5 Facts (and Caveats) About DNA Profiling

by Dr. Hsien-Hsien Lei
Posted October 24, 2007 in DNA and the Law, DNA in General

Reuters took the trouble to summarize five main facts about DNA profiling and I thought I’d comment on them.

DNA profiling was discovered in 1984 by geneticist Alec Jeffreys of Britain’s Leicester University, who first used it three years later to help solve the murders of Leicestershire schoolgirls Dawn Ashworth and Lynda Mann.

Sir Alec, a 2006 Great Briton, is actually credited with developing DNA fingerprinting specifically.

Here’s a very basic description of how DNA fingerprints were first created using restriction fragment length polymorphisms (RFLP):

1. Extract a sample of DNA from blood, hair, skin, cheek cells, etc.
2. Use polymerase chain reaction (PCR) to create more copies of the DNA to analyze.
3. Use restriction enzymes to cut the DNA strand at specific sequences, which will result in pieces of DNA of varying sizes. Since each person’s DNA unique, the enzymes will cut the DNA in different places and varying sizes of DNA pieces will result.
4. The small pieces of DNA will be passed through an agarose gel which separates them according to size.
5. Compare the DNA patterns created.

The technology makes use of the fact that small sections of DNA repeat themselves over and over in a way that is unique to each individual. The length of repeats can be measured at different locations to build up an individual’s profile.

There are actually different ways to create a DNA profile. The type of marker mentioned here is the short tandem repeat (STR). Another type of marker that can be used to create a genetic profile is the single nucleotide polymorphism(SNP).

Modern genetic tests typically look at 20 “marker” sections of DNA — 10 from the maternal and 10 from the paternal line. The chance of two strangers matching on all 20 is less than one in 1 billion. The chance of a match with a relative, though, is much higher and identical twins have identical profiles.

I’m not really sure what 20 markers this fact is referring to. Law enforcement relies on the Combined DNA Index System (CODIS) that creates DNA profiles from 13 core STR markers. Paternity tests examine as little as three markers to as many as 16. The genotypes at each of the markers would consist of data on two alleles - one from the mother and one from the father.

Scientists collect DNA direct from suspects, using a mouth swab, or from the scene of a crime, by gathering traces of blood, semen, hair or saliva. Crime scene samples are more difficult to analyze, since they may be very small and can contain the DNA of several people.

I don’t have much to say about this except that sometimes samples are just plain difficult to analyze whether or not it’s from a crime scene. It all depends on the sample from which the DNA is extracted and the informativeness of the markers being analyzed.

Governments around the world are building up DNA databases to match suspects with evidence. The United States has the largest database, with over 5 million profiles. But Britain has the highest proportion of people catalogued, with its 4 million records equal to more than 6 percent of the population.

As points of reference, Australia’s national DNA database has over 350,000 DNA samples while New York State has almost 250,000. You may also be interested in these previous posts about the UK national database:

• Be Fair - Add Your DNA to the Database
• Fake Names in the UK National DNA Database
• Police Want to Collect Abandoned DNA from Everyone

Tags: genetics, genes, dna, dna profiling, dna fingerprinting, crime

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2007-10-26 15:24:59

[...] Nobel Prize in Chemistry 1946 - “for his discovery that enzymes can be crystallized.” 5 Facts (and Caveats) About DNA Profiling - Commentary on five facts about DNA profiling that people should know. Like this? Click here to [...]

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