Biology of DNA
DNA Fingerprint
Since DNA was revealed to be the biological molecule responsible for making us unique, it’s uses as a mechanism to establish human identity and biological relationships have been widely discussed. However, it wasn’t until the discovery of the first VNTR (Variable Number Tandem Repeats) probe by Prof. Alec Jeffreys of Leicester University in 1984 when the first practical testing system became available – the DNA Fingerprint.
The term "DNA fingerprinting" was coined by Prof. Alec Jeffreys to allude to the traditional use of fingerprints as a means of human identification. As with conventional fingerprints, where various loops and ridges are compared between individuals, DNA testing relies on comparing certain DNA features called DNA markers between two (or more) DNA samples and studying similarities between them.
Two factors affect the accuracy and reliability of results obtained by DNA fingerprinting – the number of DNA markers tested and their nature. The more markers analysed, the more powerful the results of DNA analysis. There are various types of markers but currently the most widely used are Simple Tandem Repeats (STR) also called Simple Sequence Repeats (SSR) or microsatellites.
STRs (Fig.) are short, 2 to 6 nucleotide identical segments of DNA (e.g. ATG) which are aligned head to tail in a repeating fashion (Fig. A). These repeated DNA sequences are highly variable and their combination is unique for each individual.
Current methods of STR analysis are based on the Polymerase Chain Reaction (PCR) process. PCR can amplify a desired DNA sequence hundreds of millions of times in a matter of hours and is a widely used technique for the selective amplification of a particular DNA sequence of interest. It is the rapid amplification of DNA that makes PCR an indispensable tool for cases when the amount of biological material is minute (e.g. droplet of blood, a single hair strand etc).
Applications of DNA Fingerprinting
If DNA patterns between the two samples are identical, then they are likely to come from the same person. If the profiles are not identical but big similarities are found then the samples most probably come from related individuals. If population frequencies of the DNA markers analysed are known then it will be possible to determine the exact nature of relatedness.
When analysing DNA for relationship testing you are determining the chances of two individuals sharing the same DNA marker because they have a common ancestor, as opposed to chance. Because relatives have only a fraction of genetic material in common, in order to determine relatedness with a high degree of confidence more markers need to be analysed than for identity testing. Usually for paternity testing 16 markers are analysed routinely while for more complex relationships 30-35 markers are tested.
