How DNA Testing is Used to Determine Paternity and Other Biological Relationships

Published: 02nd May 2008
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In 1984 a discovery was made that highlighted the power of DNA as an indispensible tool for human identity and relationship testing. The technique - The DNA Fingerprint - was developed by Professor Sir Alec Jeffreys of Leicester University. 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.

The very first case that utilised DNA fingerprinting was Sarba Vs the Home Office, 1985, an immigration case where it was necessary to prove the direct biological relationship between Christianna Sarba and her son Andrew. By comparing Christianna's DNA sample against that of Andrew's and his three legally recognised siblings, a direct biological relationship was shown beyond a doubt.

Over the years DNA technology has progressed, and now modern techniques are so powerful that a profile can be created from a minute skin flake, a single hair or from an item handled by a person and then compared against another individual or database.

DNA fingerprinting has become routinely used for both criminal and non-criminal cases, with the aim of either establishing a match between a crime scene sample and a suspect or establishing a biological relationship between two or more individuals, the most common relationship being Paternity. With regard to a DNA paternity test, every child inherits half of its DNA from each parent i.e. 23 chromosomes from the mother and 23 from the father, this being the reason why there are two sets of alleles for each person.

Because biology is a weird and wonderful science, the inheritance patterns of the markers used do not have to correspond identically with one set/column of alleles from each parent, but rather be a combination of the two alleles of each parent. If the fathers held the a and B alleles for a particular gene and the mother C and D, the child could inherit A and C, A and D, B and C or B and D alleles.

As standard we test a system of 16 genetic markers, which is more than sufficient to establish a 99.99% inclusion probability (where the alleged father is actually the father) or a 100% exclusion probability (where the alleged father is not the father).

If there is suspicion that the alleged father could be one of two relatives (e.g. brothers) we can perform a more extensive test, looking at a greater number of markers to further differentiate between the putative fathers. The only instance where we would not be able to differentiate between individuals is if they were identical twins.
The need to perform a complex DNA relationship test arises most often in matters of child support and child custody or for immigration purposes, when:

  • A child's alleged father is unavailable but his immediate relatives are available for testing; or

  • Both the biological parents of the child are deceased or missing and a biological relative wishes to apply for custody of the child.

Since a child inherits half its DNA from each of its biological parents, we can trace certain markers to relatives of the biological parents, to determine if they contributed DNA to the child's genetic profile (presumably through the missing or deceased alleged parent or parents.)

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