Principles of DNA Evidence
DNA is sometimes referred to as a “genetic blueprint” because it contains the instructions that govern the development of an organism. Characteristics such as hair color, eye color, height and other physical features are all determined by genes that reside in just 2% of human DNA. This portion is called the coding region because it provides the instructions for proteins to create these features. The other 98% of human DNA is considered non-coding and the scientific community has only recently begun to identify its functions.
Forensic scientists, however, use this non-coding DNA in criminal investigations. Inside this region of DNA are unique repeating patterns that can be used to differentiate one person from another. These patterns, known as short-tandem repeats (STRs), can be measured to define the DNA profile of an individual.
All cells, except mature red blood cells, contain DNA. Any sweat, semen, body fluids or skin cells left behind at a crime scene can be examined for their unique STR signature to possibly link a person to the sample. While thousands of people may share several markers of their STR signature, there has been no case to date where two people have been found to have matching STR markers in all 13 areas used for comparison (except identical twins).
Different DNA, Different Uses
DNA can be found in either the nucleus of the cell (the center of the cell), or the mitochondria outside of the nucleus. Inside the nucleus, there are two types of DNA: DNA can reside in either the autosomal chromosomes or the sex-determining chromosomes. Autosomal DNA is primarily used in criminal investigations because, with the exception of identical twins, no two people have the same autosomal DNA.
However, for certain cases, such as sexual assault, examining DNA residing in sex-determining chromosomes can be very helpful during analysis. If the suspect in a sexual assault case is a male (has Y chromosomes) and the victim is female (has X chromosomes), conducting an analysis of the STR patterns in cells with Y chromosomes (referred to as Y-STR) can eliminate from the analysis the cells belonging to the female. Y chromosomes are passed through the paternal line—a brother, father and male children will exhibit the same Y chromosome.
Mitochondrial DNA (mtDNA), on the other hand, is inherited from the biological mother, so all persons related maternally have the same mtDNA (although there is a slight chance that a change in mtDNA from parent to offspring could exist). Because mtDNA is present in much higher quantities than nuclear DNA and doesn’t degrade as quickly as autosomal DNA, mtDNA is useful for identifying missing persons or unidentified remains.