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Forensic analysis of hair samples in order to extract DNA is a method commonly used for the purpose of identification in both criminal investigations as well as parental DNA testing. It is in many ways, however, the most overestimated and misrepresented DNA sample. People often assume hair samples make ideal samples for DNA testing. This notion might perhaps be due to the way hair samples are so often seen collected in detective crime fiction series and how solving the crime invariably hinges on finding such a sample. Nevertheless, whilst hair samples can be accurately used for the identification of the perpetrator, the extraction of DNA from the aforementioned sample and its inherent success depends very much on the part of the hair at hand. To better understand the complexities and challenges of carrying out a DNA test on hair, it is mandatory to analyze its structure and composition. One also needs to understand the limited distribution of DNA in the hair as well as the type of DNA available in a given part of the hair (nuclear or mitochondrial DNA).
Structure and Composition of Hair
Hairs are mainly composed of a fibrous protein known as keratin. This protein is also the main constituent of skin, animal hoofs, and nails.
Hair consists of three concentric layers. The innermost layer is known as the medulla; it is a soft structure made of rounded cells and is sometimes referred to as the marrow. The middle part of the hair, known as cortex, is what contains the pigments that give hair its color and elasticity. The outer, scaly layer is known as the hair cuticle. This outer hard and transparent part envelopes and protects the inner parts of the hair and also gives hair that characteristic lustrous sheen. Despite the fact that cells do make up parts of the hair, these cells do not contain suitable material for a DNA test. The reason for this will be explained further on in this article.
The hair root is rooted below the scalp and is enclosed in a hair follicle. This is connected to the blood stream via the dermal papilla. Certain hormones and receptors are also found in the follicles which help to regulate hair growth.
Distribution of Nuclear and Mitochondrial DNA in Hair Samples
The hair fibers (sometimes referred to as the hair shaft) are actually formed by keratinocytes, types of skin cells which are responsible for the synthesis of the protein keratin. These cells naturally die and in the process get converted from cells into a horny, tough material such as hairs. This conversion process is known as cornification, and it involves the destruction and degradation of the cell nucleus (and thus, also of the genetic material enclosed within). As a consequence of cornification, the hair shaft, which is the part of the hair that protrudes out of the scalp, does not contain any nuclear DNA.
Although nuclear DNA cannot be isolated from the hair shaft since it is mostly absent as a result of the aforementioned cornification, nuclear DNA can successfully be extracted from the hair root. The hair root contains keratinocytes, cells which are ideal for the extraction of nuclear DNA. This is not to say that cut or naturally shed hairs are entirely unsuitable for hair analysis of nuclear DNA. In a tiny number of analyses using cut or shed hairs, forensic scientists are in fact able to extract nuclear DNA. The presence of some nucleated corneocytes (biologically dead cells or keratinocytes in their last stage of differentiation) may make it possible to extract a DNA profile derived from nuclear DNA. The existence of nucleated corneocytes is known to be due to an incomplete or absent step during the process of cornification which would normally result in the degradation or destruction of the cell nucleus and DNA. Why these nuclear remnants occur is not fully understood but the phenomenon may occur in some individual’s hair.
Highly advanced DNA testing techniques, resources, money, and time are all factors which come into play when carrying out a DNA test. Forensic laboratories used by the FBI have more resources to allocate toward trying to extract a complete DNA profile from whichever sample is available, no matter how small the probability of successful extraction of DNA is. Thus, in crime scene investigations where meticulous searches by forensic teams yield a single hair, without the root or follicle, thorough and extensive DNA analysis will still be carried out despite the very low chances of success.
On the other hand, companies and laboratories offering direct to consumer paternity testing or other types of relationship testing services do not offer the possibility of hair DNA testing unless the hairs have the root attached. Without this, the chances of success are far too low. They typically discount the possibility of successful analysis using hairs that have been cut or naturally shed, accepting only the testing of hairs that obviously have the follicle attached. Identification of the hair follicle can be done by close observation of the hair with the naked eye; the follicle appears as a small gray-white ball at the tip of the hair shaft. In some cases, a magnifying glass might be required depending on whether the observer has acute enough eyesight.
Uses of Nuclear DNA in DNA Testing
Genetic identification and profiling is only possible through analysis of nuclear DNA. Despite the possibility of remnants of nuclear DNA in the hair shaft (in cut hairs or naturally shed hairs), analysis with this specific part of the hair will likely not yield results.
The minute fragments of DNA present due to a partial breakdown during the cornification process are only visible under an electron microscope.
Even when hairs do have the hair roots, the probability of successful extraction of a complete DNA profile using standard PCR technology is somewhere between 60-70%. When hairs do not have the root attached, analysis can in rare cases yield a nuclear DNA profile. This is possible in cases where living hair cells are still present at the tip of the hair shaft or where, as mentioned, there has been an incomplete breakdown of nuclear DNA during the process of cornification.
Uses of Mitochondrial DNA Found in Hair
Mitochondrial DNA is found in cell mitochondria – cell organelles located in cytoplasm responsible for supplying the cell with energy. Mitochondrial DNA is far more abundant than nuclear DNA with a typical human cell containing many thousands of copies of MtDNA as opposed to just one single copy of nuclear DNA.
In many countries, governments have access to DNA databases where the profiles of thousands of prosecuted criminals are stored. The DNA profiles are however, nuclear DNA profiles and not mitochondrial DNA profiles. This again contributes to the increased distinguishing power of nuclear DNA in crime solving. Further to this, mitochondrial DNA has no discriminating power between individuals as all people sharing a common maternal blood line will also share the exact same MtDNA profile. This means that one cannot distinguish between individuals from the same maternal line based upon analysis of their respective MtDNA profiles as profiles will be identical between maternal blood relatives.
Success Rate of Hair Samples
The success rate in terms of extraction of DNA from any samples is dependent on a number of factors:
Chemical treatment using dyes can alter the cuticle. Dyes can easily penetrate the spaces between the scaly cells forming the hair cuticle or even raise them in order to be better absorbed by the hair. Peroxides, one of the main constituent chemicals in hair dyes, heavily contribute to the degradation of DNA in hair. Peroxides act by specifically breaking the phosphodiester bonds in DNA. Once the hair is exposed to water on washing, the DNA is easily washed out of the hair fibers. The higher the number of washes, the more DNA is lost from the hairs. This loss of DNA is not only due to the degradation and breaking down of the phosphodiester bonds in DNA but also to the damage caused to the hair by simply washing it.
We cannot discount the importance and possible effects of other factors which would affect any type of DNA samples: the age of the sample, the way the sample has been collected and stored, and any external forces that may have altered the state of the DNA (for example, exposure to very high temperatures, soaps and cleaning agents, or to corrosive substances).
- Whiting DA., Holland MM., 2001. Human hair histogenesis for the mitochondrial DNA forensic scientist. Journal of Forensic Science, 46 (4), p.844–853.
- Wilson M. R., Polanskey D., Butler J., DiZinno J. A., Replogle J., and Budowle B., 1995. Extraction, PCR amplification and sequencing of mitochondrial DNA from human hair shafts. BioTechniques, 18, p.662-669
- Linch CA., 2008, The ultrastructure of tissue attached to telogen hair roots. Journal of Forensic Science, 53 (6), p.1363–1366.
- Heywood DM, Skinner R, Cornwell PA.., 2003, Analysis of DNA in hair fibers, Journal of Cosmetic Science, 54 (1) p. 21-27
- Sandra Szabo S., Jaeger K., Fischer H., Tschachler E., Parson W, and Eckhart L., 2002. In situ labeling of DNA reveals interindividual variation in nuclear DNA breakdown in hair and may be useful to predict success of forensic genotyping of hair, International Journal of Legal Medicine, 126 (1), p. 63–70.
Caroline Hughes is a pharmacist currently working as a freelance writer. The author regularly contributes to a number of online blogs and Web sites delving into various, related topics including: forensic testing, pharmaceutical research, genetic illnesses, as well as pregnancy related topics. A number of articles, resources, and information written by the author can be found at: homeDNAdirect.co.uk.