Although the results of DNA tests are reputed to be extremely accurate, there are certain problems that can, and do sometimes arise. In fact, because there are DNA testing issues and problems, it is very important to get an objective analysis of any DNA evidence, to make sure that no problems do arise.
One of the most common problems that can arise in DNA testing is a DNA mixture. This is when one or more people have contributed to a sample and three or more alleles are likely to be seen in at least one of the loci that are tested. These mixtures can be confusing and are sometimes subject to a few interpretations.
DNA testing issues and problems can also arise from degradation. When a sample is too old or exposed to harsh conditions, it can degrade – this can many times be useful information for defendants in a court case. When degradation occurs, the amplicon size increases, causing peak heights to fall progressively. Determining peak height imbalances becomes difficult to interpret, due to loss of peak height information. Allelic dropout also occurs in degraded samples.
Stutter peaks also cause DNA testing issues and problems. These stutter peaks are found in nearly every electropherogram and are small peaks that happen either immediately before or after a real peak. In DNA testing, there is a process known as the PCR amplification process. In the PCR amplification process, the polymerase can lose its place when copying a strand of DNA, and normally slips forwards or backwards four base pairs. This means that, instead of the real fragment being amplified, there is a small amount of DNA fragment copies that are either one repeat smaller or larger.
Peak height imbalance can also cause DNA testing issues and problems. These imbalances are calculated in heterozygous loci (they contain two alleles). If the difference in the height of the two peaks is greater than 30% it is generally considered that a peak height imbalance has occurred. Taking into consideration that a sample from one person should contain peaks that are almost equivalent in height, an imbalance of peaks means that the sample is a mixture.
Sometimes, a blob of dye appearing on the electropherogram output tends to show up as a wide shape, and can cover a true allele. Air bubbles, sample contamination, and urea crystals can also be the cause of large peaks that can cover the contributions made by different contributors or can even be mistaken as contributions made by either a victim or a suspect.
Pull-up, sometimes known as bleed-through, spikes, or EPT data problems, and raw data problems, are more DNA testing problems and issues that have to be dealt with efficiently in order to avoid potential adverse DNA testing results.