Effect of the activity in secondary transfer of DNA profiles

Abstract

Secondary transfer occurs when DNA is transferred from one object, surface or person to another, via an intermediate object/person [1].Currently, some lawyers are using the existence of this type of transfer as an argument to invalidate genetic analysis. In a recent case in Spain, the existence of secondary transfer was alleged from part of the defense to justify the presence of DNA of suspect on a specific object found in the crime scene.In the present work, an attempt has been made to confirm the presence of a person's DNA in an object, via an intermediate person. To test the hypothesis we used individuals who are very often in contact, such as couples. It is intended to assess if, in those cases, it is possible to obtain a genetic profile after performing a current activity such as washing hands and dry with a towel. Two experiments were performed. First, the primary transfer was evaluated. We asked 10 people (5 men and 5 women) with different daily activities, to wash their hands with neutral pH soap, and then dry their hands with a specific towel.In the second experiment, the secondary transfer was evaluated. Five couples were asked to hold their hands for 5 min. Then, they were asked to wash their hands with neutral pH soap, and dry them on a different towel each. This experiment was performed 3 times each pair. As a control, an unused piece of each towel was analysed, and saliva samples were taken from all participants, to perform genetic comparisons with the possible profiles found on the towels. DNA extracts were eluted out of the towels and genetic profiles were evaluated through the amplification of AmpFℓSTR®NGM SElect™Kit (Thermofisher™Scientific) kit markers.