DNA Profiling in Human Identification: From Past to Present

Abstract

Forensic DNA typing has been widely accepted in the courts all over the world. This is because DNA profiling is a very powerful tool to identify individuals on the basis of their unique genetic makeup. DNA evidence is capable of not only identifying the presence of specific biospecimens in a crime scene, but it is also used to exonerate suspects who are innocent of a crime. Technological advancements in DNA profiling, including the development of validated kits and statistical methods have made this tool to be more precise in forensic investigations. Therefore, validated combined DNA index system (CODIS) short tandem repeats (STRs) kits which require very small amount of DNA, coupled with real-time polymerase chain reaction (PCR) and the statistical strengths are used routinely to identify human remains, establish paternity or to match suspected crime scene biospecimens. The road to modern DNA profiling has been long, and it has taken scientists decades of work and fine tuning to develop highly accurate testing and analyses that are used today. This review will discuss the various DNA polymorphisms and their utility in human identity testing.

Keywords: DNA typing; chromosome; likelihood ratio; match probability; microsatellite; minisatellite; mitochondria; polymorphism.

Figure 1

Development in DNA techniques that facilitated the development of DNA profiling. Since the discovery of the structure of DNA, various developments in techniques to characterise DNA have facilitated the discovery of DNA profiling methods

Figure 2

Unequal cross over during meiosis

Figure 3

Strand-slippage replication resulting in addition or reduction in STR number. Also known as DNA slippage or slipped strand mispairing or DNA polymerase slippage. Most studies favour slippage as the main cause for STR mutation process. The mispairing in the region of repeat units between template DNA and the newly synthesised DNA may occur during DNA replication resulting in the addition or reduction of STR number

Figure 4

Identification of VNTR using a labelled MLP

Figure 5

General principle of the STR technique for DNA profiling. Unknown DNA samples are extracted and quantified before being amplified by PCR using specific STR primers labelled with different fluorophores. The amplified DNA samples are then electrophoresed in capillaries containing polymer to separate the labelled DNA according to size in an automated fragment analyser/gene sequencing machine. The fluorescently tagged DNA molecules are detected and the software in the genetic analyser translates fluorescence intensity data into electropherograms against a set of allelic ladders as reference. The software then labels peaks that meet certain criteria with allelic designations and the resulting electropherogram contains genotype of the individual/biospecimen for the tested STRs

Figure 6

Heteroplasmy in mtDNA. The maternal transmission of mtDNA results in homoplasmic individuals, who typically have a single mtDNA haplotype, the maternal one. Heteroplasmy is the presence of more than one type of mtDNA within an individual. A person is considered as heteroplasmic if she/he carries more than one detectable mtDNA type. Sequence chromatogram showing heteroplasmy in the HVS-I region, at the nucleotide position indicated A): G and A and B): T and C