DNA testing works by detecting certain parts or ‘markers’ in the DNA molecule that are known to vary greatly between unrelated people. Our tests compare the DNA at a number of these markers to determine if people are related. The process is called Short Tandem Repeat (STR) profiling. STR profiling is a very sensitive DNA testing procedure. DNA is extracted from the sample and a special enzyme is used to copy the genetic code at the relevant regions in the DNA. An STR profile is the result of several separate DNA tests.
In each STR DNA test performed up to two DNA markers are observed in an individual’s DNA profile. One marker will have been inherited from the mother and the other marker will have been inherited from the father. By comparing the child’s DNA profile to the mother’s DNA profile we are able to determine the maternally inherited markers in the child’s DNA profile.
The remaining markers in the child’s DNA profile must have been inherited from his/her father. If the tested man is the biological father of the child then he is expected to match all or all but one (or in very rare cases all but two) of the child’s paternal markers.
If three or more tests are observed where there is no matching DNA marker between the tested man and child then this result excludes him from paternity. In cases where the man tested is excluded from paternity, the samples are re-tested as part of our quality procedures before the report is issued. If the tested man is excluded from paternity then the report will state that the tested man is not the biological father of the child.
Paternity testing without the mother
When paternity testing is performed without the mother, certain assumptions must be made which reduce the statistical certainty of the conclusion. Without the mother’s sample it is assumed that a matching marker between father and child is the child’s paternal marker and the result is consistent with a father and child relationship.
There is a very small chance that the addition of the mother will alter the original DNA result obtained if the result is consistent with a parent and child relationship. However, if the DNA result excludes the father then addition of the mother will not change this DNA result.
Very occasionally, a natural change in one of the DNA markers may be detected which means that it does not match as expected. This complicates the analysis but generally, because many markers are tested, it does not significantly impact the statistical result in cases where both parents are tested. However, if only one parent is tested, this can lower the statistical results so that it is only possible to confirm that the two individuals are closely related, rather than conclusively confirming the nature of that relationship.
Paternity testing when the brother or father of the tested man could be the father
It is expected that the biological father will match all of the paternal markers in the child’s DNA profile. If the tested man is related as the child’s uncle then it is expected that there will be a number of tests where there are no matching paternal markers. If the result is consistent with the tested man being related as the child’s biological father then there will be two statistics quoted in the report:
If the tested man is excluded from paternity then the DNA report will state paternity is excluded. However, the DNA report may include a statistic stating the likelihood of the tested man being a close relative of the child e.g. uncle or grandfather to the child.
Sometimes no relationship can be detected between the tested man and child. Whilst this may indicate that he is unrelated to the child it is possible that the true nature of the relationship is a close relative such as uncle or grandfather but there are not enough shared markers between them to be able to detect this relationship.
DNA relationship testing, where alleged siblings are tested, is not as conclusive as parentage testing. This is due to the nature of the inheritance of DNA markers. On average, full siblings will share more DNA markers than half siblings, who in turn will share more DNA markers than unrelated individuals. However, due to the nature of inheritance, this analysis can only give an indication of the relationship - it is not conclusive.
When the father is not available for testing it is possible to test the paternal grandparents and if the mother, child and both paternal grandparents have been tested, the child’s paternal markers can be identified and compared with the DNA profiles of the grandmother and grandfather.
When only one grandparent is tested and a relationship is detected then the statistical result obtained in such cases is significantly lower than that obtained in parentage testing.
This test relies on the child’s parent and the aunt or uncle sharing enough DNA markers as siblings and then these same matching markers being inherited by the child. This type of analysis can only provide an indication of the relationship and is not conclusive. In some cases it may not be possible to detect a relationship even though one may exist. The DNA statistical results quoted in the report will be significantly lower than those obtained from parentage testing.
Combined Paternity Index
This is a ‘likelihood ratio’. It is a statistical comparison of how many times more likely it is that the tested individuals are related as claimed, against being unrelated or related as claimed against another relationship. The statistical result is a likelihood ratio calculated from the frequency of the matching DNA markers in the general population.
Please note, when more than one child is tested, the combined paternity indices can be quite different between the children. This is because they do not inherit the same DNA markers from their parents unless they are genetically identical twins.
Probability of Paternity
This is calculated from the combined paternity index and is expressed as a percentage.
Prior probability 0.5
In order to calculate a Probability of Paternity it is necessary to make certain statistical assumptions. A prior probability of 0.5 is the standard approach used in relationship analysis since it assumes that before considering the DNA results, it is equally likely that the man tested is, or is not the father.
A rare genetic change
A child will usually inherit its DNA markers from its parents unaltered. Occasionally a natural change in a marker can occur when it is inherited. Scientifically this type of change is known as a mutation. Any such apparent mutation is taken into consideration in our calculations, in accordance with accepted scientific standards and has the effect of reducing the combined paternity index.