Analysis: when we think of catching killers or solving crimes, we often think of DNA matching, but what does that actually mean?

DNA stands for deoxyribonucleic acid and is simply a long thin molecule made up of nucleic acids. DNA contains four nucleic acids: Adenine (A), Thymine (T), Guanine (G) and Cytosine(C). The order that these nucleic acids occur in your DNA is called a sequence. A length of DNA could contain over 50 million nucleic acids, but only the ones listed above, A, T, G, and C in various sequences and combinations. DNA can be found in the chromosomes in the nucleus of your body's cells. Humans have 46 chromosomes, 23 from their mother and 23 from their father. We conventionally assign these pairs of chromosomes numbers from 1 to 23.

Part of your DNA is responsible for different traits such as eye colour, hair colour, height etc. A segment of DNA that determines a certain trait is called a gene. There are large sections of DNA that do not determine traits and this area of DNA is known as non-coding region or even the "junk DNA" region. It is this non-coding region of DNA that is analysed by forensic scientists.

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From RTÉ Radio 1's Today With Claire Byrne, just what does a forensic scientist do? Interview with Chris Enright from Forensic Science Ireland

How is your DNA transferred?

A person can transfer their DNA to an object or surface if they drop their blood on the object after sustaining an injury, spit on the object or ejaculate semen onto a surface or someone. They can transfer their DNA if they touch the object or surface with their bare hands. They can also transfer their DNA by touching another person’s bare hand with their bare hand. The above transfers are referred to as primary transfer.

DNA can be left at a crime scene on a discarded plastic glove, balaclava, cigarette butt or the handle of a knife if it's used to kill or injure someone. It could also happen that the DNA which is initially transferred is subsequently transferred on to another object. This is referred to as secondary transfer or indirect transfer and is sometimes proposed by defence solicitors as the reason for the presence of the suspect’s DNA on an item.

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From RTÉ Brainstorm, how sharing your DNA information for a genetic profile may well have potentially harmful privacy infringements for you and your family

How is DNA analysed and compared

Once the DNA is recovered from the crime scene, it needs to be analysed in the laboratory. We need to go back to basics to understand how the DNA is analysed. In the forensic science lab, the scientist will look at a specific region (locus) on up 20 of the 23 the pairs of chromosomes.

At each locus, the analysis will essentially count a certain repeat pattern of the nucleic acid sequence in your DNA. For example, for chromosome pair number 1 at a specific locus, your repeat pattern could be 15 repeats on the chromosome that came from your mother and 17 repeats on the chromosome that came from your father. That would make you a 15, 17 at that locus. The same process happens for the other chromosomes being analysed.

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From RTÉ News, how new DNA tests may help in probe over IRA murders of RUC officers near Lurgan in 1982

Your final DNA profile could be something like this if 10 loci were examined,

(15,17), (9,10) (14,16) (13,18) (15,23) (19,21) (11,11) (12,16) (18,19) (16,17).

This process is undertaken for the crime scene sample and the reference sample from the suspect. The reference sample is obtained by getting a mouth swab from the suspect. The cells from the inside of the mouth are removed by a sponge swab and then sent to the laboratory. If we get the same set of numbers for the crime scene DNA and the reference sample then that is considered to be a DNA match.

The frequencies of repeat patterns at each locus are known from a population study. For example, the repeat pattern of 15 at chromosome 1 could have a frequency of 1/10 and the repeat pattern of 17 could have a frequency of 1/100. By multiplying out the frequencies at all 10 (or more) loci, we can get a frequency that is very very low. You can see how you can get to figures like one in a billion easily. The more loci that are looked at the lower the frequency values.

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From RTÉ Radio 1's Drivetime, Della Kilroy reports on an appeal from the Tuam Home Survivors' Network for the Government to arrange for DNA samples to be collected from them to determine if they match human remains discovered at the site.

If you have no specific suspect for a certain crime but do have DNA from the scene then you need to examine the DNA database to see if there is a match to someone already on the DNA database. If a suspect has been convicted of a certain crime such as burglary, their DNA will be stored on a database.

When DNA is recovered from a crime scence, the set of numbers (15, 17 etc.) is fed into the DNA database and the database is checked to see if there is anyone that matches this set of numbers. If there is a match, then the database essentially tells you who the person is that matches the DNA from the crime scene. While your DNA is what make you have blue eyes and red hair, it is also the means by which you will be caught if you are thinking of committing that perfect crime.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ