Low-Template DNA
The act of touching an object can transfer enough epithelial cells from the skin to the item to get a DNA profile. This is commonly referred to as “touch DNA”. This is not an especially good term, however, because it suggests a deliberate action and that it is done with the hands. The fact is that the act doesn’t have to be deliberate, nor does it have to be with the hands: It can be any part of the body.
In the laboratory, samples of this category are usually referred to as “low-template DNA”. This is due to the fact that there are fewer cells containing DNA in these samples than from other types of biological samples such as blood, semen, saliva, and tissue. The type of cell recovered in low-template DNA samples is predominantly dead surface skin cells that the body sheds both naturally and constantly.
From over a decade of experience, we've learned that there are six main factors that will determine whether an item of evidence will successfully yield a DNA profile:
1. Texture: Is the surface of the item textured or smooth?
Textured items have more nooks and crannies in which cells can get trapped. Smooth surfaces don’t and can’t trap the cells as well. Also, smooth surfaces are easy to wipe off.
2. Hardness: Is the surface of the item hard or soft?
Hard items can abrade more skin cells that soft items. (Think baseball bat versus pillow.)
3. Force: How much force is being applied to hold the item?
The more force required to grip the item, the more skin cells will be transferred. (Again, think baseball bat versus pillow.) This also applies to friction that can cause skin cells to be shed and transferred, like wiping your hands with a towel versus patting them dry.
4. Surface area: Is the surface area large or small?
A larger surface area means more contact and thus more transference of skin cells. Keep in mind that multiple small items can be added together, so while one .22 caliber shell casing doesn’t have a lot of surface area, ten of them do.
5. Duration: Is the contact time long or short?
The longer the duration of the contact, the greater the opportunity for skin cells to transfer to the item. Recent literature indicates that, in a non-stressful situation, approximately ten seconds of contact can transfer enough cells to be detected by today’s DNA analysis methods. Note that DNA is indifferent to time, so repeated contact with the same item over a long period of time can still deposit enough skin cells to yield a DNA profile.
6. Contamination: Could the item have been contaminated?
Contamination is always an unknown in forensic samples. For example, if a burglar wearing a mask and gloves breaks into a house and steals a necklace, he never actually touches the necklace. However, if the burglar puts the necklace in his pocket, the skin cells that were previously deposited on the inside of the pocket will transfer to the necklace, thus contaminating it with the burglar’s DNA. Hence, if the necklace is recovered, it may be possible to recover the burglar’s DNA from it. Likewise, if the jacket is recovered, the necklace’s owner’s DNA may have contaminated the pocket of the jacket.
The factors that give the best chances of DNA recovery are textured surfaces, hard surfaces, increased force, large surface area, and long durations of contact. If only one of these is present then there is a chance of obtaining a DNA profile. If more than one is present, then the chances of obtaining a DNA profile increase. However, if none of these factors are present, a partial DNA profile may still be possible. After decades of experience with DNA analysis, we’ve learned this to be absolutely true: You never know until you try.
There are other factors that can affect the successful recovery of DNA from touched objects, but these factors usually cannot be known to the collector, investigator, or analyst. The unknown factors include:
1. Does the donor have a skin condition that causes the donor to shed more skin cells than others, like dandruff, eczema, or psoriasis?
2. Was the donor sweating at the time, thus facilitating the transference of skin cells?
3. What conditions has the item been exposed to? Weather, time, and laundering can remove skin cells, while chemicals, heat, and humidity can destroy them.
4. How many people have handled the item? DNA analysis may only detect the last person – or all of them.
Finally, not all laboratories or analysts are created equal. Just because one tries and fails doesn't mean that another one will. Many analysts lack the training and/or expertise to locate and collect these types of samples. Some analysts/laboratories are overwhelmed by caseload and are therefore restricted in the number of samples that can be tested (some are even restricted as to the amount of time they can spend looking for the sample in the first place). Better collection and sample recovery is the key to getting good data.
In the laboratory, samples of this category are usually referred to as “low-template DNA”. This is due to the fact that there are fewer cells containing DNA in these samples than from other types of biological samples such as blood, semen, saliva, and tissue. The type of cell recovered in low-template DNA samples is predominantly dead surface skin cells that the body sheds both naturally and constantly.
From over a decade of experience, we've learned that there are six main factors that will determine whether an item of evidence will successfully yield a DNA profile:
1. Texture: Is the surface of the item textured or smooth?
Textured items have more nooks and crannies in which cells can get trapped. Smooth surfaces don’t and can’t trap the cells as well. Also, smooth surfaces are easy to wipe off.
2. Hardness: Is the surface of the item hard or soft?
Hard items can abrade more skin cells that soft items. (Think baseball bat versus pillow.)
3. Force: How much force is being applied to hold the item?
The more force required to grip the item, the more skin cells will be transferred. (Again, think baseball bat versus pillow.) This also applies to friction that can cause skin cells to be shed and transferred, like wiping your hands with a towel versus patting them dry.
4. Surface area: Is the surface area large or small?
A larger surface area means more contact and thus more transference of skin cells. Keep in mind that multiple small items can be added together, so while one .22 caliber shell casing doesn’t have a lot of surface area, ten of them do.
5. Duration: Is the contact time long or short?
The longer the duration of the contact, the greater the opportunity for skin cells to transfer to the item. Recent literature indicates that, in a non-stressful situation, approximately ten seconds of contact can transfer enough cells to be detected by today’s DNA analysis methods. Note that DNA is indifferent to time, so repeated contact with the same item over a long period of time can still deposit enough skin cells to yield a DNA profile.
6. Contamination: Could the item have been contaminated?
Contamination is always an unknown in forensic samples. For example, if a burglar wearing a mask and gloves breaks into a house and steals a necklace, he never actually touches the necklace. However, if the burglar puts the necklace in his pocket, the skin cells that were previously deposited on the inside of the pocket will transfer to the necklace, thus contaminating it with the burglar’s DNA. Hence, if the necklace is recovered, it may be possible to recover the burglar’s DNA from it. Likewise, if the jacket is recovered, the necklace’s owner’s DNA may have contaminated the pocket of the jacket.
The factors that give the best chances of DNA recovery are textured surfaces, hard surfaces, increased force, large surface area, and long durations of contact. If only one of these is present then there is a chance of obtaining a DNA profile. If more than one is present, then the chances of obtaining a DNA profile increase. However, if none of these factors are present, a partial DNA profile may still be possible. After decades of experience with DNA analysis, we’ve learned this to be absolutely true: You never know until you try.
There are other factors that can affect the successful recovery of DNA from touched objects, but these factors usually cannot be known to the collector, investigator, or analyst. The unknown factors include:
1. Does the donor have a skin condition that causes the donor to shed more skin cells than others, like dandruff, eczema, or psoriasis?
2. Was the donor sweating at the time, thus facilitating the transference of skin cells?
3. What conditions has the item been exposed to? Weather, time, and laundering can remove skin cells, while chemicals, heat, and humidity can destroy them.
4. How many people have handled the item? DNA analysis may only detect the last person – or all of them.
Finally, not all laboratories or analysts are created equal. Just because one tries and fails doesn't mean that another one will. Many analysts lack the training and/or expertise to locate and collect these types of samples. Some analysts/laboratories are overwhelmed by caseload and are therefore restricted in the number of samples that can be tested (some are even restricted as to the amount of time they can spend looking for the sample in the first place). Better collection and sample recovery is the key to getting good data.
