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Why Has My DNA Been Found On A Murder Weapon I Didn't Touch?

View profile for John Ellwood
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In order to answer the above question you have to in effect ask two further questions

  1. Is it certain that the sample found actually contains my DNA? and;
  2. If it does, how did it get there?

There is a lot of complex science involved in answering the two questions and in this short note it impossible to deal with all aspects of the science. DNA analysis has been in routine use in the forensic area for over 20 years. The technology has constantly improved and evolved every few years or so and is still evolving.

Most people know that DNA is a complex chemical found in the cells of the human body. It contains genetic information which determines the physical characteristics of a person. This information is in coded form, half of which is inherited from each parent. The DNA of any individual is the same in all of their cells so for example the DNA recovered from a person’s blood will be the same as that found in his or her saliva or hair roots.

Sometimes body fluid (for example blood) is found at an investigation scene. Blood is a really good source of DNA and is therefore relatively easy to test against a sample taken from a suspect. Complex science is involved in the testing. A comparison can be made in relation to the peaks of, in effect, a graph line. Where there is a clear difference in the peaks between the two samples namely, the blood found at the scene and a sample from the suspect then it is easy to exclude the suspect. Where there are identical peaks then a mathematical calculation can be carried out giving a probability. It is impossible to say for certain that it is someone’s DNA by this testing but the odds of it being someone else’s is a billion to one.

Problems occur where the sample is not so clear cut for example it might be traces of sweat. Often in such cases the sample contains a mixture of a number of people’s DNA. A number of people may have touched the same item and left the DNA sometimes as a result of mixtures. The results become very complicated indeed because you cannot tell which person contributed which peak to the testing result. Science has reached the stage where they are able to give good percentage results for a mixture of three persons’ DNA at low levels. However, if there is more than that it becomes questionable. The courts are left with very difficult decisions where all that can be said is that you cannot exclude a suspect from having touched the weapon. Sometimes people think that this means that there is evidence that they did touch it i.e. that their DNA was on the murder weapon. This is not correct and good lawyers who understand this science can deal with it. The first part of this note therefore answers the question which I have asked above.

The second part of this note deals with transfer of DNA. Let us assume we have a position whereby low level mixed DNA has been found on an item. It could have got there in more than one way. A suspect’s DNA could be on the item because he touched it. A suspect’s DNA could be on the item because he has touched someone else and that someone else has touched the item thereby in effect mixing his own DNA with the suspect’s DNA. It is possible that the suspect may have touched an article and then another person touched that article and then touched the item in question. The two incidents of non direct contact are often called secondary and tertiary deposits.

There are very few experts in this field and there are even fewer experts capable of carrying out the complex mathematics of evaluating probabilities.

I hope it will become apparent from this article that I am familiar with both the science and the law. If you are involved in a case which relies on the interpretation of complex DNA evidence, please contact me