutor’s fallacy.

Remember this is not legal advice, so be sure to contact a criminal defense attorney for questions about a specific criminal case.