Chapter 14 - Forensic Challenges: Degraded DNA, Mixtures, and LCN

Abstract

This chapter explores some forensic challenges including degraded DNA samples, mixtures, and low levels of DNA. Environmental exposure degrades DNA molecules by randomly breaking them into smaller pieces. Modern-day polymerase chain reaction (PCR) methods, such as multiplex short tandem repeat (STR) typing, are powerful because minuscule amounts of DNA can be measured by amplifying them to a level where they may be detected. The chapter illustrates the size reduction principle when creating reduced-size STR amplicons or miniSTRs. Regardless of the disadvantages, reduced-size STR assays have helped make possible some of the World Trade Center victim identifications from burned and damaged bone samples. Mixtures arise when two or more individuals contribute to the sample being tested. Mixtures can be challenging to detect and interpret without extensive experience and careful training. As detection technologies have become more sensitive, using PCR coupled with fluorescent measurements, the ability to see minor components in the DNA profile of mixed samples has improved dramatically. The three categories of mixtures are labeled as Type A, Type B, and Type C. The six primary steps for interpreting mixtures are outlined and three different approaches taken with statistical analysis of mixtures are given in the chapter. Computer programs can be used to aid the process of deciphering mixture components and determining mixture ratios and statistical calculations. Low copy number (LCN) DNA testing, sometimes referred to as “low-level DNA,” typically refers to examination of less than 100 pg of input DNA or about 15 diploid cells. The chapter also discusses the issues with LCN work and precautions to ensure optimal results with low-level D.