How DNA Evidence Works

Although most labs use either RFLP or STR techniques for their DNA analysis, there are situations that require a different approach. One such situation is when there are multiple male contributors of genetic material, which sometimes happens in sexual assault cases. The best way to resolve the complex mixture and sort out exactly which men were involved is Y-marker analysis. As its name suggests, this technique examines several genetic markers found on the Y chromosome. Because the Y chromosome is transmitted from a father to all his sons, DNA on the Y chromosome can be used to identify DNA from different males. Y-marker analysis can also be used to trace family relationships among males.

Another situation involves identifying old remains or biological evidence lacking nucleated cells, such as hair shafts, bones and teeth. RFLP and STR testing can't be used on these materials because they require DNA found in the nucleus of a cell. In these cases, investigators often use mitochondrial DNA (mtDNA) analysis, which uses DNA from a cell's mitochondria. Investigators have found mtDNA testing to be very useful in solving cold cases, which are murders, missing-person cases or suspicious deaths that are not being actively investigated. Cold cases often have biological evidence in the form of blood, semen and hair that has been stored for a long time or improperly stored. Submitting those degraded samples for mtDNA testing can sometimes break the case open and help detectives find the perpetrator.

A relatively new technique -- SNP analysis -- is also useful in certain cases where forensic labs are presented with highly degraded DNA samples. This technique requires that scientists analyze variations in DNA where one nucleotide replaces another. Such a genetic change is called a single nucleotide polymorphism, or SNP (pronounced "snip"). SNPs make excellent markers and are most often used to determine a person's susceptibility to a certain disease. But forensics labs turn to SNP analysis on occasion. For example, forensic scientists used SNP technology successfully to identify several Sept. 11 World Trade Center victims for whom other methods had failed.

In reality, analyzing a DNA sample is just a first step. Up next, we'll take a look at what happens after the analysis is complete.