When biological evidence from a crime scene is processed to isolate the DNA present, all sources of DNA are extracted. Thus, non-human DNA such as bacterial, fungal, plant, or animal material may also be present in the total DNA recovered from the sample along with the relevant human DNA of interest. For this reason, the DNA Advisory Board (DAB) Standards that govern forensic DNA testing of forensic casework require human-specific DNA quantitation (standard 9.3). This requirement ensures that appropriate levels of human DNA can be included in the subsequent polymerase chain reaction (PCR) amplification of short tandem repeats (STRs) evaluated in a DNA profile.

Equally important is the fact that multiplex STR typing works best with a fairly narrow range of human DNA. Typically 0.5 to 2.0 ng of input DNA works best with commercial STR kits. Too much DNA results in overblown electropherograms that make interpretation of results more challenging. Too little DNA can result in loss of alleles due to stochastic amplification in a low copy number regime.

In recent years, research in human DNA quantitation has focused on new “real-time” quantitative PCR (qPCR) techniques. Quantitative PCR methods enable automated, precise, and high-throughput measurements. Interlaboratory studies have demonstrated the importance of human DNA quantitation on achieving reliable interpretation of STR typing and obtaining consistent results across laboratories.

From research sponsored by the National Institute of Justice

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Simple, Rapid, and Accurate Quantitation of Human DNA

Vermont Department of Public Safety
Award Number: 2000-IJ-CX-K012
Final Report (pdf)

Quantitation of DNA for Forensic DNA Typing by qPCR: Singleplex and Multiplex Modes for Nuclear and Mitochondrial Genomes, and the Y Chromosome
California Department of Justice
Award Number: 2002-IJ-CX-K008
Final Report (pdf)

From research sponsored by the National Institute of Justice