Forensic Week

We’re all different from one another whether it is through our DNA, our fingerprints or other physical features. Forensic scientists use these physical differences to try and identify people. We can use different parts of the body to do this. For example, from a person’s hands, we can look at their fingerprints or their knuckle creases for similarities and differences.

During week 6 you learnt that the similarities and differences between you, your family and everyone else are determined by your genes. Genes are made up of DNA (can you remember what DNA is made up of?). Sometimes we leave DNA behind – we might leave it when we drink from a cup or on a door we’ve touched. Forensic Biologists can compare DNA from a crime scene to determine if it is the same as the DNA from a particular person. To do this they need to collect the DNA, extract it from the cells that contain it and then analyse the extracted DNA. DNA profiles look like peaks on a graph where the combinations of different groups of peaks are different for different people.

You can extract your own DNA at home so that it is visible to see in today’s activity.

Forensic chemists can analyse materials such as glass, paint and ink as well as identifying drugs and explosives. They sometimes need to separate the chemicals in a material in order to do this and they often use a process called chromatography. For example, the ink used in a pen is made up of different chemical molecules which can be separated from each other, as can other chemical mixtures such as paint or dyes used to colour fibres. Separating out the chemicals that make up a material allows the forensic chemist to compare materials with others that may look the same but may have different chemical compositions.

When people or things come in contact with each other, small amounts of material can be transferred. This could be for example, DNA or fibres from clothes, or soil from shoes. Transferred materials may stay (or persist) on the surface they have moved onto (such as a jumper or a chair). When forensic scientists examine pieces of clothing or shoes or other objects they will collect these transferred materials and examine them to try and work out what they are and where they may have come from.

Forensic scientists can look at particular shapes of features that can be used to identify people (for example fingerprints) or can link objects to each other (for example the pattern on the sole of your shoes). Forensic scientists look at the different details in fingerprint patterns or in footwear marks (also called shoeprints) to work out whether a fingerprint could have been left by a particular person, or a shoe could have made a specific footwear mark. In these examples the decisions made about whether the patterns match each other are mostly subjective which means that they are opinions rather than objective decisions which are based on measured facts. Whether an opinion is objective or subjective can be very important in forensic science and when presented as evidence in the courts.

Different types of evidence can have different value based on whether the evidence (such as DNA, fingerprints or knuckle creases) could come from an individual person or an individual item (like a well worn shoe) or whether evidence such as fibres or ink could have come from many items which might be mass produced like jumpers and pens. Members of a jury have to work out the value that the forensic evidence provides in helping them to decide if an accused person committed a crime or not.