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Forensic DNA Samples Used in DNA Testing



Dna test twins

There are possible hundreds of viable sources of DNA. Genetic analysis can be carried out on samples that are extremely degraded using the most advanced DNA testing methodologies available. PCR, or polymerase chain reaction, is a technique of DNA amplification and replication that enables scientists to work with tiny and often degraded samples. But how likely is it that the sample I want to use for my DNA test will work? The following are some important issues to consider:

How old is the sample? Older samples are less likely to produce a complete DNA profile than recently collected sources of genetic material

How has the sample been collected? Did you touch the sample with your hands? Is there any chance the sample might have been contaminated with somebody else’s DNA?

Has the sample been subject to any chemicals such as fabric conditioners or detergents? Has it been subject to any extreme temperatures?

All the above points are extremely important to consider and how likely the sample will yield a complete DNA profile depends on each of the above points.

Hair for a DNA test

Hair DNA testing is only feasible if the hair supplied has the root attached. The hair root is easily identifiable as it appears to the naked eye as a characteristic white ball at the tip of the hair. DNA is extracted from this hairball. If you are able to provide anything between 4 and 6 hairs, each with the follicle attached, you can expect the chances of success, in terms of extracting a complete DNA profile, to be of around 65%. You might also want to note that when hairs fall out of our bodies, they do not fall out with their root. In other words, cut hairs or hairs that have fallen out of the body (found lying around in the shower) will not be suitable for a DNA test.


Blood is a great sample to use if it is collected by a doctor and placed into an appropriate blood collection tube. However, it is unlikely that you would be able to carry out a ‘discrete’ or home paternity test by collecting a sample in this way. The chance of successful DNA extraction from a medical blood draw is around 99%. However, you will have problems with packing and posting the sample to the laboratory as blood cannot be sent as a regular parcel. It is considered a hazardous substance and will require appropriate packaging, labeling, and official authorization.
Bloodstains on the other hand are far easier to send off for laboratory testing. The chances of success with a bloodstain are however lower and stand at around 75%.

DNA testing with Urine

There is DNA in the urine. But urine is composed of urea, ammonia, and uric acid and water. The medium in which DNA is found in urine is thus problematic. DNA degrades very quickly in such a chemical solution. It may be possible to extract a DNA profile from a urine sample but the urine would need to be tested immediately. The chances of success would be extremely low. If the urine sample were to be collected from a toilet bowl it would be impossible to use for a DNA test – the loo is most likely contaminated with bleaching agents and other strong chemicals and moreover, the urine would be heavily diluted meaning the volume of genetic material would be even smaller.

A used Glass

We often see the scene on a thriller or detective film: a lip mark on a glass becomes a source of evidence, placed into a sterile plastic bag, and sent off for laboratory analysis. So how is the DNA on a lip impression on a drinking glass analyzed? Well typically, the part where the lip mark is evident is rinsed with distilled water. Following this, a swab is gently rubbed to and fro at the point where the person’s lip has come into contact with the glass. The rubbing of the swab against the glass should collect a sample of the drinker’s cheek cells.