0140 29466
info@alphabiolabs.ie
0 Items
The DNA Science
Sample Collection
Sample Collection is a critical part of the process. Most of the DNA samples processed for relationship testing are buccal cell samples, whereby a simple to collect, a cell sample is taken from the inside of the cheek. However, other biological samples can also be used. Sample collection is important, as not only does this provide the laboratory with the information on the test participants but a sample of poor quality can also lead to delays in reporting.
Dependent on the type of test required samples can be taken by the sample donor for “Peace of Mind” testing or for legal DNA testing an impartial third party is responsible for taking the sample. Who takes the sample is important, and for legal testing, it is essential that a “Chain of Custody” of the sample is maintained. A GP or nurse is the most common sample collector for a legal DNA test. This helps to maintain the “Chain of Custody”, which means that the sample can be proven to have come from the correct person and that it has not been tampered with. Furthermore, DNA tests submitted for legal purposes cannot be cheated or falsified by someone impersonating the true sample donor.
“Peace of Mind” DNA tests can be performed on samples collected and provided by the sample donors themselves with no proof that the samples are genuine. Peace of Mind tests is performed based on the information provided by the test participants and cannot be used for any legal purposes such as changing birth certificates.
The Laboratory Process
Once the samples have been received in the laboratory the first step is to extract the DNA from the sample. This process is dependent on the type of sample submitted, but essentially the aim of the process is to obtain pure DNA. Our laboratory uses one of the newest DNA extraction techniques available to process buccal cell samples ensuring a high level of accuracy and improved sample quality.
The high-quality DNA samples obtained during extraction are then subjected to Polymerase Chain Reaction (PCR). PCR is a biochemical technology in molecular biology which amplifies DNA extracted from a person’s buccal cell sample, generating thousands to millions of copies of a particular DNA sequence.
Our laboratory is one of only a few accredited to test 24 DNA Short Tandem Repeats (STR) markers as standard; most relationship tests are performed using only 16 STR markers. STR markers are repeated sequences of DNA which come in various sizes and are classified according to the length of the core repeat units, the number of repeat units, and the overall length of the repeat region.
DNA regions with short repeat units (usually 2-6 bp in length) are called Short Tandem Repeats (STR). The variations in length are known as alleles and it is this variation which makes relationship testing possible. STRs are found surrounding the chromosomal centromere (the structural centre of the chromosomes). STRs have proven to have several benefits that make them especially suitable for human identification.
STR Markers
STRs have become popular DNA markers because they are easily amplified by PCR without the problem of differential amplification; that is, the PCR products for STRs are similar in amount, making analysis easier.
Because of their smaller size, STR alleles can also be separated from other chromosomal locations more easily to ensure closely linked loci are not chosen. Closely linked loci do not follow the predictable pattern of random distribution in the population, making statistical analysis difficult. An individual inherits one copy of an STR from each parent, which may or may not have similar repeat sizes. The number of repeats in STR markers can be highly variable among individuals, which make these STRs effective for human identification purposes.
For human identification and relationship testing purposes, it is important to have DNA markers that exhibit the highest possible variation to discriminate between samples. Our PCR process specifically targets 24 STR markers within the genome (a person’s total DNA) which are informative for relationship testing. These DNA markers have been specifically selected to provide the highest discrimination available, meaning that the results generated provide as much information as possible to provide a conclusive result.
After PCR, each sample is profiled using a Genetic Analyser. This process separates the DNA fragments representing each of the tested STR markers by size and builds a DNA profile of the sample donor.
The DNA profiles produced in this way can then be used to perform Human Identification and Relationship analysis such as Paternity and Sibling testing.
Relationship tests are completed by comparing the observed DNA profiles from a group of people and considering the probability of any potential relationships. Paternity Tests are completed by comparing the STR markers seen in the child with those seen in the parents.
A child inherits 50% of its DNA from each parent; this inherited DNA includes the STR markers which are carried on the parents’ chromosomes. The child will carry a unique combination of DNA from the parents including the variations in length (alleles) of the STR markers.
In a Paternity Test each STR marker is compared to identify the alleles which have been inherited by the child from the parent and the Paternity index (PI) calculated; this is the statistical strength or weight of that marker in favour of or against parentage. The PI for each marker is then multiplied to give the Combined Paternity Index (CPI); the higher the CPI the more likely it is that the parents are the true biological parents of the tested child.
In general, when the true biological father is tested it is possible to identify the paternal allele at each of the tested STR markers within the child. When the tested man is not the true biological father, there will be several STR markers that do not match within the child’s profile.
The CPI value is used to calculate the Probability of Paternity. For a man to be confirmed or included as the true biological father a Probability of greater than 99.99% is required.
