Human and animal hair in forensic evidence: Problems, troubleshooting and workarounds

During the course of a criminal investigation, many types of physical and biological evidence are encountered. One of the most common is hair evidence. The identification and comparison of human and animal hairs can be helpful in demonstrating physical contact with a suspect, victim and crime scene. A common forensic approach should proceed from the morphological and microscopic examination to DNA analysis. The nondestructive techniques (morphological and microscopic examination) are a vital step when it is necessary to exclude the synthetic origins of the fibers and to distinguish between human and animal hairs. But the technique is also important to compare the features of the questioned hairs with those of hairs from a known individual. Human hairs are easily distinguishable from those of other animals because they present some typical microscopic characteristics. Today, nuclear and mitochondrial (mtDNA) testing can provide additional information to microscopic examination. In human hairs, routine forensic method for DNA testing (STR analysis of nuclear DNA) is possible when the root and/or adhering tissue are present. However, shed hairs, which are often associated with a crime scene, may harbor no nuclear material. While the nucleus degrades as the hair shaft hardens during keratinization, cellular mitochondria and mtDNA remain relatively intact, making mtDNA analysis of hair shafts possible. Instead the microscopic identification of the species of animal hairs can be complicated because of a general trend of reduced training of and lack of experienced hair examiners. In this case the findings of microscopy may be inconclusive to discriminate one species from the others. When conclusive results about species cannot be performed through microscopic examination, the DNA analysis offers the best opportunity to answer the question: "What species is this?". In this case the mtDNA sequencing analysis has to be not only useful in degraded remains but also in the identification of the species. In this review we report the main applications of human hair analysis to forensic investigations and the recent development in species identification of hairs found at the crime scenes. Moreover I describe the new prospects offered by the new generation sequencing methods (NGS) and the genomewide association study applied to hair shaft in forensics science