Guest Blogger: Karl M McDonald: DNA Testing Methods, Part 1

20 Jan

Method used in Forensic Analysis and Crime investigations

CSI and the many other forensic fictions we so often see on TV have spurred an interest in forensic DNA testing to solve criminal investigations. However, these TV serials also sometimes provide incomplete or inaccurate information and details about the genetic aspect of forensic investigations. Moreover, we rarely get to get a glimpse into some important factors live the validity of a DNA sample and the method chosen for laboratory analysis. Let’s take a look.


Blue research photo


Methods of DNA analysis 

Any suspected trace of genetic material at a crime scene needs to of course be analyzed in a laboratory. Nuclear DNA is relatively stable due to the strength of hydrogen bonding in base pairs on the helical structure of DNA. However, there are some external factors which come to play and which can affect the validity of the forensic DNA sample:

1–The type of genetic samples (whether it is blood, semen, nails clippings or hair. Different samples provide different chances of successful extraction of a DNA profile)

2–The age of the sample and the conditions, environmental and chemical, to which it has been subject. Has the sample been exposed to very high temperature? Have any caustic agents been used on it?

3–The way in which the sample is collected. The forensic team must be scrupulous and meticulous, following protocol so as not to contaminate the sample. The Meredith Kercher case in Italy is a good example of how callous forensic sample collection can lead to unviable results.

Whilst we are brought to believe that DNA testing is infallible, the truth is somewhat different. The criminal justice system and the individuals that make it up may not be fully aware of the complications and intricacies of DNA evidence. There may be misevaluation of forensic evidence by lawyers involved. Moreover, the statistical calculations undertaken by genetic testing facilities ignore or often fail to exclude two possibilities:

  • A possible match between the DNA profile extracted from the suspect and the real perpetrator of the crime.
  • Whether the possibility exists of the laboratory concluding a match between the profile of the suspect and the forensic evidence at the crime scene, when in fact, the match is not complete.

The following are the two main methods used in forensic analysis

RFLP or Restriction Fragment Length Polymorphism is a method of analysis that is based on the comparison of variations in DNA sequences. This method of analysis is used less frequently since it is only possible if large quantities of DNA are available. This means that tiny blood stains or single hairs will not be suited for this type of analysis.

PCR or Polymerase chain reaction is a method of DNA replication and amplification which enables scientists to create thousands of copies of DNA. This makes it a much more effective method when compared to RFLP as it makes sample analysis possible even with tiny, degraded quantities of DNA.

Mitochondrial DNA analysis is used in cases where there is no viable nuclear DNA. Mitochondrial DNA is a special type of DNA found in a different cell component to nuclear DNA. This type of DNA is exclusively passed on down the maternal line. Mitochondrial DNA is extremely stable and there are moreover many more copies of this existing in the cells when compared to nuclear DNA.


Bio: Karl M McDonald is a free lance writer specializing in the field of genetics and DNA testing. Articles by the author can be found on many blogs and info sites, including the article knowledge base for homeDNAdirect.  Karl currently lives in West Sussex, UK with his wife, kids and 2 dogs.



Posted by on January 20, 2013 in DNA, Guest Blogger


3 responses to “Guest Blogger: Karl M McDonald: DNA Testing Methods, Part 1

  1. Teresa Reasor

    January 20, 2013 at 10:13 am

    Excellent information!!! Thanks so much for sharing your expertise.
    Teresa R.


  2. amorycannon

    January 20, 2013 at 8:31 pm

    Reblogged this on Amory Cannon and commented:
    Great information on DNA analysis for those who are interested.


  3. Clare K. R. Miller

    January 21, 2013 at 3:24 pm

    This is fascinating. I knew about mitochondrial DNA, but I didn’t know there were two different ways of analyzing DNA based on sample size. I’m definitely looking forward to the next part!



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