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Why A DNA Test Result Might Be Inconclusive

posted by DNA Identifiers @ 1:56 PM
Friday, September 11, 2009

By Jessica

Why a DNA Test Result Might be Inconclusive Most of the time, a DNA test will provide a conclusive result, whether it confirms a biological relationship or identifies an unknown sample. Sometimes, however, an inconclusive result may be obtained. Why might that be? There may be many reasons, but here are several.

First of all, some samples may not yield sufficient DNA profiles, which would mean that there would be a lack of sufficient data for use in calculating a conclusive result. Insufficient data may tend to come from non-standard or forensic evidence samples, which may be old or degraded. Time and storage conditions can greatly affect the viability of DNA samples. Usually, new samples must be obtained, if available, in order to determine a conclusive result or, perhaps, other, more sensitive testing methods may be utilized.

In a parentage test, specifically one between an alleged parent and child only, a mutation can sometimes be found. Mutations are differences in alleles, which usually vary by one unit, i.e. alleged parent has a 10 allele and the child has an 11 allele. Known mutations have a specific frequency in any given racial population. When that frequency is figured into the Probability of Parentage, it will lower that percentage and sometimes can cause it to fall below 99%. In this instance, it would be recommended to have the other, known parent provide a sample, or if that is not possible, perform extended testing to see if the

Probability of Parentage increases.

In other relationship tests, like those between siblings or grandparents and grandchildren, the

Probability of Relationship may fall into an inconclusive range. This is because these individuals have a second degree relationship and will likely share fewer similarities in their DNA profiles than would a

parent and child
(first degree relationship). If, for example, a siblingship test resulted in a 70%

of Relationship, it would be considered “uncertain” that the individuals would be related as such. If the individuals were, in fact, biological siblings, testing of their parent(s) would result in an increased

of Relationship, thereby confirming the fact that they were siblings. Extended testing of additional markers might also be an option for increasing

of Relationship in a situation like this.

Lastly, contaminated samples can be cause for inconclusive results. If a sample is contaminated, showing DNA from more than one person, a mixed DNA profile will result and may be reported as inconclusive, depending upon what the situation is. In forensic cases, contaminated samples are common, such as in the case of a perpetrator/victim scenario. Known DNA samples from the perpetrator and victim would have to be obtained and tested to determine whether they could be contributors to the mixed DNA profile. Samples can also become contaminated by handlers or improper storage and/or exposure.

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