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DNA profiling
SOCIETY N E W S M E E T I N G S
Edited by AS Fawcett
DNA profiling A joint meeting of the Forensic Science Society and the British Academy of Forensic Sciences was held at New Scotland Yard on the evening of 30 June 1988. All available places for the meeting had been taken up within a short time of its announcement, such is the current interest in DNA profiling. The meeting began with a slide presentation by Mr T Howitt (Home Office Forensic Science Laboratory, Birmingham) and this was followed by questions from the audience answered by a panel consisting of Mr T Howitt, Dr D Werrett (Home Office Central Research Establishment, Aldermaston), Mr B Parkin and Dr B Sheard (Metropolitan Police Forensic Science Laboratory, London), and Miss M Pereira CBE, (former Controller of the Home Office Forensic Science Service). This report was prepared for the Society by Mr R Chapman. Mr Howitt started by outlining how antigen and enzyme variation has been used to link suspects to crimes. Progress has been made by getting more blood groups out of less blood stain. However, only when the sickle cell trait or the sex of a donor was detected in a stain could the police be directed towards a suspect. The double helix structure of DNA was described as consisting of units called bases strung together to form a code. This code is unique in each person excepting identical twins; it is 3000 million bases long and can be cut into lengths of various sizes by restriction enzymes. The profiling technique was then explained. After the DNA has been isolated from a stain and cut up, the fragments obtained are separated by size on an electrophoresis gel. The position of the DNA is marked by combination with a probe. The probe is a small length of DNA made of bases which will only bind the probe to complementary bases on the target DNA. The probe itself is radioactive so the position of fragments on the gel may be visualised by autoradiography. The result is a ladder-like series of bands. The bands are inherited and therefore can be used for determining paternity, and the probability of having a particular set of bands can be calculated. Depending on the condition of the DNA analysed, 4 to 16 bands may be obtained with a frequency of occurrence in the range 1 in 200 to 1 in 2 billion. @ Forensic Science Society 1988
37 1
The questions to the panel began with a topical enquiry about the amount of material required. A DNA profile can be obtained from a single buccal swab or 7 to 10 hair root sheaths. It was suggested that a picture of the suspect could be built up if probes to physical features such as sex or eye colour became available. The problem of recording results was discussed. DNA profiles can be stored on computer but this would only be useful if the type of probe were standardised nationwide. Some people were worried about slight technical errors. These cause no problems; a profile is so highly discriminating that the chance of two samples matching by accident is very small. However, there was no satisfactory answer to the question of defence experts reproducing results, because the probe currently in use has been patented by ICI. Despite the success of DNA profiling, traditional bloodgrouping would still be used for screening samples, and in cases where insufficient or no DNA was found. The meeting ended on an amusing note considering the case loads which exist in most laboratories. It was suggested that DNA profiling be conducted on the entire population!
Edited by AS Fawcett
DNA profiling A joint meeting of the Forensic Science Society and the British Academy of Forensic Sciences was held at New Scotland Yard on the evening of 30 June 1988. All available places for the meeting had been taken up within a short time of its announcement, such is the current interest in DNA profiling. The meeting began with a slide presentation by Mr T Howitt (Home Office Forensic Science Laboratory, Birmingham) and this was followed by questions from the audience answered by a panel consisting of Mr T Howitt, Dr D Werrett (Home Office Central Research Establishment, Aldermaston), Mr B Parkin and Dr B Sheard (Metropolitan Police Forensic Science Laboratory, London), and Miss M Pereira CBE, (former Controller of the Home Office Forensic Science Service). This report was prepared for the Society by Mr R Chapman. Mr Howitt started by outlining how antigen and enzyme variation has been used to link suspects to crimes. Progress has been made by getting more blood groups out of less blood stain. However, only when the sickle cell trait or the sex of a donor was detected in a stain could the police be directed towards a suspect. The double helix structure of DNA was described as consisting of units called bases strung together to form a code. This code is unique in each person excepting identical twins; it is 3000 million bases long and can be cut into lengths of various sizes by restriction enzymes. The profiling technique was then explained. After the DNA has been isolated from a stain and cut up, the fragments obtained are separated by size on an electrophoresis gel. The position of the DNA is marked by combination with a probe. The probe is a small length of DNA made of bases which will only bind the probe to complementary bases on the target DNA. The probe itself is radioactive so the position of fragments on the gel may be visualised by autoradiography. The result is a ladder-like series of bands. The bands are inherited and therefore can be used for determining paternity, and the probability of having a particular set of bands can be calculated. Depending on the condition of the DNA analysed, 4 to 16 bands may be obtained with a frequency of occurrence in the range 1 in 200 to 1 in 2 billion. @ Forensic Science Society 1988
37 1
The questions to the panel began with a topical enquiry about the amount of material required. A DNA profile can be obtained from a single buccal swab or 7 to 10 hair root sheaths. It was suggested that a picture of the suspect could be built up if probes to physical features such as sex or eye colour became available. The problem of recording results was discussed. DNA profiles can be stored on computer but this would only be useful if the type of probe were standardised nationwide. Some people were worried about slight technical errors. These cause no problems; a profile is so highly discriminating that the chance of two samples matching by accident is very small. However, there was no satisfactory answer to the question of defence experts reproducing results, because the probe currently in use has been patented by ICI. Despite the success of DNA profiling, traditional bloodgrouping would still be used for screening samples, and in cases where insufficient or no DNA was found. The meeting ended on an amusing note considering the case loads which exist in most laboratories. It was suggested that DNA profiling be conducted on the entire population!