DNA forensics and seduction by television
Discussion
Police drama on TV (especially) gives the impression that DNA analysis takes minutes. Well, it doesn't.
Forensic DNA analysis of sexual assault evidence requires differential extraction of the DNA from sperm and vaginal epithelial cells (to produce separate male and female fractions of DNA), PCR amplification, and separation of the PCR products. This takes time and is a somewhat imperfect method (but it's the best so far). Some DNA other than from the alleged rapist's sperm will likely contaminate the tested sample. Where more than one man has been involved, the sample will be a combination of all the alleged rapist's DNA, the victim's DNA, the DNA from others with whom both the victim and the alleged rapists have had sex recently and others with whom they may have come (no pun intended, BTW) into contact. Separating these out takes a not-inconsiderable time.
Comparing samples of DNA from the victim and from the alleged rapist(s) also takes time.
These times are (AFAIK) measured in not much less than two days. The more complex the evidence, the longer the time. The longer the time, the more reliable the evidence.
This morning the main UK TV news programmes are claiming that two fishermen arrested in Thailand within the past 24 hours (apparently) have been confirmed as the rapists of murdered tourist Katherine Horton by DNA evidence. It is unlikely that DNA could have been properly analysed in such a short space of time. For completeness, it has also been reported that they have confessed to her murder.
Once again the media fail to question the veracity of such reports. This plays into the hands of Nu Labia who must rejoice every time DNA evidence is even suggested as "solving" a crime as this is interpreted and presented as supporting the case for the National ID Register.
Streaky
Forensic DNA analysis of sexual assault evidence requires differential extraction of the DNA from sperm and vaginal epithelial cells (to produce separate male and female fractions of DNA), PCR amplification, and separation of the PCR products. This takes time and is a somewhat imperfect method (but it's the best so far). Some DNA other than from the alleged rapist's sperm will likely contaminate the tested sample. Where more than one man has been involved, the sample will be a combination of all the alleged rapist's DNA, the victim's DNA, the DNA from others with whom both the victim and the alleged rapists have had sex recently and others with whom they may have come (no pun intended, BTW) into contact. Separating these out takes a not-inconsiderable time.
Comparing samples of DNA from the victim and from the alleged rapist(s) also takes time.
These times are (AFAIK) measured in not much less than two days. The more complex the evidence, the longer the time. The longer the time, the more reliable the evidence.
This morning the main UK TV news programmes are claiming that two fishermen arrested in Thailand within the past 24 hours (apparently) have been confirmed as the rapists of murdered tourist Katherine Horton by DNA evidence. It is unlikely that DNA could have been properly analysed in such a short space of time. For completeness, it has also been reported that they have confessed to her murder.
Once again the media fail to question the veracity of such reports. This plays into the hands of Nu Labia who must rejoice every time DNA evidence is even suggested as "solving" a crime as this is interpreted and presented as supporting the case for the National ID Register.
Streaky
The methods that you described are very straight forward and could be done easily in 24 hours. The only problem I have with such a short time frame is the obvious lack of repeats. I'm not a forensic scientist but when we do genotyping of transgenic animals and studying polymorpisms in patients I won't trust a result until I have at least 3 completely independent data sets. This case looks like a rushed job. I good solicitor would have a field day with shakey forensic evidence..... then again what's the chances of these guys getting a solicitor at all let alone a good one?
You are right in thinking that governments like to trump DNA evidence as the solution to crime/terrorism. Its a powerful technique but should only ever be used to build a body of evidence rather that the whole case!
>> Edited by MilnerR on Monday 9th January 12:22
You are right in thinking that governments like to trump DNA evidence as the solution to crime/terrorism. Its a powerful technique but should only ever be used to build a body of evidence rather that the whole case!
>> Edited by MilnerR on Monday 9th January 12:22
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